Question

With a neat circuit diagram, explain the working of an nPn transistor in CE mode as an amplifier with input and output waveform.

Answer 1

In NPN transistor, two n-type semiconductors(emitter and collector) are separated by p-type semiconductors(base).

$\text{Operation:}$

The basic circuit diagram for studying input characteristics is shown in the circuit diagram. The input is applied between base and emitter, the output is taken between collector and emitter. Here emitter of the transistor is common to both input and output and hence the name Common Emitter Configuration.

Input characteristics are obtained between the input current and input voltage at constant output voltage. It is plotted between VBE and IB at constant VCE in CE configuration.

Output characteristics are obtained between the output voltage and output current at constant input current. It is plotted between VCE and IC at constant IB in CE configuration.

$\text{Input characteristics:}$

1. Connect the circuit as shown in the circuit diagram.
2. Keep output voltage VCE = 0V by varying VCC.
3. Varying VBB gradually, notes down base current IB and base-emitter voltage VBE.
4. Step size is not fixed because of the non-linear curve. Initially, vary VBB in steps of 0.1V. Once the current starts increasing vary VBB in steps of 1V up to 12V.

Repeat above procedure (step 3) for VCE = 5V

$\text{Output characteristics:}$

1. Connect the circuit as shown in the circuit diagram.
2. Keep emitter current IB = 20A by varying VBB.
3. Varying VCC gradually in steps of 1V up to 12V and note down collector current IC and Collector-Emitter Voltage(VCE).
4. Repeat above procedure (step 3) for IB = 60µA, 0µA.