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194 Chapter 12. Ampliﬁers and Oscillators

Vi Vo

Figure 12.1: Symbol of ampliﬁer

The additional power content in the output signal is supplied by
the DC (Direct Current) power source used to bias the active device. A
block schematic for an active ampliﬁer is given in ﬁgure 12.2.

DC
Power source

Signal input Amplifier Signal output

Figure 12.2: A schematic of active ampliﬁer.

The output waveform of an ampliﬁer must be an exact replica of
the input waveform. It preserves the details of the signal waveform
and any deviation of the output waveform from the shape of the input
waveform is considered as distortion.

12.2 Characteristics of an ampliﬁer

Following are the important desirable characteristics of an ampliﬁer.

V ◦
1. Gain A = should be large.
V i
2. Input resistance R i should be large.

3. Output resistance R ◦ should be small.

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