For the cases
Implement the transfer function as a direct form circuit. Verify the circuit by PSpice simulation.
Determine the loop gain twice: once analytically, using the procedure described in class, and once by PSpice simulation. Compare these results to verify the loop gain.
Use the loop gain to determine the phase margin of the circuit (and the transfer function).
Report your results in a “3-level report”.
EE446 Report
To: J.A. Svoboda
From: (groups consisting of 1, 2 or 3 students)
Date: March 7, 2008
Subject: Loop gain and phase margin
Introduction: The input to the amplifier shown in Figure 1 is the source voltage, vi. The output is the node voltage, vo.
Figure 1. An amplifier circuit.
The op amp in Figure 1 is a compensated op amp satisfying the following specifications:
| Specification | Max | Typical | Min | Units |
| input offset voltage | 2 | | | mV |
| bias current | 25 | | | nA |
| input offset current | 5 | | | nA |
| input resistance | | | | |
| output resistance | 0 | | | |
| (dc) open-loop gain | | | 20,000 | V/V |
| GBP | | | 100,000 | Hz |
| saturation voltage | | 15 | | V |
| saturation current | | 10 | | mA |
Conclusion: The closed loop gain of the amplifier is A(f) = … and the loop-gain is T(f) = …. The phase margin of the amplifier is … so the amplifier is (un)stable.
Results:
R1: Circuit analysis shows that t he closed loop gain of the amplifier is A(f) = … and the loop-gain is T(f) = ….
R2: PSpice simulation shows that t he closed loop gain of the amplifier is A(f) = … and the loop-gain is T(f) = ….
R3: The simulation and circuit analysis (dis)agree.
R4: The phase margin of the amplifier is … so the circuit is (un)stable.
Findings:
F1.1 Analysis of the amplifier circuit shows that Aideal = …
F1.2 Analysis of the amplifier as a feedback circuit shows that β = …
F1.3 The open-loop gain of the op amp is a(f) = …
F1.4 The loop gain is T(f) =
F1.5 The closed-loop gain of the amplifier is A(f) = …
F2.1 The PSpice simulation shown in Figure 2 … The plot of |A(f)| versus f shown in Figure 3 … From the plot …. A(f) = …
F2.2 The PSpice simulation shown in Figure 2… The plot of |T(f)| versus f shown in Figure 4 … From the plot …. T(f) = …
F3.1 The closed-loop gain, A(f), obtained by circuit analysis (dis)agrees with the simulated closed-loop gain because…
F3.2 The loop gain, T(f), obtained by circuit analysis (dis)agrees with the simulated closed loop gain because…
F4.1 The PSpice simulation shown in Figure 2… The magnitude and phase of T(f) versus f are shown in Figure 5 … From these plots …. The phase margin of the amplifier circuit is .…
