Loop gain and phase margin

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, v_i. The output is the node voltage, v_o.

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 A_ideal = …

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 .…