Download Connect the bipolar junction transistor (BJT) into the circuit shown in

The BJT as an Electronic Switch.
Objective:
To observe the operation of a bipolar junction transistor (BJT) when used in an
electronic switching application.
Theory:
The collector current flowing in a BJT is controlled by the much smaller base current.
The relationship between base current and collector current in a BJT is given by
I C   .I B
Where:
IC is the current flowing into the collector terminal of the BJT
IB is the current flowing into the base terminal of the BJT
β is the current gain of the BJT – usually 100 or more.
This behaviour is useful in switching applications where it is desired to turn on or off
a large load current using a small switching current – e.g. a PLC output signal turns
on or off a large motor.
Procedure:
1. Connect the bipolar junction transistor (BJT) into the circuit shown in the
schematic diagram.
2. Set the d.c. supply voltage, VCC, to 15 volts and observe that the LED turns
on.
3. Measure V1 and V2 using a DMM.
V
4. Calculate the base current I B  1
RB
V
5. Calculate the collector current I C  2
RC
I
6. Calculate the current gain of the BJT   C
IB
7. Is IB greater than or less than IC?
8. Repeat steps 2-7 with VCC set to 15 volts. Compare the two calculated values
for β.
9. Now, remove RB and observe the LED turn off. Explain why the LED turns
off even though it is still connected to the power supply.
10. Connect two wires where RB was previously located and pinch them between
your finger and thumb – without allowing them to connect together directly.
Observe the LED turning on. Explain why this happens and suggest an
application of the effect that is observed here.