Sample problems
Topic: Operational Amplifiers
Statement of problem
The opamp in the circuit is ideal.

+
a
v
+
−
b
v
+
−
o
v
+
−
18
k
Ω
80
k
Ω
64
k
Ω
72
k
Ω
54
k
Ω
9
V
9
V
−
b
i
a
i
The signal voltages
a
v
and
b
v
are 500mV and 1200mV, respectively.
(a) Calculate
o
v
in volts.
(b) Find
a
i
and
b
i
in microamperes.
Solution
(a) The idea is to find out
,
,
o
v
v
v
+
−
, respectively. Due to the virtual open circuit property,
the circuit looks like

+
a
v
+
−
b
v
+
−
o
v
+
−
18
k
Ω
80
k
Ω
64
k
Ω
72
k
Ω
54
k
Ω
9
V
9
V
−
b
i
a
i
v
+
v
+
From the right hand side circuit, it is easy to write down the KCL equation at the positive
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