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Sample problems
Topic: Operational Amplifiers
Statement of problem
The opamps in the circuit are ideal.

+

+
a
i
5
V
−
5
V
−
5
V
5
V
400
mV
30
k
Ω
120
k
Ω
1
k
Ω
90
k
Ω
15
k
Ω
500
mV
(a) Find
a
i
.
(b) Find the value of the right source voltage for which
0
a
i
=
.
Solution
(a) The idea is to find out output voltages of the two opamps, and then to find out the
current in between. The left opamp is virtually grounded, so the current flowing through
the left source is
1
500
33.3
15
mV
iA
k
μ
−
==
−
Ω
Then its output voltage is obtained using KVL
11
90
3
o
vikV
=−×
Ω=
(1)
Repeating the same process for the right source and opamp, its output voltage is obtained
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