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5.8

AMPLIFIERS AND OPERATIONAL AMPLIFIER CIRCUITS

[CHAP. 5

VOLTAGE FOLLOWER

The op amp in the circuit of Fig. 5-18(a) provides a unity gain ampliﬁer in which v2 ¼ v1 since v1 ¼ vþ , v2 ¼ v and vþ ¼ v . The output v2 follows the input v1 . By supplying il to Rl , the op amp eliminates the loading eﬀect of Rl on the voltage source. It therefore functions as a buﬀer. EXAMPLE 5.13 (a) Find is ; vl ; v2 ; and il 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 is ¼ 0

v1 ¼ vs

v2 ¼ v1 ¼ vs

il ¼ vs =Rl

The voltage follower op amp does not draw any current from the signal source vs . Therefore, vs reaches the load with no reduction caused by the load current. The current in Rl is supplied by the op amp. (b) With the op amp removed [Fig. 5-18(b)], we have is ¼ il ¼

vs Rl þ Rs

and

v1 ¼ v2 ¼

Rl v Rl þ Rs s

The current drawn by Rl goes through Rs and produces a drop in the voltage reaching it. depends on Rl .

Fig. 5-18