Draw a labelled circuit diagram to study the characteristics of an npn transistor in common emitter configuration. Draw its input and output characteristics, and explain how β is found from them.

1. 1 · State the idea. In CE mode the emitter is common to the input (base) and output (collector) circuits.
2. 2 · Draw the circuit. [Diagram: npn transistor with the emitter common; base circuit with VBB, potentiometer, μA meter and VBE voltmeter; collector circuit with VCC, potentiometer, mA meter and VCE voltmeter]
3. 3 · Explain the input characteristic. Keep VCE fixed and vary VBE: IB stays near zero to about 0.5 V, then rises steeply. ri = ΔVBE/ΔIB.
4. 4 · Explain the output characteristic. Keep IB fixed and vary VCE: IC rises quickly, then levels off. A larger IB gives a higher curve.
5. 5 · Draw both sets of curves. [Graphs: IB (μA) against VBE (V), rising after about 0.6 V; IC (mA) against VCE (V) for IB = 20, 40 and 60 μA]
6. 6 · Find β. At one fixed VCE, read IC on two curves: β = ΔIC/ΔIB.
7. 7 · Close in one line. So, the output curves show that a small change in IB gives a large change in IC.

Scoring tip: Label every output curve with its IB value, and use μA for IB and mA for IC. Missing units cost marks.