Voltage Gain of Emitter Follower
Fig. 1.11 shows the emitter follower circuit. Since the emitter resistor is not bypassed by a capacitor, the a.c. equivalent circuit of emitter follower will be as shown in Fig. 1.12. The ac resistance rE of the emitter circuit is given by
In order to find the voltage gain of the emitter follower, let us replace the transistor in Fig. 1.12 by its equivalent circuit. The circuit then becomes as shown in Fig. 1.13. Note that input voltage is applied across the ac resistance of the emitter circuit i.e., (r’e + RE). Assuming the emitter diode to be ideal,
Output voltage, Vout = ie RE
Input voltage, Vin = ie (r’e + RE)
Voltage gain of emitter follower is
In most practical applications, RE >> r’e so that Aν =1.
In practice, the voltage gain of an emitter follower is between 0.8 and 0.999.
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