BASIC 
ELECTRONICS COURSE 
Page 16 INDEX

 

 

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OUR FIRST TRANSISTOR CIRCUIT

Suppose we want to turn on a globe via a transistor. This may seem a complex way to do things but it demonstrates the operation of a transistor. Suppose the switch is not large enough (strong enough - i.e: the contacts are not large enough to take the high current) to turn on the globe. This is highly unlikely but it demonstrates the reason for the inclusion of the transistor.  A transistor is capable of turning ON when a very small current is fed into the base. This means the current through the switch will be very small while the current through the globe and transistor will be very large - maybe about 250 times larger.

We have shown on the previous pages that a resistor is capable of supplying a large or small current depending on its value. We showed that if a high value resistor was included in series with a LED, it would receive only a small current and not be very bright. This is the type of resistor we need for the transistor. It has to be a high value so that only a small current flows into the base. Actual values will come later as they will depend on the circuit you are designing.

In the animation below, the component names are identified in the LHS frame and when the switch is closed, a low current flows in the base circuit and a high current in the collector-emitter circuit.
The base current actually joins the high current and flows to the supply via the 0v rail - thus creating a complete path.

The Right-hand frame shows the direction of conventional current flow and this is the direction "we talk about" when discussing circuits. 
Electrons, the moving particles of electricity, actually flow in the other direction (called ELECTRON FLOW direction) but since components have arrows on them to show the direction of "current flow" we stick to the easy method of describing things. 
Electricity actually flows very fast (at the speed of light) because an electron is pushed in at one end of the wire and another electron comes out the other end. 
The diagram shows the large and small current flow and the direction of the flow.
"Voltage rail" is the positive rail and means any voltage suitable for the globe. This can be 3v to 12v and must be DC.   "0v"  means the negative of the battery supplying power to the circuit.  

The circuit above is a simple "on-off" circuit because the transistor can only turn the globe on and off. This type of circuit is also called a DIGITAL circuit since a digital circuit has only two states ON and OFF. 
This is the simplest digital circuit you can create. It is called a REPEATER circuit as the transistor is simply repeating the operation of the switch. In digital terms: switch = 0,  globe = 0.    switch = 1   globe = 1. 
A transistor is capable of doing much more than turning a globe on and off. It is capable of dimming it to any value of brightness.

Question 56: How can you dim the globe?

Ans: Use a higher value resistor for the base resistor. With a higher value, less current will flow into the base and the transistor will not be capable of delivering enough current to turn the globe on fully. 

The transistor is simply amplifying the current in the base circuit and every transistor has a maximum amplifying factor. If this value is say 250 and the current required by the globe for full brightness is 250mA, we have to deliver 1mA into the base.  If the resistor is chosen so that it delivers less than this, the globe will be dull. 
To create a circuit to do this we can select different-value resistors and try them one at a time or add a variable resistor in series with the base resistor. (You cannot leave out the original base resistor and replace it with a variable resistor as the transistor will be damaged when the variable resistor is reduced to a low value when the shaft is rotated in one direction).  
A variable resistor is called a POTENTIOMETER. To see the effect of adding extra resistance to the base circuit, click: "View."

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Question 57: Name the three leads of a transistor.

Ans: Collector, base, emitter.

Question 58: Name the input lead of a transistor:

Ans: base

Question 59: The globe will be bright when the resistance of the potentiometer is  high/low?

Ans: low

Question 60: What is the direction of current flow?

Ans: From positive to negative

Now we will add a capacitor to the circuit to show the effect it has on the brightness of the globe, when it is connected for the first time. We have already discussed how it works and the animation shows the globe starting with full brightness when the uncharged capacitor is connected to the circuit. The reason is the uncharged capacitor "looks like" a very low-value resistor to the rest of the circuit and that's exactly like the potentiometer turned to low-resistance. 
As the capacitor charges, the globe dims and finally goes out. It goes out because the capacitor does not pass DC. It only allows a current to flow when it is charging. 

There is one thing you have to know about the animation above. The effect shown only works the FIRST TIME you close the switch because the capacitor is uncharged at the beginning. If the switch is opened, the capacitor will be fully charged and when the switch is closed for the second time, the globe will remain unlit. You have to discharge the capacitor for the circuit to work the second time. 

Discharging the capacitor means to connect its two ends together. This can be done mechanically with a switch or by electronic means. If the two ends are connected together directly, it is called SHORT-CIRCUITING the capacitor. If the two ends are connected via a resistor or some other device, it is called DISCHARGING the capacitor. If you want the capacitor to repeat the effect shown above, it must be discharged between cycles. This is one of the hardest things to do in electronics, but it must be done (even partially discharged) for the capacitor to work in the next cycle. More about this later.

There are two more things you have to know about a transistor to understand how it works. These are "characteristics" of a transistor (just like the voltage drop across a LED or the voltage drop across a diode) and cannot be changed. They are the "secret" to knowing how a circuit works.  

The two characteristics are :
1. The base "turn-on" voltage, and 
2. The collector-emitter voltage drop. 

In the animations above we have shown the transistor "turning on" when a voltage is supplied to the base and a current is delivered that is sufficient to turn the transistor on. 

There is one point we haven't mentioned. The voltage on the base must be 0.7v for the transistor to turn on. If the voltage supplied to the base is below 0.6v, the transistor DOES NOT TURN ON AT ALL. This is an amazing characteristic and we can show it in the following animation. There is a very small gap between 0.6v and 0.7v where the transistor changes from "not turned on all all" to "fully turned on" and maybe we can say the transistor is partially turned on at 0.65v. 

For this experiment we use a voltage divider resistor. This is a potentiometer connected between positive and negative rails and as the shaft is rotated, the output will vary from 0v, to full rail voltage. We only want a voltage between 0.6v and 0.7v and the pot (potentiometer / voltage divider resistor) has to be turned very carefully. 

The second characteristic of the transistor is the voltage between collector and emitter when the transistor is fully turned on. This state is called "SATURATION" the voltage is approximately 0.3v. The animation below shows a multimeter connected between the collector and negative (0v) rail. When the transistor is off, the voltage on the collector will be rail voltage (in this case 6v). When the transistor is partly turned on, the voltage will be about 3v. 

There's another way to see what is happening between the transistor and globe. You can think of the two components as resistors. In the first frame, when the transistor is not conducting, it can be thought of as a high value resistor connected to a low value resistor (the globe). This produces a high voltage (rail voltage) at their join. 
In the second frame, the resistance of the transistor is approx equal to that of the globe and the voltage at their join is about half-rail voltage. In the third frame the resistance of the transistor is considerably less than the globe and the voltage across the transistor is very small. This is shown in the animation below:

Now we come to combining the facts we have learnt and see why the "turn-on voltage for the base" and "collector-emitter voltage" is so important for the operation of some transistor circuits. There are basically two types of transistor circuits. DIGITAL and ANALOGUE. Digital circuits work on the principle of the transistor being either fully ON or OFF. Analogue circuits allow the transistor to be partially turned on. Digital circuits are used in computers for storing information and transferring it from one area to another. Some oscillators, such as the multivibrator, are digital in operation as the transistors are either ON or OFF.

Analogue circuits are used for audio  and some types oscillators such as sine-wave oscillators. The next circuit demonstrates how one transistor (Q1) can control another transistor (Q2). It is fortunate Vbase turn-on voltage = 0.7v and Vcollector-emitter = 0.3v otherwise many transistor circuits would not work!
In the circuit, Q1 is turned on and off via the potentiometer as shown in the animation above. We know the collector voltage will change from rail voltage when turned off to 0.3v when turned on. The base of Q2 is connected directly across the collector-emitter terminals and when Q1 is turned on the voltage across it is lower than the 0.7v required to turn on Q2 and thus Q2 is turned OFF. If you are constructing this circuit, it will be difficult to see the change in voltage on the collector of Q1 because the base of Q2 is connected directly to it and the voltage will only rise to 0.7v when Q1 is off. Note also, that the globe is off when the voltage on the base of Q1 increases and on when the voltage on the base of Q1 is zero. This is the opposite to the animation above because Q2 is INVERTING THE SIGNAL. In other words, Q2 causes INVERSION. 

Question 61: When the voltage on the base is 0.6v, the transistor is: ON, Off?
Ans: The transistor is OFF. 

Question 62: To turn a transistor on FULLY, the voltage on the base must be:
Ans: 0.7v

Question 63: When a transistor is turned-on, the voltage between the collector and base is :
Ans: 0.3v

ADDING A CAPACITOR

There is just one more thing we have to cover before we can go into explaining how various different circuits work. It's the secret of discharging a capacitor so an oscillating waveform can be passed to a transistor. We say above, a capacitor will only operate the first time in an oscillating circuit. Once it is charged, it does not provide the required effect. The solution is to discharge it so it can be ready for the next cycle. To understand how this is done, we need three more animations. 

The first animation shows the capacitor charging when the voltage on the left-hand side rises. When the voltage falls, the capacitor is fully (or nearly fully) charged and you can think of it as a tiny battery of say 5v (if the supply to the circuit is about 6v).
Replace the capacitor with the 5v battery and now turn the potentiometer to take the left-hand side down to the 0v rail. What happens to the right-hand side of the battery? It drops by 5v and actually goes 5v below the 0v rail. The base sees this -5v and and as we explained above, the transistor does not turn on unless a +0.65v is present on the base. This means the 5v battery will not discharge into the base of the transistor and it will remain charged. 

There is no point is raising the left-hand side of the battery (capacitor) to the positive rail as the capacitor is charged and will not turn on the transistor. This is shown in the animation below:

 The solution is to put a resistor between base and 0v rail to discharge the 5v battery (capacitor). The capacitor (battery) does not have to fully discharge but it best to discharge it as much as possible. This will depend on the value of the capacitor and resistor.  

Now you can see how a capacitor puts negative voltage on the base of a transistor. It would be almost impossible to show this effect without the aid of animation. 

The animation above is showing three things. The potentiometer moving up and down is exactly the same as a waveform entering the circuit. We saw this effect above and performed it with the pointer of the mouse. The waveform could be a sinewave or audio. Any oscillating waveform will create the same effect with the capacitor. The length of the battery represents the charge in (or on) the capacitor and you can see how the capacitor fills and empties as the wave enters the circuit. The transistor only turns on when the wave is  0.65v higher than the voltage in the capacitor and this might occur when the waveform is 3v or 4v. It all depends on the size of the capacitor and base resistor.

When the wave is falling, the charge in the capacitor is removed by the base resistor and this continues until the wave turns the transistor on again. If the incoming frequency is 1,000Hz (1kHz), the action is occurring 1,000 times per second. If the frequency is 100MHz, it is occurring 100,000,000 times per second!

Question 64: How does the capacitor in the animation above, get charged?

Ans: The incoming waveform is the "supply" and the capacitor is charged via the base resistor and the 0V rail completes the circuit. 

Question 65: Give two names for the waveform produced by the potentiometer in the animation above, moving up and down:

Ans:      AC   sinewave, 

Question 66: Explain why a capacitor passes AC:

Ans: Electricity flows in and out a capacitor when a waveform is delivered to one of its leads and the other components in the circuit see this as exactly the same as the flow of electricity. 

Question 67: How does the capacitor in the animation above gets discharged? 

Ans: Via the base resistor and through the 0v rail. 

Question 68: When the waveform in the animation above starts to fall, why does the transistor turn off immediately?

Ans: The waveform only has to fall 0.1v and the transistor changes from "FULLY TURNED ON" to "NOT TURNED ON."
 

This completes some of the most important concepts to understanding how a wide range of transistor circuits work - especially oscillator circuits such as the feedback amplifier and multivibrator. These are the next two circuits we will be covering. 



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