Download Problem 4.14 For the op-amp circuit shown in Fig. P4.14: (a) Obtain

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Problem 4.14 For the op-amp circuit shown in Fig. P4.14:
(a) Obtain an expression for the current gain Gi = iL /is .
(b) If RL = 12 kΩ, choose Rf so that Gi = −15.
Rf
_
is
+
Rs
vo
iL
RL
Figure P4.14: Circuit for Problem 4.14.
Solution: (a) This is an inverting amplifier circuit with input voltage
υs = is Rs .
R f is R s
Rf
υo = −
υs = −
= −Rf is .
Rs
Rs
Also,
υo = iL RL .
Hence,
Gi =
iL
υo
−Rf is −Rf
=
=
=
.
is
R L is
R L is
RL
(b) If RL = 12 kΩ, and Gi = −15,
−15 = −
Rf
,
12 × 103
or
Rf = 180 kΩ.
c
All rights reserved. Do not reproduce or distribute. 2013
National Technology and Science Press
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