Page 75 - Schaum's Outline of Theory and Problems of Electric Circuits

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Ampliﬁers and

Operational Ampliﬁer

Circuits

5.1 AMPLIFIER MODEL
An ampliﬁer is a device which magniﬁes signals. The heart of an ampliﬁer is a source controlled by
an input signal. A simpliﬁed model of a voltage ampliﬁer is shown in Fig. 5-1(a). The input and output
reference terminals are often connected together and form a common reference node. When the output
terminal is open we have v 2 ¼ kv 1 , where k, the multiplying factor, is called the open circuit gain.
Resistors R i and R o are the input and output resistances of the ampliﬁer, respectively. For a better
operation it is desired that R i be high and R o be low. In an ideal ampliﬁer, R i ¼1 and R o ¼ 0 as in Fig.
5-1(b). Deviations from the above conditions can reduce the overall gain.

Fig. 5-1

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