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Using a Transistor For Current Gain

Experiment E5.

Many years ago it was found that a thin springy wire poking into a crystal of germanium has strange properties. It allows current to flow one way but not the other. This cats whisker point contact diode is famous for its use in the very early radio receivers.

Then it was discovered that two cats whisker diodes created on one crystal of germanium interact in such a way that the current in one affects the current in the other. The first crude transistor had been created.

We are going to use the very simple circuit on the left to demonstrate how a transistor can be used to amplify current.


The picture on the left shows the most common arrangement of transistor connections. The leads are pointing upwards.
Wiring the circuit.....

Switch off the 5 volt supply by moving the slide switch to the left then follow these instructions:-

1. If you are following on from the previous experiment remove all the components except S1. Remove the yellow link connecting H6 to J9. Remove the bare wire link connecting J11 to Z11.

If you are starting with a blank plugboard fit push button switch S1 between H1 & H3 and F1 & F3.

2. Bend and trim the leads of a 100k resistor as shown in the first experiment. Fit R1 between I3 and I7.

3. Take a BC548 NPN transistor. Fit Tr1 with its base in J7, emitter in Y9, and collector in J8. The leads are just long enough to stretch this far so make sure that all the leads enter the sockets deep enough to make contact.

4. Take a 1k resistor which you have already formed and trimmed. Fit R2 between G8 and G12.

5. Take a red LED that you have already formed and trimmed. Fit D1 with the short lead in H12 and the long lead in H16.

6. Fit bare wire link from J16 to Z16.

Testing the circuit.....

Switch on the 5 volt supply.

Press push button S1 and the LED will glow.

Study the circuit. A transistor is a device which amplifies current. A small current into the base causes a large current to flow into the collector. In this circuit we are using the transistor as a switch. When the push button is pressed a small current flows into the base of the transistor via the 100k resistor. The transistor then tries to draw a large current but that is limited by the 1k resistor to about 3 milliamps.

The push button switch is switching a tiny current but the current that flows into the LED is about 3 milliamps. We are using a small current to control a bigger current. This is what we mean when we say that the transistor has current gain.

Longer time constants.....

In experiment E3 we used a 100uF capacitor to light an LED. The LED takes 3 mA which discharges the capacitor quite quickly. If we wanted the LED to stay on for several seconds using that simple circuit we would need to use a much large value capacitor which would be bigger and more expensive.

Now that we have a device with current gain we can make the time constant much longer using the same capacitor. To do this we draw a much smaller current out of the capacitor and use our current amplifier to drive the 3 milliamps that the LEDs needs.

Switch off the 5 volt supply to the plugboard. Add the capacitor as follows:-

7. Fit a 100uF capacitor with its short lead (-) in Z4 and long lead in J3.

Switch on the 5 volt supply to the plugboard. Press the push button for a very short time. The LED will light and stay at full brightness for about half a minute and then slowly turn off.

The transistor that we are using has a current gain in our circuit of about 100 so the LED stays on 100 times longer.

You may have noticed that in the picture there is another resistor R3 which we have not used in the circuit which we wired on the plugboard. So think about this question. How much current flows into the capacitor in the circuit we have wired at the instant the push button is pressed?......

The answer is that the current at that moment is only limited by the maximum current that the 5 volt line can supply. If you have a high power 5 volt supply as your source the current may be as high as several amps, but the push button switch contacts are able to handle just 50 milliamps.

If your power supply can provide a high current it will flow for a very short time but even so the switch contacts will burn a little each time and they will wear out relatively quickly.

We know that our circuit will only be used a few times so we do not need to worry. A professional electronic engineer would wire the circuit as shown in the picture.

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