BASIC
ELECTRONICS COURSE
Page 23 INDEX

So far we have showed how a transistor responds to a signal delivered to it from an external source. But the input signal can come from the transistor itself. This is called FEEDBACK and there are two types of feedback:
1. Positive feedback
2. Negative feedback.

Just as in the real world, everybody likes "positive feedback," it makes them feel bigger, better and more important. In electronic circuits positive feedback INCREASES the amplitude of the waveform being produced. Negative feedback REDUCES the amplitude being processed. The different between positive and negative feedback is the PHASE of the signal. If the phase of the signal (the feedback signal) is "out of phase," the result is NEGATIVE FEEDBACK. If the feedback signal is "in phase," the result is POSITIVE FEEDBACK.

NEGATIVE FEEDBACK
If the feedback signal is rising  when the transistor requires a falling signal, the output will decrease and this is known as NEGATIVE FEEDBACK. In other words the feedback signal is operating against the amplification of the stage.
There is a hidden advantage in this. If the stage produces distortion, the output signal will have "bumps" and "spikes" that are not present in the input waveform and by passing the output back to the input, these "spikes" will decrease the input waveform just at the points where the spikes are appearing and the result is they will be eliminated!

POSITIVE FEEDBACK
If the feedback signal is rising at the same time when the transistor requires a rising signal, the output will increase and this is known as POSITIVE FEEDBACK. The phase of the signal will be "in phase" and the result is the circuit will oscillate.
For a transistor to operate as an oscillator it must be provided with POSITIVE FEEDBACK. The problem is: How does the circuit start to oscillate
All transistors produce a small amount of noise within the transistor and this will appear in the output. If this noise is fed back to the input AT EXACTLY THE RIGHT INSTANT (as a positive feedback), the output will be more noisy and this can increase to a very high level to produce noise called "random noise" or "white noise."  Turning a transistor ON also produces a "spike" in the output and this can be used to start the oscillation.
If we add components from the output to the input we can control the speed at which the transistor turns on and off. This creates the FREQUENCY at which the circuit will oscillate.  In electronics, the "natural waveform" or the "natural oscillation of a circuit" is the SINEWAVE.
We have already seen how a rising and falling voltage produces a sinewave and this effect can be produced by a single transistor plus a number of controlling components.
There are many ways of  passing a signal from the output of a transistor to the input and we will study two of these, then get "hands-on" experience with some projects.

SINEWAVE OSCILLATORS
We will study the PHASE-SHIFT OSCILLATOR and COLPITTS OSCILLATOR.

THE PHASE-SHIFT OSCILLATOR
This circuit is easy to recognise by the three equal-value capacitors and two equal-value resistors connected to the base of the transistor. We have already mentioned the signal at the output of a transistor connected in COMMON EMITTER mode is FALLING when the signal at the input is RISING. This is called OUT OF PHASE signals and you cannot connect the output directly to the input to get the transistor to oscillate.
If you do, the rising output will be fed into the base to turn the transistor ON and this will reduce the output. The signal has to be delayed by a short period of time to allow the output voltage to rise and then the transistor can be turned ON to reduce the voltage on the output.
This delay is created by a set of capacitors and resistors on the base. We have already mentioned the fact that a capacitor takes a period of time to charge and this feature is utilized in the PHASE-SHIFT OSCILLATOR circuit. The value of the components create a time-delay and this sets the frequency at which the circuit operates.   But it's a bit more complex than a single "time-delay" arrangement. Each capacitor is doing something different at each part of the cycle.

The interesting point to note is the feedback signal only has to be about 1/50th of the collector signal for the circuit to operate as the gain of the transistor is about 50 to 100. This means there can be a lot of ATTENUATION (reduction) in the signal and the circuit will still operate.
There are two paths between the collector and the emitter. The "1M path" and the "three 22n path."
The signal through the 1M finds it difficult to pull the base up or down "quickly" because the 22n capacitor has a very large "holding effect" on the signal.
The other path is via the three 22n capacitors and this can operate on the base very easily.
We will start at the beginning. When power is applied, the transistor is not turned on and the voltage on the collector is RAIL VOLTAGE.
This voltage is passed to the three uncharged capacitors and they pull the base up very quickly to turn the transistor ON. The secret to the operation of the circuit is the voltage across the second 10k resistor. During this time the voltage across it is 0.7v and this reduces slightly as the first capacitor charges, so that the "hard turn-on" of the transistor is reduced.
The collector voltage is allowed to gradually rise and as the charge on the first two capacitors increases, the charging current reduces and this means the voltage across the second 10k resistor  reduces. This reduction in voltage is passed to the base of the transistor via the 3rd 22n capacitor and the transistor turns off a little.
As the first 2 capacitors charge, the current through the 2nd 10k resistor reduces and this turns off the transistor MORE and MORE.
Finally the two capacitors are charged and the voltage on the 2nd 10k resistor does not alter.
The 1M base-bias resistor now comes into operation by turning the transistor ON and the collector voltage falls. The "falling effect" is passed directly to the base via the three capacitors with the result that they turn the transistor off slightly. The 1M takes over by charging the 22n and this controls the rate at which the transistor turns off.
The falling collector voltage discharges the first 22n via the first 10k resistor and the 2nd 22n discharges via the combination of both 10k's.
As you can see, the operation of the circuit is much more complex than explained in any text book and its important to know exactly what is happening so that when you look at the waveform on a CRO, you understand how the waveforms are produced.

The animation above shows the output voltage of the Phase shift oscillator is rising and falling. We have already seen a rising and falling voltage produces a sinewave as shown in the diagram opposite.  This is how a rising and falling voltage produces a sinewave.

THE COLPITTS OSCILLATOR
Another circuit that produces a sinewave is the Colpitts Oscillator. It is recognised by a small capacitor tapping the TUNED CIRCUIT to monitor the waveform.

THE COLPITTS OSCILLATOR CIRCUIT
The colpitts oscillator consists of a tuned circuit made up of inductor L1 and capacitor CT (sometimes called the tuning capacitor) and a transistor in common-base mode. At the frequency of operation for the circuit, the capacitor C1 on the base of the transistor prevents the base moving (rising and falling) and this puts it in the common-base configuration.
Resistor Rb is the base bias resistor. It is designed to turn the transistor on at the beginning of the cycle.
Re is the emitter resistor and keeps the emitter from the 0v rail so the emitter can be injected via capacitor Cf (the feedback capacitor) to keep the oscillator operating.

This circuit has two features we will cover in detail. They are:
1. EMITTER FEEDBACK
2. The TUNED CIRCUIT

EMITTER FEEDBACK
For a circuit to be self-oscillating it must have positive feedback. This can come in a number of forms. It can be intentional or due to poor design. If a circuit is not designed correctly it can self-oscillate. This is an undesirable situation as the frequency at which the circuit oscillates is unknown and uncontrollable. We are dealing with CONTROLLED FEEDBACK.
Using controlled feedback we can provide positive feedback to either the base or emitter.
Up to now we have shown how delivering a waveform to the base of a transistor will allow it to amplify the signal but the same effect can be gained by injecting the waveform into the emitter.
The base and emitter are effectively tied together and separated by 0.7v. Any waveform delivered to the base can be delivered to the emitter but if it is delivered to the emitter, it must be 100 times "more powerful" to produce the same voltage shift.

THE TUNED CIRCUIT
The TUNED CIRCUIT also called the PARALLEL RESONANT CIRCUIT or TANK CIRCUIT and the two components that make up this arrangement called PASSIVE devices. In other words they are not active (amplifying) devices such as transistors and each component by itself cannot amplify, but when they are placed together, they perform an amazing feat. They AMPLIFY a signal (a pulse) delivered to them and turn the pulse into a natural curvy waveform called a sinewave.

The two components we are talking about are: a coil and  a capacitor.
When they are placed in parallel, they produce a circuit that has a natural frequency of oscillation.
Depending on where it is placed in a schematic (circuit diagram), it can be called a RESONANT CIRCUIT or TANK CIRCUIT.  TANK CIRCUIT is reserved for its placement in the output of a transmitter, where the effect of the circuit is to store energy (like a tank of water) that has been delivered in short bursts and deliver it over a long period of time.

Before we go into the discussion it's best to see the coil and capacitor in action. Watch the animation below and see the "charges of electricity" moving from the capacitor to the coil and back again. It's the time take to charge the capacitor and then go to the coil to produce magnetic flux, that creates the gradual rise and fall (voltage rise and fall) across the combination.

THE TUNED CIRCUIT

To start the circuit into operation, a short burst of energy has to
be applied. This has not been shown in the animation above, but
if you mouseover the frame opposite it will be pulsed with a
burst of energy. The energy from the pulse goes into the capacitor
because it is uncharged and readily accepts the pulse. The coil,
on the other hand, has a characteristic called IMPEDANCE and
this prevents it from accepting the initial burst of energy.
It only likes to receive energy at an initial SLOW RATE and that's why it accepts the energy LATER in the cycle, as shown above.

After a burst of energy is pumped into the circuit, the coil and capacitor respond by producing a sinewave.
This is an amazing effect for two seemingly simple components and is one of the earliest phenomena to be discovered. Without it, electronics would never have got off the ground; certainly not in the radio field.

It is the basis of all transmitters as well as many other types of radio circuits. The name "Tank Circuit" came from ham radio operators who used a coil and capacitor in the output of their transmitters to improve the output. The circuit stores bursts of energy from the output stage like a tank and delivers it smoothly to the antenna.
The way it does this is the energy is firstly passes into the capacitor. The energy (the voltage) is also presented to the coil and it is converted to magnetic flux. This flux cuts the turns of the coil and produces a voltage that is of opposite polarity and this has the effect of pushing against the incoming voltage. That's why the capacitor is first to receive the energy.
As it charges, the coil gradually begins to accept a flow of energy and since  the coil has a very low resistance, it eventually takes over to take the charge from the capacitor.
The coil converts the energy into magnetic flux and this is passed to the surrounding air or into the core material of the coil. While the coil is receiving energy and producing magnetic flux, the flux is called EXPANDING FLUX. But the capacitor soon runs out of energy and the flux cannot be maintained.
The flux surrounding the coil COLLAPSES and produces COLLAPSING FLUX and the magnetic flux lines cut the turns of the coil to produce a voltage (and current) in the coil that IS OPPOSITE POLARITY to the original voltage.
Refer to the animation above and see the energy coming out of the coil is opposite to that entering it. This is one of the amazing features of a coil and you can see the capacitor being charged in the OPPOSITE DIRECTION by the coil.
When the magnetic lines fully collapse, the capacitor is fully charged (but slightly less than before due to the losses in the circuit) and the cycle repeats, this time with the voltage from the capacitor in the OPPOSITE DIRECTION to the original charge.
This action will repeat a number of times. Each time the amplitude of the voltage will be slightly less (due to losses in the circuit).
All it requires is the initial pulse to be presented to the circuit AT THE CORRECT TIME and the circuit will repeat its transfer of energy form coil to capacitor and back again.
This is done by a circuit monitoring the waveform and turning on at exactly the correct instant so that the losses are replaced and the full amplitude is maintained.

The result is a sinewave that can have an amplitude greater than the applied voltage. This is amazing and is due to the collapsing magnetic flux producing a voltage that is higher than the delivering voltage. The voltage mainly depends on the speed at which the field collapses. If this voltage is double the applied, the quality factor or "Q" factor is 2. If the voltage is 10 times, the Q factor is 10.
Some parallel resonant circuits can have a Q factor of 20, 50, 100, 500 or more. It all depends on the coil and the percentage of energy tapped off for monitoring etc.

The tuned circuit is very important. Even though it appears to be very simple, it takes a lot of skill to design for the correct Q.
The physical size of the coil and capacitor must be worked out as well as the correct value and placement of the parts as the current that circulate between the two can be higher than the current entering the circuit!
A high Q arrangement is required in a receiver to obtain good selectivity - so that adjacent stations can be separated from one another. A low Q will cause them to come through in a jumble.
A tank circuit in the output of a transmitter will have a low Q as most the energy will be transferred to the antenna during each cycle and its main function it to match the output stage to the antenna,
rather than provide a high Q factor.

The animation below shows the voltage across the combination is changing direction very quickly to produce a SINEWAVE. This is shown on the CRO (Cathode Ray Oscilloscope). The waveform on the CRO will be stationary when it is set-up correctly. The red mark on the screen indicates the peak, zero and minimum values of the waveform and these correspond to the voltage on the capacitor at different points in the cycle.

Once the Tuned Circuit is given a pulse, the energy will flow back and forth between the two components an infinite number of times, except the fact that the voltage decreases on each cycle due to the losses (mainly in the coil) when the energy is converted into magnetic flux then back into "electricity."

Our next project, METAL DETECTOR-1 is based on a Colpitts Oscillator. This is a very simple but effective project using a single transistor to generate magnetic flux in a 12cm (5inch) diameter coil. An AM radio is placed near the coil and tuned until a tone of the lowest frequency is heard.
The Metal Detector project becomes a RADIO STATION and transmits to the AM radio. When the radio is "tuned in" the tone from the speaker is a very low frequency.
When a piece of metal is brought near the coil, the Metal Detector changes frequency and this is picked up by the radio and a whistle is heard. The coil is very sensitive and a small coin can be detected at a range of 10cm. You can use this project to hunt for coins at the beach.  On the next page we cover METAL DETECTOR-1.

Question 102: Name the two types of feedback.

Ans:  Positive and Negative

Question: 103 Name the type of feedback that increases the gain of the stage.

Ans:   Positive.

Question 104: Name the two sinewave oscillator circuits we have studied:

Ans: Phase Shift, Colpitts

Question 105: How do you recognise a Phase Shift Oscillator?

Ans: The two equal-value resistors and three capacitors on the base.

Question 106: Name the timing components in the Colpitts Oscillator:
Ans: The capacitor and coil in the tuned circuit.

Question 107: In the Colpitts oscillators, is the tuned circuit a parallel tuned circuit or series tuned circuit?

Ans: Parallel tuned circuit

Question 108: What are two other names for the PARALLEL TUNED CIRCUIT:

Ans: Tank Circuit, resonant circuit.

Question 109: What is the "quality factor" of a tuned circuit known as:

Ans: The "Q Factor."

Question 110: When a coil collapses, is the direction of the collapsing voltage produced by the coil the same as the voltage energising the coil?

Ans: No.  The collapsing voltage is REVERSE to the supplying voltage.

Question 111: Name the output pin of the transistor for the output of the Phase Shift Oscillator and Colpitts Oscillator:

Ans: Collector.