Page 85 - Schaum's Outline of Theory and Problems of Electric Circuits

AMPLIFIERS AND OPERATIONAL AMPLIFIER CIRCUITS
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5.8 VOLTAGE FOLLOWER [CHAP. 5

The op amp in the circuit of Fig. 5-18(a) provides a unity gain amplifier in which v ¼ v since
1
2

þ

þ
v 1 ¼ v , v 2 ¼ v and v ¼ v . The output v 2 follows the input v 1 . By supplying i l to R l , the op amp
eliminates the loading effect of R l on the voltage source. It therefore functions as a buffer.
EXAMPLE 5.13 (a) Find i s ; v l ; v 2 ; and i l in Fig. 5-18(a). (b) Compare these results with those obtained when
source and load are connected directly as in Fig. 5-18(b).
(a) With the op amp present [Fig. 5-18(a)], we have
i s ¼ 0 v 1 ¼ v s v 2 ¼ v 1 ¼ v s i l ¼ v s =R l
The voltage follower op amp does not draw any current from the signal source v s . Therefore, v s reaches the
load with no reduction caused by the load current. The current in R l is supplied by the op amp.
(b) With the op amp removed [Fig. 5-18(b)], we have
v s R l
i s ¼ i l ¼ and v 1 ¼ v 2 ¼ v s
R l þ R s R l þ R s
The current drawn by R l goes through R s and produces a drop in the voltage reaching it. The load voltage v 2
depends on R l .

Fig. 5-18