INTERFACING
Interfacing simply means: "connecting."  When a circuit connects a device (such as a microphone), to an amplifier, it is called INTERFACING. The characteristics of the microphone are matched to the input requirements of the amplifier.  Or a relay may need to be connected to the output of an amplifier (that does not have enough current to turn the relay ON).
In most cases, the output of a circuit or a "pick-up" device (sometimes called a TRANSDUCER) does not have enough VOLTAGE or VOLTAGE-SWING or AMPLITUDE to drive the next circuit or device and it needs an amplifier.
That's why we have to add a circuit between.
The circuit we add has a number of names:
When it increases the CURRENT, we call it a BUFFER.
When it matches a high impedance to a low impedance or a low impedance to a high impedance, we call it IMPEDANCE MATCHING.
Or when we need an increase in voltage, it is called an AMPLIFIER.
In ALL "stages" (common-base, common-collector and common-emitter) the current is increased.
Interfacing can be as simple as adding a resistor or capacitor, but this is usually called "connecting" or "coupling".

We have learnt that all devices and circuits have an ability to deliver a "waveform" or "amplitude" or "voltage" and this can be weak or strong according to the amount of current it can deliver.
We have also learnt that this current may be delivered from the load resistor or from the device itself. It does not matter how the current is delivered; the size of the current (the amount of current) is important.

We have also covered the fact that the input to a circuit (or "stage") requires current and when these two are equal, the matching is ideal.
But this rarely happens.
If the input requires more current, the voltage (or voltage-swing) from the previous circuit or device will be reduced. If the input requires less current, the voltage-swing will be affected a very small amount. But in ALL cases the voltage-swing will be reduced - because you ARE supplying SOME energy to the stage that follows.

Interfacing is not easy.
You have to know the output voltage of the device and the current it can supply.
The current it can supply is related to its OUTPUT IMPEDANCE.
OUTPUT IMPEDANCE basically means its output resistance.  A low resistance or LOW IMPEDANCE means it is capable of delivering a HIGH CURRENT. A high-impedance device cannot deliver very much current. A stage with a high output impedance cannot deliver very much current.
All these terms are relative. When we say: "cannot deliver much current" the value of current can be less than 1uA or 50mA.  It depends on the circuit we are discussing and if you are working with low-current circuits or power circuits.
We have also learnt that the input impedance of a stage can be high or low and the voltage-swing it will accept can be small or large.  (for instance, an emitter follower stage will accept a large input voltage).
This gives us a wide range of values (parameters) that may need to be joined together - INTERFACED.
In some cases the output voltage of a device or circuit will be HIGH and by connecting a capacitor between the two stages, the output voltage will be "absorbed" in the capacitor and the energy from the output stage will be transferred. The "energy" is a combination of the voltage-swing and the current. 
But if the output voltage is very small, we may need to amplify it to deliver a high voltage to a device.

This is the case in the following requirement.
A piezo diaphragm or electret microphone is required to be interfaced to the input of a microcontroller.
The output of these devices is about 10mV and the input of a microcontroller requires about 3.5v (3,500mV).
This involves an amplification (gain, amplification factor) of 10:3500 = 350 and requires two stages of amplification.
The output of a piezo and microphone are classified as high impedance and the input of a microntroller is also high impedance.
This means the two amplifying stages can be low-current stages (also called high-impedance stages) and the load resistors can be high-value (about 22k - 100k).
The following two circuits have been designed for this application:

Fig 79d.

Fig 79d. In this circuit the first transistor is self-biased and the 2M2 base bias resistor turns the transistor ON and the voltage on the collector is only about 1.8v. This means the collector has to drop by only 1.2v for the second transistor to turn off and the 100k will produce 5v on the input to the microcontroller. If the transistor has a gain of 100, the electret mic or piezo has to produce a 12mV signal to activate the circuit. When the load resistor is increased to 100k, the collector has about 850mV on it, and it only has to drop 300mV for the signal to enter the microcontroller. This makes the 100k load resistor produce a more-sensitive circuit. When no audio is being detected, the output of the second stage is 0v.

Fig 79e.

Fig 79e. This circuit has been taken from Fig 71acc. It is a bootstrap circuit and produces a very clever "switch." The circuit sits with the first transistor turned ON and the second turned OFF as can be seen in the first line at the top of the output waveform - up to the red dot. When a signal is picked up by the microphone (this is the red dot on the waveform), a negative-going signal of about 100mV will turn the transistor off slightly and the second transistor will turn ON. The 4u7 will be "pulled down" and completely take over from the signal from the microphone. It will turn the first transistor off more and the second transistor will be turned ON more. This will continue until both have completely changed states.

They will stay like this until the 4u7 is charged in the opposite direction and the base of the first transistor sees 0.7v. This causes the second transistor to turn off and the 4u7 rises and turns the first transistor ON more. The 4u7 gets slowly discharged and the circuit remains in this state. The circuit produces a very clean output every time it detects audio. The duration of the low in the graph can be shortened by reducing the value of the electrolytic.

 

Fig 79f

Fig 79f  interfaces a phase-shift oscillator (see Fig 90) to a speaker. This is a very difficult thing to do as the phase-shift oscillator has an output that is very close to rail-to-rail and any loading of the output will cause it to stop working. In an attempt to interface the oscillator to a speaker we have added an emitter follower transistor and a 1k separating resistor, plus a 100R in series with the speaker. This should give a loading of about 20k and the circuit should work. Otherwise the 10k will have to be reduced or the 100R increased.

ANALOG TO DIGITAL

Many of the circuits we have described convert an ANALOG signal to a  DIGITAL signal. These are called ANALOG TO DIGITAL CONVERTERS but we have not given them this specific name because we have been concentrating on other features. We will now cover the concept of Analogue to Digital Conversion. An ANALOGUE signal rises and falls but doesn't have any defined amplitude or frequency. This signal cannot be delivered reliably to a circuit that requires a DIGITAL SIGNAL as the amplitude may not be large enough. A DIGITAL CIRCUIT requires a digital signal and this type of signal is either a constant HIGH or LOW and the amplitude must be very close to rail voltage or almost 0v. And it must change from one state to the other very quickly. Delivering a high amplitude analogue signal may be recognised by a digital circuit when it reaches a peak or goes to 0v, but this is not guaranteed or reliable. In addition we may want the signal to be a CONSTANT HIGH when the audio is present. This is what an ANALOG TO DIGITAL circuit will do. It will produce a constant HIGH when audio is present and ZERO (LOW) when the audio is not present. Or pulses that are nearly rail voltage and very close the 0v. Recapping: To convert an analogue signal to a digital signal we need to deliver ZERO OUTPUT (called a LOW output) when the signal has a small amplitude and a HIGH output when the signal has a high amplitude. To do this we use a common-emitter stage, as it has a high voltage-gain and this is what we need. There are many ways to convert an Analogue signal to a Digital signal but the basic way is to amplify the signal by a large amplification-factor so the resulting waveform will rise to the voltage of the rail (or even higher). It cannot go higher than rail voltage but you will see what we mean in a moment.  This is normally called "over-driving" the signal and if this is done in an audio circuit, the result is distortion. But we are not going to listen to the output, so we take advantage of this feature to produce a DIGITAL OUTPUT.

Fig 80a

Fig 80a shows an analogue signal. It is made up of lots of sine-waves and may be as high as 2v or only a few millivolts. Low-level signals are generally expressed in mV, to make them instantly recognisable and easy to talk about.  In general this type of signal will be too small to be detected by a Digital Circuit. A Digital Circuit needs a signal greater than about 3,500mV so the waveform appears on the input line as a HIGH, during the peak of its excursion. It should be nearly 5,000mV for reliable detection.

Fig 80b. A Digital Circuit will detect a waveform larger than 4.5v as a HIGH and less than 0.5v as a LOW

Fig 80b.  Only the large excursion(s) will be detected by a Digital circuit as the other parts will not rise high enough to be detected. To increase the analogue signal to as much as 5,000mV, an amplifier is needed.

Fig 80c.

The amplifier maybe one or two stages, depending on the amplitude of the original signal. Each stage of an amplifier will increase the size of the signal about 70 times. If you are very lucky, you may get an amplification of 100x (100 times). Thus a 5mV signal with one stage of amplification will produce a 350mV to 500mV signal. This is not sufficient to be detected by a Digital Stage. Another stage will easily produce a full 5,000mV signal.  The second stage only needs to amplify the signal about 10 to 12 times and a higher gain simply drives the waveform into "bottoming" and "cut-off" as shown in fig 80c.

Fig 80d.

This means the waveform will be "clipped" at the top and bottom and converted to a fairly "square-ish" shape. Suppose you have a waveform that is higher than 5mV (say 30 - 50mV) and want to know if it will trigger a Digital Circuit with a single stage of amplification.  Connect the components as shown in Fig 80d and write a program to illuminate a LED when the waveform is detected.

There is only one problem with the circuit in Fig 80d. At the end of a whistle or speech, the LED may be illuminated or extinguished. It all depends on the last cycle of the waveform. The circuit sits with the output approx mid-rail and the micro does not know if this is a high or low, and takes the reading by the direction of the last cycle.  Some of the inputs of a micro are Schmitt Triggered. This means a HIGH has to be 85% to 100% of rail voltage for it to be seen as a HIGH and between 20% and 0% to be seen as a LOW.  The non-Schmitt Trigger inputs see a LOW as 20% to 0% and a HIGH as above 2v for 5v operation.  If the last cycle went from zero to mid-rail the micro will see the waveform as a low on Schmitt Trigger inputs and a HIGH on the other inputs. This problem can be overcome by adding a second stage that only produces a LOW when audio is detected. It also increases the amplitude of the audio to guarantee triggering of the Digital Circuit. This is shown in Fig 80e.

Fig 80e.

The second transistor in Fig 80e is called a DIGITAL STAGE. This simply means a biasing resistor is not connected to the base of the second transistor so it turns on fully when a signal greater than 650mV is detected and is turned off at other times. This stage is ideal for a micro or other Digital Stage as only two voltage levels are delivered. Either 0v or rail voltage (5v).The other advantage is it does not take any quiescent (idle) current.  This stage is only suitable if you are sure you have plenty of "over-voltage" to drive the transistor into saturation. By this we mean you must have at least 1v (1,000mV) drive signal so you can be sure the transistor will turn on (saturate). The fast rise and fall times means you have a "clean" HIGH and LOW.

Fig 80f.	Fig 80f couples a magnetic pick-up to the amplifying circuit so  the biasing of the first transistor can be determined by the value of the base-bias resistor. The coil cannot be connected directly to the transistor as the low impedance (resistance) of the coil will upset the bias on the base.
With this arrangement, the descending part of the input waveform of a few millivolts will turn off the transistor, while the ascending part of the waveform will not have any effect. A coil of wire of any size will be suitable and to make it an effective collector of magnetic flux, it should have a magnetic core such as ferrite. No other impedance-matching is necessary.

OSCILLATORS

There are over 20 different types of oscillators and many more variations. We cannot cover them all - so we will concentrate on the most often-used and explain how they work. Oscillators consist of one or two transistors. They start-up by one or more components in the circuit producing "noise" or a spike from the "mains" when the circuit is turned on. Some oscillators will not  start-up if the supply is increased gradually. When a spike or noise is detected, the rest of the circuit amplifies it. In most cases the noise comes from the circuit being turned ON but it can also come from the noise generated within the junction of a transistor. This noise is random and of little use, but it is fed to components such as coils and capacitors as they have the ability to produce a waveform that rises and falls smoothly and this is amplified to produce the output. When coils, crystals, capacitors and resistor are combined with transistors, many different effects and waveforms can be created and this all comes under the heading of OSCILLATORS. And the circuits are all amplifiers. An amplifier can be turned into an oscillator by providing POSITIVE FEEDBACK. The purpose of providing NEGATIVE FEEDBACK is to prevent oscillation.  The purpose of providing POSITIVE FEEDBACK is to create oscillation.  Positive feedback is when you take a point that is rising a large amount and pass it to a point that is also rising at the same time but only a small amount. In other words, the feedback line must be able to help or assist the small-signal line. If it does not assist the small-signal line,  NO oscillation will occur.   Some oscillators have a name - either after their inventor, by the way they are configured or by the shape of the wave. Some have 5 names. Some have no particular name and are just called Feedback Oscillators (positive feedback).

Fig 80.  A Feedback Oscillator

Fig 80. The 10n capacitor provides the positive feedback to keep the circuit oscillating.

Fig 81.  A feedback oscillator

Fig 81. The 10n capacitor provides the positive feedback to keep the circuit oscillating.

Fig 82.  The positive feedback line creates the CALL tone

Fig 83.

Fig 83. When the third transistor is turning OFF, the collector voltage is rising and this is passed to the base of the first transistor, to turn it ON. This is how the circuit keeps "cycling" or oscillating.

Fig 83a.  Globe flashes at 1Hz

Fig 83a. The high-gain amplifier we studied in Fig 66, for example, has negative feedback to prevent oscillation. By using positive feedback we can turn the high-gain amplifier into an oscillator. This circuit is simply a high-gain amplifier with both transistors turning ON via the 1k and 100k resistors. This makes the voltage on the collector of the BC557 rise and the 22u and 4k7 passes this rise to the base of the BC547 to turn both transistors ON more and more until they are fully turned ON. The 22u charges a little more and this reduces the current into the base of the BC547 to turn it off a little. This effect is passed to the collector of the BC557 and the two transistors start to turn OFF. When they are fully turned off, the cycle repeats by the transistors being turned on via the 1k and 100k.

Fig 83aa.  Simple Tone Oscillator

The 2-transistor amplifier we studied in Fig 42 can be changed slightly to drive a speaker. The two common-emitter transistors turn on together and the 22u is "lifted" to turn on the NPN transistor harder. Both transistors turn on until fully saturated and this puts current though the speaker. The 22u charges a little more and this reduces the current into the base of the NPN transistors, turning it off a slight amount. The PNP is turned off a small amount and they both keep turning off until fully turned off. The 10k and 50k start to charge the 22u to repeat the cycle. The 22u produces positive feedback. It can be replaced by values from 100n to 22u to change the frequency of the tone.

 

The two circuits above are examples of LOW IMPEDANCE outputs. If the load (the globe or speaker) is increased above about 47 ohms, the circuit will not work. They simply "lock-ON." This is because the  capacitor (electrolytic) must be pulled down by the load at a very critical point in the cycle. In addition, the 100k "turn-on" (or 50k and pot) resistor must be a very high value. If it is too low, the circuits will "lock-ON." The critical point is this: When the circuit is fully turned-ON, the right side of the capacitor is near rail voltage and it is being charged via the bas-emitter junction of the first transistor. As it becomes fully charged, the current into the base of the first transistor reduces slightly and the transistor turns off slightly.  This effect is passed to the second transistor and it turns off slightly too. The right lead of the capacitor drops and this lowers the left lead to turn off the first transistor slightly more. This is the beginning of the "turn-off section" of the cycle. If the second transistor did not have a very heavy load (low resistance load), the slight turning-off of the two transistors would not lower the capacitor and they would both remain ON.

You can see the importance of FEEDBACK in a circuit. Some circuits will not work without feedback and some will distort. Sometimes the feedback is POSITIVE and sometimes NEGATIVE. The trick to understanding a circuit is to locate the feedback (component or "line") and work out what it is doing.

Fig 83b.  Positive feedback comes from the 22u electrolytic. This is a very unusual circuit. Normally the feedback is obvious.

Fig 83b. Here's an oscillator circuit. We know it must have feedback to operate, but where is the feedback? In this circuit the 4 electrolytics are equivalent to miniature rechargeable batteries. When the circuit is turned on, they all get charged to a voltage according to the surrounding components but the 22u is the important component. The base of the BC557 sits at 4v and the emitter must rise to 4.6v for the PNP transistor to turn on. When it does, it turns on the BC547 and this transistor puts a load of 220R across the circuit. This reduces the voltage across the 470k/1M voltage divider and the base if the BC557 sees a lower voltage. During this time the 22u is acting as a miniature supply and maintaining the voltage of 4.6v on the emitter. The BC547 turns ON more and more and even though the voltage on the 22u drops, the circuit turns ON and this takes more current from the 6v battery and produces a click in the speaker.

THE SQUARE-WAVE OSCILLATOR

Fig 84.

When two transistors are cross-coupled as shown in Fig 84, you can safely assume the circuit will oscillate. The frequency of oscillation will depend on the value of the components but the oscillator is known as a FREE-RUNNING OSCILLATOR or ASTABLE (ay-stable) MULTIVIBRATOR and the output is a square wave. It will have an equal-mark-space ratio if the components are the same value. This circuit is also called a FLIP-FLOP.

Fig 85.

Fig 85. By rearranging the components in Fig 84, we can draw the circuit as one common-emitter stage driving another common-emitter stage with a 100u providing positive feedback. The circuit relies on the power being turned on quickly for it to start up. Both transistors will turn ON but one will turn on faster than the other and prevent the other turning on. The 100u connected to the turned-on transistor will start to charge in the opposite direction and the second transistor will start to turn ON. This will pull the 100u lower and the first transistor will start to turn OFF. This will continue until both transistors have changed states.

Fig 86.

Fig 86. Here is the ASTABLE MULTIVIBRATOR with the LEDs in the emitters instead of collectors (as is normal). The frequency of oscillation is approximately 1 second. The 330 ohm resistors set the LED current to 12mA for a 6v supply.

Fig 87.

Fig 87. The ASTABLE ("ay" - meaning not-stable) MULTIVIBRATOR circuit is rich in harmonics and is ideal for testing amplifier circuits. To find a fault in an amplifier, connect the earth clip to the 0v rail and move through each stage, starting at the speaker. An increase in volume should be heard at each preceding stage. This Injector will also go through the IF stages of radios and FM sound sections in TV's.

Fig 88.

Fig 88. The astable multivibrator can be made with PNP transistors.

Fig 89.

Fig 89. A circuit can be made with one NPN and one PNP transistor. It ceases to be a FLIP FLOP or Multivibrator as both transistor turn on at the same time and the circuit becomes a Relaxation Oscillator.

THE SINE-WAVE OSCILLATOR - also called the PHASE-SHIFT OSCILLATOR
A Sine-wave Oscillator can be made with a single transistor. 

Fig 90.  The Sinewave Oscillator

Fig 90. This circuit produces a sinewave very nearly equal to rail voltage. The important feature is the need for the emitter resistor and 10u bypass electrolytic. It is a most-important feature of the circuit. It provides reliable start-up and guaranteed operation. For 6v operation, the 100k is reduced to 47k. The three 10n capacitors and two 10k resistors (actually 3) determine the frequency of operation (700Hz). The 100k and 10k base-bias resistors can be replaced with 2M2 between base and collector. This type of circuit can be designed to operate from about 10Hz to about 200kHz.

Fig 91.

Fig 91. The phase-shift oscillator has 3 "sections" made up of a 10n capacitor and 10k resistor. This "section" is shown above and each "section" produces a delay or "phase-shift" of about 60° but the total must be 180°. The base and collector of a common-emitter stage are 180° out-of-phase, so the signal entering the base is 360° (IN-PHASE with the output). This creates POSITIVE FEEDBACK. This concept is very hard to understand so we need to explain it in simple terms. Points Y and Z are the ends of a long piece of rope and the three resistors are weights tied to the rope. You shake the rope up and down at Y and Z moves up and down at a later time in the cycle. You know this because you can make a wave travel down a rope. Exactly the same thing happens with a signal that enters at Y. It takes time for the peak to reach Z. Now consider the circuit at switch-on. The caps are uncharged and the 10k collector resistor pulls the three capacitors high. Taking into account the voltage-dividing effect of the three lower 10k resistors, the collector is possibly at about 2v. The three 10k resistors start to charge the three 10n caps and the voltage on the base falls. This makes the collector voltage rise. This continues until the collector cannot rise any further and the capacitors continue to charge and the voltage on the base drops. The 100k base resistor takes over and starts to discharge the 3rd capacitor and turn the circuit on. The collector voltage drops and the energy in the three capacitors get passed into the base to fully turn the transistor ON. This all happens in a "sliding motion" that produces a sweeping output called a SINEWAVE. It is a very "delicate" oscillator and any change to the LOAD (10k) may stop its oscillation. How to read the Graph: Get a ruler and hold it "up and down" on the page (or on the screen) so you view the right-hand edge of the ruler and can only see the word "phase" and "60° "  Now slide the ruler to the right and you will see the graph "A" gradually rising. Keep moving the ruler to the right and you will see graph "B" gradually rising. This is how you "interpret" the graph and see how graph "B" lags (is behind) graph "A."  If you don't read the graph correctly, it looks like graph "B" is in front of graph "A" - but this is not the case.

THE BLOCKING OSCILLATOR

Fig 92.  Fig 92a.  Fig 92b.

Fig 92. The BLOCKING OSCILLATOR circuit uses a transformer  to produce POSITIVE FEEDBACK to the base. The circuit starts by Rbias charging Cbb to deliver voltage to the base of the transistor via Rb. The transistor turns on and produces expanding magnetic flux in the primary of the transformer. This flux cuts the turns of the secondary (or feedback) winding and increases the base voltage and CURRENT. The voltage out of the top of the secondary winding is prevented from "disappearing" by Cbb. The transistor keeps turning ON until it cannot turn on any more. At this point, the current through the primary is a maximum but it is not expanding flux and its effect is not passed to the secondary winding. The base ceases to see its turn-on current and the transistor turns off abruptly. The heavy current through the primary is producing a very strong flux and it collapses, producing a voltage in both windings  of opposite polarity and very high amplitude. Fig 92a shows the base being "capacitor injected." This saves one capacitor and can produce a higher output. All the values and the transformer needs adjusting for the performance required. The start of each winding is shown with a dot. This assumes the windings are wound in the same direction. Figs 92b,c shows alternative ways to produce a blocking oscillator. The difficulty with producing a Blocking Oscillator is getting a suitable transformer.           Fig 92c

Fig 93.

Fig 93. A simple BLOCKING OSCILLATOR circuit can be made with a 10mH inductor and 80 turns of very fine wire wound on top. The piezo diaphragm reacts to the very high "FLYBACK VOLTAGE" produced by the primary when the transistor turns off. This type of circuit is often used to produce very high voltages.

Fig 94.

Fig 94. This LED Torch circuit uses the "flyback" voltage of a BLOCKING OSCILLATOR to illuminate a 3.6v super-bright LED from a 1.5v supply. Note: the 10n capacitor prevents the energy from the feedback winding being lost. All the energy from the feedback goes into the base of the transistor to turn it on hard.

Fig 95.

Fig 95 shows a Blocking Oscillator producing a regulated 5v from a 1.2v supply.

Fig 96.  2-transistors in PUSH-PULL - as a Blocking Oscillator circuit Fig 96a.

Fig 96. A simple extension of the Blocking Oscillator in Fig 92c above, is shown in this diagram. It consists of two BLOCKING OSCILLATOR transistors that are turning each other off. The circuit starts by one transistor being slightly faster than the other. It turns ON and produces magnetic flux that cuts the turns of the other half of the primary winding to increase the voltage from the battery and at the same time it reduces the voltage to the base of the other transistor - because the transistor allows only a very small voltage to appear across the collector-emitter terminals when it is turning ON.  It keeps turning on until it is fully ON. At this point the flux is no longer expanding and the generated voltage in the winding that supplies the base voltage (and current) ceases. This turns it off a small amount and the magnetic flux starts to collapse and produce voltages in the opposite direction. The voltage (and current) to the base is less than before and this turns the transistor off more. The voltage to the base of the other transistor starts to rise and that transistor takes over. The two transistors operate in PUSH-PULL mode. To reduce the wasted power in the 220R resistors, Fig 96a uses Darlington transistors and 2k2 0.5watt resistors. The circuit is used to drive a CFL lamp from a 12v battery.

The difficulty with producing a Blocking Oscillator is getting a suitable transformer. A similar "flyback voltage" can be obtained from an inductor. This will need an oscillator made up of two transistors to drive the inductor.

Fig 97.

Fig 97.  This circuit is a "Buck Converter" meaning the supply is greater than the voltage of the LED. It will drive one high-power white LED from a 12v supply and is capable of delivering 48mA when R = 5R6 or 90mA when R = 2R2. The LED is much brighter when using this circuit, compared with a series resistor delivering the same current.
But changing R from 5R6 to 2R2 does not double the brightness. It only increases it a small amount.
The inductor consists of 60 turns of 0.25mm wire, on a 15mm length of ferrite rod, 10mm diameter.   Frequency of operation: approx 1MHz. This circuit draws the maximum current for a BC 338.
When the circuit is turned on the 330p gets charged. This turns on the BC547 and keeps the BC338 off. When the 330p is charged the BC547 is not turned on as much and the 2k2 can start to turn on the BC338. It pushes the charge on the 330p into the base of the BC547 to keep it off. The 330p gets discharged by the 330R and the voltage across the *R resistor turns on the BC547 to turn off the BC338. The 1N4148 absorbs the high-voltage flyback pulse. The circuit is only active for a very short period of time and off for a longer period of time. This delivers a small amount of energy to the high powered LED and allows the LED to be connected to a 12v supply (via the circuitry).
THE FLYBACK OSCILLATOR

Fig 97a

A flyback oscillator is any oscillator where the transistor turns off quickly and abruptly during part of the cycle and allows the energy (the flux) in an inductor to collapse suddenly to produce a high voltage IN THE OPPOSITE DIRECTION. The circuit in Fig 97a consists of a transformer with a feedback winding of 40 turns. It can be constructed as a piece of test equipment to test transistors, zeners and LEDs.

THE BOOST CIRCUIT or BOOST CONVERTER

Fig 97aa

Any circuit that converts a low voltage to a higher voltage is classified as a BOOST CONVERTER or BOOST CIRCUIT. Fig 97aa will drive a super-bright white LED from a 1.5v cell. The 60 turn inductor is wound on a small ferrite slug 2.6mm dia and 6mm long with 0.25mm wire. The main difference between this circuit and the two circuits above is the use of a single winding and the feedback to produce oscillation comes from a 1n capacitor driving a high gain amplifier made up of two transistors. The feedback is actually positive feedback via the 1n and this turns on the two transistors more and more until finally they are fully turned on and no more feedback signal is passed though the 1n. At this point they start to turn off and the signal through the 1n turns them off more and more until they are fully turned off. The 33k turns on the BC557 to start the cycle again.

THE BUCK CONVERTER

Fig 97b

Any circuit that converts a high voltage to a lower voltage is classified as a BUCK CONVERTER. Fig 97b will drive a 1watt white LED from a 12v supply and is capable of delivering 300mA. The driver transistor is BD 327 and the inductor is 70 turns of 0.25mm wire wound on the core of a 10mH inductor.  The voltage across the LED is approx 3.3v - 3.5v  The 1R is used to measure the mV across it.  300mV equals 300mA LED current. The diode MUST be high speed. A non-high-speed diode increases current 50mA. This circuit is a very good design as it does not put peaks of current though the LED.

MORE OSCILLATORS
The Armstrong, Clapp, Colpitts, Hartley, Wien Bridge and even unknown oscillators like the one below all use capacitors, resistors and coils to create a circuit called a RESONANT CIRCUIT and these two components produce a sinewave when they receive a pulse of energy.

Fig 98.

Most oscillators are described on the web and you can decide which type you need for your particular application.

OSCILLATOR SUMMARY
Look for a TUNED CIRCUIT and feedback line. It will be an oscillator.
Most have a high-impedance output and must be connected to a circuit that will not "load" them (and reduce the amplitude of the output) or prevent them oscillating.  But some oscillators have a very low output impedance and can drive a low-impedance device. You have to be aware of these features.


IMPEDANCE MATCHING

Every electronic component has a value of resistance. You can measure this value with a multimeter. But sometimes the value changes according to the light it receives, the frequency it is operating-at, or the voltage it is connected-to, or the sound it receives, or its temperature or many other influences. Sometimes the output from a circuit might be 2v, but if you put a low-impedance device such as a speaker on the output, it "kills" the sound. Or you may have a nearly flat 9v battery. It measures 5v with a multimeter but when you connect a 3v motor, it does not work. These are both examples of poor IMPEDANCE MATCHING - yes, the battery has a HIGH Impedance and that's why it cannot deliver the current required by the motor. What is IMPEDANCE MATCHING? Impedance Matching is is connecting two items together so: "THEY WORK." Some devices PRODUCE or DELIVER a signal or a voltage or a current. Some devices ACCEPT a signal or voltage or current. We need to connect these types of devices together. Let's start with those that DELIVER: An amplifier may be able to produce an output of 2v, but when a low-impedance device (low resistance device) such as a speaker is connected, it cannot deliver the CURRENT needed to drive the speaker. The same with a flat 9v battery. It cannot deliver the CURRENT needed to drive a 3v motor. You cannot "test" or measure the output capability of a device. You must connect it to the input of the project you are designing and measure the waveform or voltage being delivered (or transferred). If the voltage or waveform is considerably less than when it is not connected, you have decide if the attenuation (reduction) is acceptable. The ideal situation is NO attenuation - but in nearly all cases you will get some attenuation. There are no "rules to follow" and every case is different. However when the output of a device is NOT reduced when it is connected to a circuit, the two items are said to be IMPEDANCE MATCHED. There are three ways to "Match Impedances:" 1. via a resistor. This is generally a poor way to match impedances and is very inefficient. But it may be the only way to connect two devices. 2. via a capacitor. This can be a very good way to match impedances. 3. via a transformer. Generally the most efficient way to match impedances but requires a lot of calculation and expense in getting the transformer designed and manufactured. Impedance Matching can also be referred to as "MATCHING" and the simplest example is connecting a 6v globe to a 12v battery. This is called "Resistance Matching" or "Current Matching" or "Voltage Matching" because the resistance, voltage and current are known quantities. [To connect a 6v globe to a 12v battery you can use a resistor or put two 6v globes in series. Using a resistor will be very difficult because a globe requires about 6 times normal current to allow it to start illuminating and then it takes the "rated current."]   But when when a device produces a signal; the voltage, resistance and current changes during the production of the signal and because these values are not constant, we use the term IMPEDANCE MATCHING. Impedance really means "resistance that changes during the production of the waveform." Impedance matching can be worked out mathematically, but you need to know all the parameters of the device and the circuit you are connecting together. This is rarely possible to obtain. Rather than calculate the result, it is much easier and more-accurate to connect the two items and view the result on a CRO (Cathode ray Oscilloscope). But if you cannot do this, simply connect them and listen or view the output from a speaker, relay or LED etc.      We have already studied "Impedance Matching" in the circuits above, but did not identify the concept. We will now go over some of the circuits and show where impedance matching took place: Fig 6 In Fig 6, the transistor matches the HIGH IMPEDANCE of your finger to the LOW IMPEDANCE needed to turn on the LED. The transistor converts the 50k resistance of your finger to less than 0.5k (due to the gain or amplification of the transistor of about 100 -200).  You can also say it matches the HIGH RESISTANCE of your finger to the LOW RESISTANCE needed to turn on the LED. Fig 64 In Fig 64, the transistor matches the LOW IMPEDANCE of the speaker to produce a HIGH IMPEDANCE output on the "out" terminals, suitable for delivering to the input of an amplifier. The transistor converts the 8 ohms of the speaker to more than 800 ohms (possibly 1600 ohms) due to the gain or amplification of the transistor (about 100-200) and at the same time the 0.5mV produced by the speaker is amplified to about 40mV to 80mV.    Fig 71f The 100n capacitor in Fig 71f  matches the impedance of the electret microphone to the input impedance of the transistor. The impedance of the electret mic is about the same as the input impedance of the transistor but the mic needs about 0.5mA to operate and the voltage on the base of the transistor needs to be very accurately set for "self bias." A capacitor separates these slightly different DC values while passing the AC signal..   Fig 71e Sometimes Impedance Matching is needed to separate or remove the DC component of a signal. In Fig 71e, the electrolytic matches the LOW IMPEDANCE output of the amplifier to the LOW IMPEDANCE of the speaker. The two impedances are almost identical and you could connect the speaker directly to the output of the amplifier, but the output has a voltage of approx mid-rail and this would enter the speaker and shift the cone when no audio is being reproduced. And the speaker would only be able to amplify the negative part of the waveform. To remove the DC component of the waveform, an electrolytic is included.

SATURATING A TRANSISTOR
This is the last topic for discussion because it is the last thing to attend to when designing a circuit.
We have explained the fact that a transistor turns ON when the base voltage is above 0.7v and the current though the collector-emitter leads is approximately 100 times the base current.
This means a transistor with a gain of 100 will deliver 100mA to a collector LOAD when 1mA enters the base.
This is theoretically true and will occur in nearly all cases, but some devices such as motors and globes need a lot more current to get them started or to get them to turn ON because the cold resistance of a globe is only about 1/5 its hot resistance. This means a 100mA globe needs 500mA to get it to start to glow.
The same with a motor. The starting or "stalled current" is 5 times more than the operating or "running current.
On top of this the transistor might not have a gain of 100 and the voltage may be slightly higher than expected. All these things means the transistor must be turned ON with more than 1mA into the base.
If you deliver 2mA, it does not mean the transistor will deliver 200mA to a LOAD. If the load requires 100mA, delivering 2mA to the base will simply turn the transistor ON harder and the collector-emitter voltage will be slightly lower, but the load will still draw (or take) 100mA.
But the devices we mentioned above require 500mA to get them started, so the base current needs to be 5mA.
Now, here's the unfortunate part, 5mA base-current will not deliver 500mA collector current. The transistor needs more than 5mA base-current to get it to deliver this HIGHER current. It needs about 7mA.

This process can be proven by experimentation.
Simply increase the base current until the device is turned ON, then measure the base current. Add 1mA to 3mA to guarantee reliability and the circuit is complete.
This process is called SATURATING A TRANSISTOR or GUARANTEEING TURN-ON, or FULLY SATURATING the TRANSISTOR or FULLY TURNING the TRANSISTOR ON.

HYSTERESIS
Hysteresis is a feature of a circuit. It is when the circuit turns on at a particular voltage and turns off when a higher or lower voltage is reached. The gap between these two voltage-levels is called the HYSTERESIS GAP.
This is a very handy feature.
It prevents an effect called "hunting."
If a circuit turns on at say 6v, and turns off at 5.7v, any slight variation in the supply voltage will cause the output to change state. This may produce an undesirable effect of the circuit turning "on and off" at the wrong time due to supply voltage fluctuations. By increasing the gap between these two voltages, the circuit will remain in one state or the other - until the input voltage (the controlling voltage) increases or decreases.
The Schmitt Trigger (Fig 79a) is an example of a circuit using Hysteresis.
Any circuit with a positive feedback line, introduces the effect we are talking about.
The feedback line has the effect of assisting the input voltage.
In other words, it widens the gap between an ON state and an OFF state. 
This is called POSITIVE FEEDBACK because it ADDS to the effect of the input voltage.
Even when the input voltage is falling, the feedback improves the ON or OFF state by taking the circuit past the point where the change takes place. 
Rather than thinking of the feedback as "positive," consider it as AIDING.
All HYSTERESIS feedback is AIDING or ASSISTING the effect you are trying to produce. 
 
This circuit uses Hysteresis. The main "assisting component" is the 22k.

Fig 103cc

This is how the circuit works: When the circuit is turned on, the base of the second transistor gradually develops 0.6v and the transistor turns ON. The voltage between collector and emitter is about 0.2v and the third transistor is OFF. When the first transistor receives an AC signal, an increasing voltage on the base causes the collector voltage to reduce and the charged 4u7 electrolytic moves towards the 0v rail. The negative lead of the 4u7 goes below the 0v rail by about 0.6v. This allows the second diode to discharge the 10u electrolytic and the 0.6v on the base of the second transistor is reduced. Let's say it is reduced to 0.55v. This causes the second transistor to turn off and the positive lead of the 1u electrolytic rises toward the 12v rail. The negative lead of the 1u rises too and this makes the transistor turn ON. In this process the 1u starts to charge and it has the effect of slowing down the "turning ON" of the second transistor. But the pulses keep coming from the first transistor and 10u is kept discharged. The 1u keeps charging and eventually it is fully charged and now the pulses from the first transistor can finally turn off the second transistor. The third transistor is turned ON and the 22k connected to the collector of the third transistor reduces the voltage on the base of the second transistor by about 0.15v This helps the pulses from the first transistor to keep or put a low voltage on the base of the second transistor and even if these pulses stop, the voltage on the base will take time to rise via the 15k and this is called the HYSTERESIS GAP. When the circuit changes state, the pulses from the first transistor will discharge the 10u and this will be "fighting against" the 15k and 22k resistors that will be trying to charge the 10u.

VOLTAGE TO CURRENT CONVERTER
This sounds very complex but it is very simple.
The simplest voltage-to-current component is a resistor.
A resistor performs lots of different jobs, depending on the circuit.
One of its jobs limits the current to a LED. It is called a CURRENT LIMITING RESISTOR. It can also be called a VOLTAGE TO CURRENT CONVERTER.
Here's how it works:

Fig 103d. A resistor is a VOLTAGE TO CURRENT CONVERTER

A red LED must be delivered a voltage of exactly 1.7v for it to work. In other words it must be connected to a 1.7v supply.
But a 1.7v supply is very hard to obtain, so we use a 3v supply and a dropper resistor.
The resistor converts the 3v to 1.7v.
This is easy to understand because the 3v supply is fixed at 3v and when a voltage is delivered to the red LED it develops exactly 1.7v across it. The resistor sits between the 3v and 1.7v
When the voltage of the supply is increased, The voltage across the LED remains at 1.7v and the voltage across the resistor increases. This is shown in the diagrams above.
When the voltage across a resistor increases, the current through it increases. That's how we get 3mA, 7mA and 10mA. This is called VOLTAGE TO CURRENT CONVERSION.
The VOLTAGE on the input goes up and down and the CURRENT through the LED goes up and down.
The input CURRENT will also go up and down but we are only covering the fact that the input VOLTAGE rises and falls and the output CURRENT RISES and falls.
Any circuit that produces this effect is called a VOLTAGE TO CURRENT CONVERTER.
A transistor can also be connected to produce VOLTAGE TO CURRENT CONVERSION.
The following circuit is an emitter-follower. It is also a VOLTAGE TO CURRENT CONVERTER.  A rising and falling voltage on the input creates a rising and falling CURRENT on the output.
It also produces a rising and falling voltage on the output but we are only concerned with the fact that the circuit produces a rising and falling CURRENT on the output when the input VOLTAGE rises and falls.

Fig 103e. An emitter-follower is a VOLTAGE TO CURRENT CONVERTER

The circuit in Fig103e requires say 1mA input current. The output current will be 100mA. The circuit has the capability of increasing the current or AMPLIFYING the current. The resistor circuit above does not AMPLIFY the current. It is only a voltage-to-current converter.
The transistor performs a VOLTAGE TO CURRENT CONVERSION and also produces CURRENT AMPLIFICATION.
A common-emitter stage also performs VOLTAGE TO CURRENT CONVERSION.

Fig 103f. A common-emitter stage is a VOLTAGE TO CURRENT CONVERTER

A slight increase in the voltage on the base of a common emitter transistor will increase the current through the load by a large amount.
As you can see, there are lots of circuits that perform VOLTAGE TO CURRENT CONVERSION but we usually identify them for other features and that's why the term VOLTAGE TO CURRENT CONVERSION is rarely mentioned.
There are also special circuits (using op-amps) to perform precision voltage-to-current conversion, but we are concentrating on transistor stages.

 
 CURRENT TO VOLTAGE CONVERTER
A resistor can be used as a CURRENT TO VOLTAGE CONVERTER.
Fig 103g shows a resistor called a SENSE RESISTOR.
It is a low-value resistor in series with one line of a circuit and its function is not to change the operation of the circuit in any way.

Fig 103g.  Measuring the "sense resistor"

Its function is to produce a very small voltage across it and this voltage is detected by a circuit (basically a voltmeter (or milli voltmeter).
When a current flows though a resistor, a voltage is produced across the resistor. You can also say a VOLTAGE DROP is produced across the resistor. If the resistor is exactly 1 ohm, a voltage of 1v will be produced across it when 1 amp is flowing or 1mV is produced for each 1mA of current. Using a 1 ohm resistor produces an easy conversion.
If the circuit is 24v or 50v, a loss of 1 volt will not be noticed.
But if the circuit has a lower voltage, (say 5v) the resistor will be need to be a lower value so the drop across the sense resistor does not upset the operation of the circuit.
The actual value of the resistor is not important for this discussion, It can be 1 ohm or 0.1 ohm.
The important point is to understand the function of a Sense Resistor.
In the circuit above, if the globe is replaced by a 20watt or 50watt, globe, the current through the sense resistor will increase. We measure the voltage (in millivolts) across the resistor and we convert the value into CURRENT.  This is a CURRENT to VOLTAGE CONVERSION.
A transistor can be used as a CURRENT TO VOLTAGE detector. Fig 103h shows a 1 ohm sense resistor connected to a transistor. When the circuit is turned ON, the charging current (the current flowing into the battery) will be high and when the voltage across the sense resistor reaches 0.65v, the transistor turns ON and the voltage on the collector reduces. This turns on the red LED and reduces the voltage on the ADJ terminal of the LM317T regulator and the regulator outputs a lower current to the batter. This is how the circuit limits the charging current. The resistor is converting the current flowing through the circuit (and into the battery) into a voltage, and the transistor detects the voltage. The transistor is not detecting or measuring the current. It has absolutely no idea of the amount of current flowing. It is detecting the voltage across the resistor. The resistor is performing the CURRENT to VOLTAGE conversion.

Fig 103h.  The 1ohm Sense Resistor.

SQUEALING, BUZZING, OSCILLATING,
and MOTOR-BOATING
We have studied POSITIVE FEEDBACK and the effect it produces.  It turns an amplifier into an oscillator.
The following circuit will not work:

Fig 104a.

The three stages of amplification will produce so much gain that the circuit will self-oscillate. The output will be a "buzzing-sound" and the fault will be impossible to find because it comes from within the design of the circuit. The first thing you must do is add "power-supply decoupling."
The unwanted sound produced by the circuit is called MOTOR-BOATING and is generated in the "front-end" by very small noises or "disturbances" and amplified by the stages that follow.
Fig 104b shows where the noise starts. It can be produced by the electret microphone or by the noise in the junctions of the first transistor (due to current flowing in the collector-emitter circuit).

Fig 104b.

This waveform will be very small and almost impossible to detect via any test-equipment, but it will start in the first stage and pass through the coupling capacitor as shown in Fig 104c.
The next stage will amplify this "noise" and it will be amplified further by the following stages.
There will be some slight cancellations from the various stages as the signal will be "out-of-'phase" but the end result will be a "putt-putt-putt" or squealing from the output.
The general term for this is called MOTOR-BOATING and is due to the high gain of the circuit.
The noise will appear on the power rail and get passed to the front-end where it will be amplified more.

Fig 104c. The positive feedback loop producing "Motor-boating"

This effect can be reduce and eliminated by a term called DECOUPLING.
Decoupling is achieved by adding capacitors [electrolytics] (and resistors) across the power rails so that each stage is effectively powered by a separate supply.
Adding an electrolytic can sometimes make a big difference and sometimes it will make no difference.
It all depends where it is connected and the value.

Fig 104d.

Fig 104d shows an electrolytic connected across the power rails. This is called DECOUPLING THE POWER RAILS and effectively tightens up the power rails so that any noise on the positive rail is removed. 
But, as you can see, the power rails extend to the first transistor and although the rails may be "tight" near the battery they can "move" near the first stage.
This is due to the wiring between the stages or the tracks on the PC board. That's why an electrolytic across the battery may have little effect on removing our motor-boating problem. 

Fig 104e.

Fig 104e shows an electrolytic connected across the supply that feeds the electret microphone and 1k2 resistor to separate the supply we have just created, from the main supply rail.
We have effectively created a separate power supply. It is fed by a 1k2 and kept "tight" by the 10u capacitor.
The electrolytic does not have to be a high value because the electret mic takes very little current and the voltage-waveform (the AC signal) produced by the microphone is very small (about 20mV).
These two items very effectively decouple the microphone from the supply rails so the microphone has its own supply. The 1k2 resistor does most of the "separation." The voltage-drop across it will be very small and it will not affect the operation of the circuit, but the small voltage-drop will prevent any noise on the power rails being fed to the microphone via the 10k resistor.

Fig 104f.

To remove any slight motor-boating problems (if they still exist); a power-supply filter (called power-supply decoupling) made up of a 1k2 and 10u can be placed after the first amplifier stage as shown in Fig 104f.
By selecting the value of capacitance and resistance, this arrangement will remove almost all motor-boating problems. It is a very-effective form of suppression.
Decoupling is most-effective on the pre-amplifier stages, however every circuit is different and these two components only deal with the low-frequency motor-boating type of instability. Some circuits also produce high-frequency oscillations (about 1MHz) and these need removing by a different value of capacitor-feedback.

BREAKDOWN and ZENER MODE
There are two conditions or states where a transistor can be instantly damaged. This is due to voltage applied in the wrong direction or the application of voltage that is higher than the rating of the transistor.
Voltage will kill a transistor faster than excess current.
A high voltage spike can damage a transistor instantly.
However if the excess voltage does not have enough current to damage the transistor, it will recover and we can use this feature in a circuit.
Breakdown and zener mode are different.
In breakdown mode, suppose we have a transistor that has a specification of 85v for the voltage it will withstand between the collector and emitter as shown in Fig 104g: It will  "resist" a voltage of 85v and this voltage will appear across the collector-emitter leads. When the voltage increases to 86v, 87v ... the transistor will suddenly breakdown and only a few volts will appear across it. This fires the trigger transformer in the circuit above.
If the current is very small, the transistor will not be damaged and when the voltage is removed and a lower voltage applied, it will operate as an undamaged device. 
  In zener mode, the base-emitter junction is connected to a voltage higher than 9v via a resistor. The junction will breakdown and a voltage of about 7v will appear across the base-emitter leads and the excess voltage will be dropped across the resistor.
The zener-effect or zener mode can be used to produce white noise or a 7v zener reference. Fig 104h shows the first transistor with the base-emitter junction reverse-biased to produce a "noisy zener" via the 1M feeder resistor. The noise is picked off via the 100n and amplified by the remainder of the circuit.
 

TRANSISTOR TESTER

This circuit is basically a high gain amplifier with feedback that causes the LED to flash at a rate determined by the 10u and 330k resistor. Remove one of the transistors and insert the unknown transistor. When it is NPN with the pins as shown in the photo, the LED will flash. The circuit will also test PNP transistors. To turn the unit off, remove one of the transistors.

ZENER TESTER
The maximum voltage a transistor can withstand is called the ZENER VOLTAGE of the transistor.
It is Vce - the voltage between (across) collector and emitter.  It is also the maximum supply voltage or circuit voltage or the voltage generated by an inductor in the collector-circuit and can be tested via the following circuit. This circuit will also test ZENER DIODES and LEDs.

TRANSISTOR and ZENER TESTER CIRCUIT
The circuit is a flyback oscillator. This type of oscillator energises an inductor then turns off very quickly and the magnetic field (flux) produced by the inductor collapses and produces a very high voltage in the opposite direction. The maximum voltage produced by the circuit depends on the "maximum voltage capability" of the transistor. The voltage produced by the inductor is over 120v but the transistor will zener at a voltage lower than this and thus the output voltage will be determined by the characteristic of the transistor. A diode on the output of the inductor passes this high-voltage-spike to a 1u electrolytic, which stores the energy and provides a high voltage output. The circuit will test transistors up to 120v and zeners up to the voltage produced by the transistor. The project is built on a strip of PC board cut into lands with a file or saw.  The following diagrams shows the parts placement and connecting the 5 button cells to the board. The project can be built in an evening and added to your TEST EQUIPMENT.
	TESTING A TRANSISTOR When testing a transistor, fit it into the pins marked C B E. If you have a LED connected to the LED terminals, it will glow. If you remove the LED and measure the voltage across the 1u electrolytic, it will provide the maximum working voltage for the transistor.   TESTING A ZENER When testing a zener, place it in the pins provided. If the zener is around the wrong way, the voltage across it will be less than 1v. When it is placed correctly, you can read the zener voltage with a high impedance multimeter such as a digital meter. TESTING A LED When testing a LED, fit it into the pins for the LED with the cathode lead (the shorter lead) to the left. It will glow very dim because the dropper resistor is very high and only allows 4 - 6mA to flow. This will give you a good idea of the relative brightness of a LED when compared to others in a batch.