Exercise 5 - Faculty of Engineering, Ain Shams University

Ain Shams University
Faculty of Engineering
ECE Dept.
CHEP
Electronic Circuits
(COMM 361)
Spring 2015
Dr. Sameh A. Ibrahim
Exercise 5
The Opamp as a Black Box
1. Looking at Equation (1), an adventurous student decides that it is possible to achieve a zero gain
error with a finite A0 if R2/(R1 + R2) is slightly adjusted from its nominal value.
(a) Suppose a nominal closed-loop gain of α1 required. How should R2/(R1 + R2) be chosen?
(b) With the value obtained in (a), determine the gain error if A0 drops to 0.6 A0.
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛
=
𝐴0
𝑅2
𝐴
𝑅1 +𝑅2 0
1+
(1)
2. The input/output characteristic of an op amp can be approximated by the piecewise-linear
behavior illustrated in Fig. 1, where the gain drops from A0 to 0.8A0 and eventually to zero as |Vin1
– Vin2| increases. Suppose this op amp is used in a noninverting amplifier with a nominal gain of 5.
Plot the closed-loop input/output characteristic of the circuit. (Note that the closed-loop gain
experiences much less variation; i.e., the closed-loop circuit is much more linear.)
3. Determine the closed-loop gain of the circuit depicted in Fig. 2 if A0 = ∞.
4. Due to a manufacturing error, a parasitic resistance RP has appeared in the adder of Fig. 3.
Calculate Vout in terms of V1 and V2 for A0 = ∞ and A0 < ∞. (Note that RP can also represent the
input impedance of the op amp.)
5.
Plot the current flowing through D1 in the precision rectifier of Fig. 4 as a function of time for a
sinusoidal input.
6.
(Simulation) Assuming an op amp gain of 1000, plot the input/output characteristic of the precision rectifier
shown in Fig. 5.
Figure 1
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Figure 2
Figure 3
Figure 4
Figure 5
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