Problem 4.15 Determine the gain G = υL /υs for the circuit in Fig. P4.15, and
specify the linear range of υs for RL = 4 kΩ.
ip
vs‘
+
_
Vcc = 6 V
vo 4 kΩ
vL
70 kΩ
5 kΩ
20 kΩ
RL
vs
10 kΩ
Figure P4.15: Circuit for Problems 4.15 and 4.16.
Solution: The basic op-amp circuit is that of a noninverting amplifier. We first
convert the input from υs to υs′ .
Since ip = 0,
υs′ = υs
υ0 =
20k
= 0.8υs
(5 + 20)k
(70 + 10)k ′
υs = 8υs′ = 8 × 0.8υs = 6.4υs
10k
Voltage division at the output side gives
υL =
υo RL
υo
=
,
RL + 4k
2
Hence,
G=
for RL = 4 kΩ.
υL 6.4
= 3.2.
=
υs
2
The linear range of υs is governed by the requirement that |υo | ≤ Vcc = 6 V, or
|υs | ≤
or
6
= 0.9375 V,
6.4
−0.9375 V ≤ υs ≤ 0.9375 V.
c
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