Describe the function of a transistor as an amplifier with the neat circuit diagram. Sketch the input and output wave form.

Explanation

Transistor as an amplifier:
A transistor operating in the active region has the capability to amplity weak signals.Amplification is the process of increasing the signal strength (increase in the amplitude). If a large amplification is required, the transistors are cascaded with coupling elements resistors, capacitors, and transformers which is called as multistage amplifiers.
Here, the amplification of an electrical signal is explained with a single stage transistor amplifier as shown in figure (a). Single stage indicates that the circuit consists of one transistor with the allied components. An NPN transistor is connected in the common emitter configuration.

To start with the point or the operating point of the transistor is fixed so as to get the maximum signal swing at the output (neither towards saturation point nor towards cut-off). A load resistance, `R_(C)` is connected in series with the collector circuit to measure the output voltage. The capacitor `C_(1)` allows only the ac signal to pass through. The emitter bypass capacitor CE provides a low reactance path to the amplified ac signal. The coupling capacitor `C_(C)` is used to couple one stage of the amplifier with the next stage while constructing multistage amplifiers. `V_(S)` is the sinusoidal mput signal source applied across the base-emitter The output is taken across the collector-emitter.
Collector current,`I_(C)=betaI_(B)[because beta=(I_(C))/(I_(B))]`
Applying Kirchhoff.s voltage law in the output loop, the collector-emitter voltage is given by
`V_(CE)=V_(CC)-I_(C)R_(C)`
Working of the amplifier:
During the positive half cycle
Input signal `(V_(S))` increases the forward voltage across the emitter-base. As a result, the base current `(I_(B))` increases. Consequently, the collector current `(I_(C))` increases `beta` times. This increases the voltage drop across R which in turn decreases the collector-emitter voltage `(V_(CE))`. Therefore, the input signal in the positive direction produces an amplified signal in the negative direction at the output. Hence, the output signal is reversed by `180^(@)` as shown in figure (b).
• During the negative half cycle
Input signal `(V_(S))` decreases the forward voltage across the emitter-base. As a result, base current `(I_(B))` decreases and in turn increases the collector current `(I_(C))`. The increase in collector current `(I_(C))` decreases the potential drop across `R_(C)` and increases the collector-emitter voltage `(V_(CE))`. Thus, the input signal in the negative direction produces an amplified signal in the positive direction at the output. Therefore, `180^(@)` phase reversal is observed during the negative half cycle of the input signal.