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224 Chapter 12. Ampliﬁers and Oscillators

12.13 Oscillators

An oscillator can be described as a source of alternating voltage. An
ampliﬁer delivers an ampliﬁed version of input signal while oscillator
generates an output waveform without an input signal. The additional
power content in the output signal is supplied by an external DC power
source.
The oscillator requires no external signal to initiate or maintain the
energy conversion process. Instead, an output signal is produced as
long as a DC power source is connected. Figure 12.23, depicts the
comparison between ampliﬁer and oscillator.

DC source

Signal input
Signal input Amplifier Signal output

(a)
DC source

Oscillator Signal output

(b)

Figure 12.23: Comparison between ampliﬁer and oscillator

12.14 Principle of oscillators

Oscillators are ampliﬁers with positive feedback. Consider a feedback
ampliﬁer with an input signal V in and output V ◦ as shown in Figure
12.24.
A is the open loop gain of the ampliﬁer. Without feedback, output
voltage of ampliﬁer is V ◦ = A × V in . Since positive feedback is used,
feedback voltage V f is added with input signal V in . Thus the input to
the ampliﬁer V e = V in + V f .

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