Electrical Engineering ⇒ Topic : Transistor as an Amplifier

TRANSISTOR AS AN AMPLIFIER

It is possible to increase the signal level with the help of an amplifier. With the help of an amplifier, it is possible to get a larger signal output from a small signal input. The emitter base junction and collector base junction of the transistor are made forward biased and reverse biased respectively to make the transistor work as an amplifier.

Figure (a) shows the common emitter amplifier circuit using voltage divider bias. When the input signal is absent and DC signal is present in the circuit, this is called zero-signal or no-signal condition or quiescent condition for the amplifier. The dc collector-emitter voltage

                                                Figure (a) Common emitter amplifier

(V_CE) and, the dc collector current (I_C) and dc base current (I_B) form the quiescent operating point for the amplifier. The AC signal by application of AC sinusoidal voltage at the input is superimposed on this dc quiescent operating point for the amplifier. Therefore, the base current varies sinusoidally as shown in Figure (b).

                                                 figure (b)  I_BQ is the quiescent DC base current.

The output is linearly proportional to input because the transistor is biased in the active region. The output current, i.e., the collector current is β times larger than the input current for the common emitter configuration. Therefore, the collector current will also vary sinusoidally about its quiescent value I_CQ as shown in Figure 1 (a). The output voltage will also vary sinusoidally as shown in Figure 1 (b).

                                                      FIGURE (1)  Variation of i_c and v_CE about I_CQ and V_{CEQ r}espectively.

Figure (2) shows the variation of collector current and the voltage between collector and emitter due to change in the base current. The collector current varies above and below its Q-point value in-phase with the base current and the collector-to-emitter voltage varies above and below its Q-point value 180⁰ out-of-phase with the base voltage shown in Figure (2).

       After completion of one cycle of input, one cycle of output will also be completed. Hence,the frequency of output signal is same as that of input signal. Therefore, frequency of output signal does not change in the amplification process. Only the magnitude of output becomes larger than the input.

      Figure (3) shows the practical circuit for common emitter transistor amplifier consisting of different circuit components. The following are the functions of these components:

Biasing circuit: The resistances R₁, R₂ and R_E form the voltage divider biasing circuit for CE amplifier which sets the proper operating point for the CE amplifier

         Figure (a) Variation of base current, collector current and collector-emitter voltage swings.

                                Figure (b) Practical CE amplifier circuit.

Input capacitor C₁: This capacitor couples the signal to the base of the transistor blocking any DC component present in the signal. It passes only ac signal for amplification.

EmitteI nput capacitor C₁: This capacitor couples the signal to the base of the transistor blocking any DC component present in the signal. It passes only ac signal for amplification.

Emitter bypass capacitor C_E: This capacitor C_E is connected in parallel with the emitter resistance RE, to provide low reactance path to the amplified ac signal. Otherwise, the amplified ac signal passing through R_E will cause a voltage drop across it and hence reduction of output voltage will occur.

Output coupling capacitor C₂: This capacitor couples the output of the amplifier to the load.r bypass capacitor CE: This capacitor CE is connected in parallel with the emitter resistance RE, to provide low reactance path to the amplified ac signal. Otherwise, the amplified ac signal passing through RE will cause a voltage drop across it and hence reduction of output voltage will occur.

Output coupling capacitor C₂: This capacitor couples the output of the amplifier to the load.