Download ECE 3110 Spice Assignment: Feedback Amplifier Design and Compensation and more Assignments Electrical and Electronics Engineering in PDF only on Docsity!
UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
ECE 3110 Spice Assignment #
FEEDBACK AMPLIFIER
Create a .cir file for the following circuit. For the opamp, use the 741 macromodel posted on the class website. Please number the nodes in the circuit as shown below.
R R
C C
R
R
10K 10K
200K
x R y
v C
i
1.1K
.01μF .01μF (^) .01μF 10K
3 4 R 5 R 6
0
0
v
1
(^1 )
1 2
(^3 4 5 ) 8 0 9
10K 10K
Fig. 1.
2
6
(20 points) For the open loop amplifier above, simulate the magnitude and phase response. Then simulate the transient response to a 50-mV, 50-Hz, square wave. Report the rise and fall times. R
R
x y
v (^) i a
b
1 -A(s)^9 vo Fig. 2. Uncompensated shunt feedback amplifier.
Shunt Feedback The circuit you just built is meant to act as an op-amp, where vi (in Fig. 1) is the negative input, and the positive input is implicitly grounded. This circuit has a high input resistance, low output resistance, and reasonably high gain. The
capacitors C 1 -C 3 limit the high-frequency response of this amplifier, playing the role that internal transistor parasitic capacitances play in actual IC (integrated circuit) op- amps.
In this part of the experiment we will close a feedback loop around the amplifier in several different ways and characterize its performance for each different feedback example. You will find that stability is not always assured, and the frequency- dependent properties of the op-amp (our Fig. 1 circuit) have a large effect on this stability or instability.
(10 points) Connect a shunt feedback network around the amplifier, as shown in Fig. 2. The triangular block represents the entire 3-stage amplifier of Fig. 1. The triangular block represents the entire 3-stage amplifier of Fig. 1 (i.e., our “op- amp”). Note that this corresponds to the familiar “inverting amplifier” op-amp configuration. What would you expect the gain to be in this case? Terminals x and y should be left open. Note that the 200-Kilohm resistor labeled R 0 in Fig. 1 should not be removed.
(15 points) First, referring to Fig. 2, let Ra = 1 kΩ and Rb = 3.3 kΩ. Ground the input and run a transient simulation to observe the output. Does the feedback amplifier oscillate? If so, plot the waveform at vo, and report the peak amplitude and the period.
(15 points) Change the input to a 0.1 volt 50 Hz square wave and plot the output. Report the steady-state output voltage (flat portion of the waveform), and the voltage of the first (+) and second (-) peaks, and the ringing period.
Dominant Pole Compensation
Compensation is a technique used to make op-amps stable under closed-loop feedback conditions. Typically, a pole is introduced which is lower than the other
x y
v (^) i a
b
-A(s) (^) vo
R = 10K
R Cx
Rx
1K 1 9
Fig. 4. Zero cancellation compensation.
Adjust Rx and Cx for optimum performance, that is minimum overshoot consistent with smallest ringing and fastest rise and fall time.
(10 points) Record the rise and fall time and overshoot of the waveform you consider optimum (in response to the same square wave as above simulations). Include a plot of the output waveform. Record the corresponding values of Rx and Cx.
What to turn in:
- Your netlist (.cir file).
- AC and transient response plots for the open loop amplifier.
- Plots of transient responses from all three remaining sections (shunt feedback, dominant pole, and dominant pole with zero cancellation). Report the steady-state output voltage (flat portion of the waveform), and the voltage of the first (+) and second (-) peaks, and the ringing period for each configuration. Did the ringing period of the closed loop amplifier correlate with the ringing period of the compensated amplifer?
- Anything else used to complete the assignment.
