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Problem 4.32 Design a circuit containing a single op amp that can perform the
following operation: υo = 3 × 104 (i2 − i1 ), where i2 and i1 are input current sources.
Solution: This is a difference amplifier circuit with current sources. Converting the
difference-amplifier circuit in Table 4-2, we have:
R2
i1
R1
−
vo
+
i2
R3
where
i1 =
R4
υ1
,
R1
i2 =
υ2
.
R3
Application of Eq. (4.41) gives:
R1 + R2
R4
R2
υo =
υ2 −
υ1
R3 + R4
R1
R1
R3
R4
(R1 + R2 )i2 − R2 i1
=
R3 + R4 R1
Since the gain factor given in the problem statement is the same for both i1 and i2 , we
need to satisfy the conditions:
R4
R3
(R1 + R2 ) = R2
R3 + R4 R1
and
R2 = 3 × 104 = 30 kΩ.
Choose R1 = 10 kΩ (arbitrary choice),
R3 R4
(1 + 3) = 30 kΩ
R3 + R4
or
R3 R4
= 7.5 kΩ.
R3 + R4
Choose R4 = 10 kΩ and R3 = 30 kΩ.
Summary:
R1 = R4 = 10 kΩ,
R2 = R3 = 30 kΩ.
Since R3 and R4 are in parallel, they can be replaced by an equivalent resistor whose
value is
(10 × 30)k
R3 k R4 =
= 7.5 kΩ.
10 + 30
c
All rights reserved. Do not reproduce or distribute. 2013
National Technology and Science Press
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