The transistor is biased to operate in the active region. The micro ammeter and voltmeter are used in the base-emitter circuit to measure the base current (IB) and the voltage between base and emitter. Similarly, milliammeter and voltmeter are connected in collector-emitter circuit to measure the collector current (IC) and voltage between collector and emitter (VCE).
Input characteristics are graphs between VBE and IB at different constant values of VCE. To plot input characteristics, the potentiometer R1 in the emitter-collector circuit is adjusted till the voltmeter shows constant value. Then potentiometer in the emitter-base circuit is adjusted in such a way that base-emitter voltage is zero. For this value, base current is also observed to be zero.
Keeping the VCE constant, VBE is increased gradually and change in base current is noted with the help of micro ammeter. To plot input characteristics at VCE = –2 V, the potentiometer in emitter-collector circuit is adjusted till the voltmeter in the same circuit reads 2 V. Then, potentiometer in the emitter -base circuit is adjusted to make VBE zero. Then VBE is increased gradually, keeping VCE constant.
Similarly, the input characteristics of the transistor in the CE configuration can be drawn for different values of VCE.
The nature of input characteristics is similar to the forward characteristics of p-n junction diode. The base current remains zero as long as the base voltage is less than the barrier voltage (for silicon transistor, it is ~0.7 V). As the base voltage exceeds barrier voltage, current begins to increase slowly and then rises abruptly.
These curves are similar to the ones obtained for the CE configuration for n-p-n transistor.
From the reciprocal of the slope of the curve of input characteristic, the a.c input resistance of the transistor can be calculated.
Rin = ∆VBC/∆IB
In this configuration, Rin is typically of the order of one kΩ.
These are graphs between collector-emitter voltage (VCE) and the collector current (IC) at different constant values of base current (IB). To draw these characteristics, VCE is made zero and VBE is adjusted till the micro ammeter in the base-emitter circuit is set to read a constant value.
Thus, VCE is adjusted to make IB constant at a particular value. Now keeping IB constant, VCE is increased from zero in a number of steps and the corresponding collector current IC is noted with the help of milliammeter connected in series with collector.