Bredhurst Receiving and Transmitting Society

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Transistor junctions can be measured for go/no go situations, but the test gives no indication of what the current gain maybe.

A simple circuit can be constructed as shown with a fixed base current of 5 micro amps the collector current can be measured and so give an indication of current gain.

Try testing a number of transistors and work out their current gain. Results from a batch of transistors may be as follows.

An approximate gain table
Meter reading hfe
1mA 200
0.9mA 180
0.8mA 160
0.7mA 150
0.6mA 120
0.5mA 100
0.4mA 80
0.3mA 60
0.2mA 40
0.1mA 20

The results obtained in the table are achieved because the two base resistors maintain a current flow of 5 micro amps.

To carry out the same tests with PNP transistors use the same circuit but just reverse the supply connections. (A PNP transistor has to be biased on from the negative rail).

RULES FOR SUBSTITUTION a transistor in a circuit.

1. The substitute transistor must be of the same variety (ie.silicon, NPN, switching as opposed to amplifying etc).

2. The substitute transistor should have about the same hfe value.

3. The substitute transistor should have the same or better ratings of maximum voltage and current.

4. The substitute transistor must physically fit - same case style and pinout and size :-)

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