Transistor as an amplifier

Transistor as an amplifier

A transistor operating in the active region has the capability to amplify 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 like 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 9.36(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 Q 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 C₁ allows only the ac signal to pass through. The emitter bypass capacitor C_E 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 input signal source applied across the base-emitter. The output is taken across the collector-emitter.

Collector current, I_C = 

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 (Vs ) increases the forward voltage across the emitter-base. As a result, the base current (I_B ) increases. Consequently, the collector current (I_C ) increases β times. This increases the voltage drop across R_C (I_C R_C ) 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 180º as shown in Figure 9.36(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, 180º phase reversal is observed during the negative half cycle of the input signal as well as shown in Figure 9.36(b).