Transistor characteristics

The characteristics of the npn transistor can be found using the circuit shown in Figure 1

Three characteristics are usually measured and these are shown in Figure 2:

(a) the variation of base current (I_B) with base- emitter voltage (V_BE),
(b) the variation of collector current (I_C) with base current, and
(c) the variation of collector current with collector- emitter voltage (V_CE).

Figure 2(a) shows the 'switching on' of the transistor. There is no significant base current until the base-emitter voltage reaches about 0.6 V.

Figure 2(b) shows how the collector current varies when there is a change in the base current. You can see that a change in base current (ΔI_B) of a few microamps will produce a collector current change (ΔI_C) of a few miiliamps. This shows the amplifying action of the transistor and also that this amplification is current- controlled rather than voltage-controlled. The ratio of the change in collector current to the change in base current is called the current gain of the transistor, and is written as h_FE.

This usually has a value of between 100 and 200 for an npn silicon transistor.

Figure 2(c) shows the variation of collector current with collector-emitter voltage. There are several curves, each representing a different base current.