The characteristics of a transistor describe how the currents through it – the current through the base or collector or emitter – depends of the voltages between the various terminals – the base/emitter voltage, the collector/emitter voltage or the base/collector voltage. Not all of these are independent. The currents are not independent for example because the emitter current is the sum of the base and collector currents, so we need only consider two voltages and two currents. In fact we consider only
The variation of base currentwith base/emitter voltage
The variation of collector currentwith base current
The variation of collector currentwith collector/emittor voltage
We can find the characteristics of a transistor using the circuit below.
The graphs below are typical. The second graph clearly shows the transistor acting as an amplifier, since a small change in base current (measured in) produces a much larger change in collector current measured in