Question

(a) Draw a circuit diagram of a transistor connected as a common emitter amplifier including bias resistors, coupling and decoupling capacitors. (10)

(b) Explain the operation of the circuit components. (6)

Answer 1

Part (a)

A single NPN transistor (or PNP) with the emitter common to input and output.
Bias resistors: R1 and R2 form a potential divider across the supply (Vcc) to set the base voltage and hence the quiescent operating point. Rc is the collector load resistor, Re is the emitter resistor (with a decoupling capacitor Ce across it) for d.c. stabilisation.
Coupling capacitors: C1 couples the input signal to the base, and C2 couples the amplified output from the collector to the next stage or load. These block d.c. and pass only the a.c. signal.
Decoupling capacitor: Ce across Re bypasses the a.c. signal to earth so that the a.c. gain is not reduced by negative feedback from Re, while still providing d.c. bias stabilisation.
Supply: Vcc connected through Rc to the collector; emitter through Re to earth; base biased by R1 (to Vcc) and R2 (to earth).
Output taken from the collector.

Part (b)

R1 and R2: potential divider sets the base potential so the transistor operates in the active region (base-emitter junction forward biased, base-collector reverse biased), giving a stable quiescent point.
Rc (collector load): converts the collector current variation into a voltage variation at the output. The amplified output voltage is developed across Rc.
Re (emitter resistor): provides d.c. negative feedback for bias stabilisation - if collector current rises, the voltage across Re rises, reducing base-emitter voltage and opposing the rise, stabilising the operating point against temperature and transistor variations.
Ce (emitter decoupling capacitor): presents a low impedance to the a.c. signal, so the a.c. signal is not attenuated by Re; it restores the full a.c. voltage gain while keeping d.c. stabilisation.
C1 (input coupling capacitor): blocks the d.c. bias from the signal source and passes only the a.c. input signal to the base.
C2 (output coupling capacitor): blocks the d.c. collector potential from the load and passes only the amplified a.c. signal.
The transistor: amplifies the small base current/voltage signal; a small change in base current produces a much larger change in collector current (current gain beta), giving voltage and power amplification. The output at the collector is 180 degrees out of phase with the input.

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