A transistor as a current amplifier
Practical Activity for 14-16
A minute current in the base-emitter circuit is used to control a much larger current in the collector-emitter circuit.
Apparatus and Materials
For each student group
- NPN transistor (mounted if possible)
- Ammeters, 0-100 mA., 2 (depending on the transistor)
- Cell, 1.5 V in holder
- Cells, 1.5 V in holder, 4 (or stabilized low voltage DC supply)
- Resistor (680 ohms, 1 W)
- Lamp in holder, 6 V 60 mA.
- Leads, 4 mm, 10
- Crocodile clips, 3 (if necessary)
Health & Safety and Technical Notes
It will help if the transistor is mounted on a base with three 4 mm terminals. Otherwise, make connections to it using crocodile clips.
- In the transistor, a minute current in the base-emitter circuit is used to control a much larger current in the collector-emitter circuit. Arrange the components of the circuit as in the diagram, and connect them as follows.
- Connect the rheostat to form a voltage-divider across the 1.5 V cell.
- Connect the slider terminal to the fixed resistance of 680 Ω., one ammeter (range 100 mA.) and the base terminal of the transistor.
- Connect one end of the voltage-divider to the emitter terminal of the transistor.
- Connect the collector terminal of the transistor to the other milliammeter (range 100 mA.), the small lamp and the 6-volt battery and back to the emitter (which is already connected to the voltage-divider).
- Now try the following experiments:
- Firstly, leave the base circuit open, with no connection to the base. You will see no detectable current in the collector circuit.
- Join up the base circuit. The voltage for a suitable base current is less than 1 volt. Start with no voltage from your voltage-divider, and increase the voltage until the lamp in the collector circuit glows.
- Read the milliammeter in that circuit.
- Look at the other milliammeter, in the base-emitter circuit. Is any current flowing to the base? If there seems to be no current, try switching the supply on and off, and see whether the milliammeter's pointer moves at all.
- Your transistor is
amplifying current. Comparing the two milliammeter readings gives you an idea of the amplification.
- Increase the base current a little, causing an increase in collector current. The ratio of the two currents will remain approximately constant.
- The collector current will level off at about 60 milliamps, which is the limit imposed by the lamp in the circuit. Any further increase in the base current will have no further effect.
- Students may not have experimented with transistors previously. You could describe a transistor to them like this:
- A transistor is a tiny chip of semi-conductor materials. It is rather like a sandwich of a piece of cheese between two slices of bread.
- The transistor's emitter corresponds to a thin slice of bread
- The transistor's base corresponds to the cheese
- The transistor's collector corresponds to a thick slice of bread.
- The transistor needs a small voltage to cause a base current to flow: less than 1 volt. That is easily obtained from a voltage-divider (
potentiometer) across a cell. -You may need to ensure that students are familiar with the use of a three-terminal variable resistor for tapping off a voltage in this way.
- Students should plot a graph of collector current against base current. This is one of several possible characteristic graphs for a transistor.
- The experiment could be extended with the addition of voltmeters. Students could then look at, for example, how the base current and collector current depend on the input voltage Vinput.
This experiment was safety-tested in October 2006