ECE 3434
Homework 7 (Spear quest 7)
version 2.2
(This is a mildly adulterated version of the original ece3434 Slone amplifier exercise of 2005). In this
instance we shall investigate usage and effect of feedback within the Slone 12W amplifier via a
set of pSPICE simulations.
---------------------------------------------------------------1. (a) (10 pts) This part is an investigation of the performance of the Slone 12 W amplifier with feedback,
which is its normal mode of operation. Display a figure of the unadulterated circuit as your figure 7-1.
Execute an AC sweep from 0.5Hz to 5MHz with an 0.5mV VAC source. We later will remove feedback
for which you should find that the gain of the amplifier becomes extra large, but only for a small
bandwidth. For Vin = 0.5mV input the output amplitude should fall well within the power supply rails.
Except for feedback the amplifier output would blow you away at some frequencies and leave others at
marginal amplitudes, and otherwise be a very troubled circuit. Feedback promotes good digestion and
regularity of the bowels of your power amplifier.
(10 pts) Display Vout vs frequency on a semi-log plot. From the display, show cursor measure of
voltage gain of the circuit at midband and include a screenshot as your figure 7.2a. In a new window
display a DB( ) log-log plot of Vout vs. frequency as your figure 7.2b and use your cursors to estimate the
frequency of the dominant high freq pole and the asymptotic slope (in dB/decade) from the amplitude
curve at high frequencies. Place the two figures side-by-side, either by use of pSPICE or by use of the
cut/paste using the ‘Paint’ utility and shrink fit. Be sure to include clear and visible cursor coordinates
(5pts) From the display estimate the frequency of the dominant low freq pole from the voltage amplitude
curve at low frequencies. Determine the amplifier bandwidth according to the simulation.
Include all of the extracted values of interest in your figure titles. It is good form to do so.
(5pts) Calculate the gain-bandwidth product (with feedback) of your amplifier and (also) include this
value in your figure title.
(b) (10 pts) In a new window, use the P( ) function to display a semi-log plot of output phase P(V(Vout))
vs. freq. Use the cursors to identify the frequency range over which the phase error of the output voltage
is 10o or less. Include a screenshot as your figure 7.2c. Be sure to include clear and visible cursor
coordinates that display the relevant information.
2. (a) This part is an investigation of the performance of the Slone amplifier with signal feedback
removed but with DC feedback retained to maintain proper biasing. To accomplish this mode of
operation modify the appropriate portion of the Slone schematic as follows:
Shift resistors R11 and R12 to make room for a new capacitor C13 = {Cx}. Note use of parameterized
values (for which you will need to include a PARAM element). Initially set Cx = 1.0 pF for which
impedance will be large with respect to resistor R12 even for very high frequencies. The resistance R11 =
12 k isolates the output stage from R12 and C13 so that the output stage is essentially unaffected by the
insertion of C13. Repeat the AC sweep analysis and compare the results with those of part (a) with those
from the modified circuit to verify that Cx = 1.0 pF produces no change in the output of the amplifier.
Q3
feedback
Q2N3906
R12
330
C6
220u
CMAX
C13
1p
C9
47u
CMAX
feedback
R11
12k
Q9
Q2N3439
R15
100
D1
D1N4148
Q13
Q2N3906
P1
1k
D3
D1N4148
R20
100
Figure 7.1 The site of interest (image contributed by. Eric Tramel, F’06) .
For plot of the gain in dB, be sure to remember to divide the output by the input, e.g. dB(V(R39:2) /
0.5mV)
And a warning: (Because of the size of the schematic, etc.) you won't be able to do this homework at
home So if you start at 7PM, you will need to be prepared to spend the rest of the evening at Simrall with
a chocolate-walnut muffin as your only comfort).
To remove the signal feedback without affecting the bias feedback, set Cx = 100 farad, (**Recall that
PSpice interprets the letter F, upper or lower case, as the prefix femto, so you should enter into PSpice a
value for Cx of 100 rather than the value of 100F. ) For 100 farads the impedance is much smaller at
audio frequencies than the value of resistor R12 so that C13 appears as a short circuit and no signal
feedback from the output stage can appear at the base of Q3 which is the (-) (= inverting) input of the
differential pair.
(5pts) Display the Slone circuit section as modified circuit as your figure 7-3.
(b) (10pts) Display the Vout vs. frequency on a semi-log plot. From the display, use the cursor to
identify the voltage gain of the amplifier at midband. Include a screenshot as your figure 7-4a. In a new
window, use the DB( ) to create a log-log plot of Vout vs. frequency. From the display estimate the
frequency of the dominant pole and the asymptotic slope (in dB/decade) of the voltage amplitude curve at
high frequencies. Also estimate the frequency of the dominant low freq pole of the voltage amplitude
curve at low frequencies. Show your results as figure 7-4b. (Include cursors).
(5 pts) Determine the amplifier bandwidth and include the value of the bandwidth in your figure title.
Calculate the gain-bandwidth (with feedback) product for your amplifier and include in your figure title.
Place the two figures side-by-side, either by use of pSPICE or by use of the cut/paste using the ‘Paint’
utility and shrink fit. Be sure to include clear and visible cursor coordinates.
(5 pts) In a new window, use the P( ) function to display a semi-log plot of phase shift vs. frequency.
Include a screenshot as your figure 7-4c. Use the cursors to determine the frequency range over which
the phase error of the output voltage is 10o or less. Be sure to include clear and visible cursor coordinates
with your figure.
3. (a) (10 pts) Set up a summary table (your table 7-1) to compare the results from Parts 1 and 2 with
feedback and without feedback, for
Gain
Bandwidth
gain-bandwidth product
frequency of the dominant low frequency pole
frequency of the dominant high frequency pole
slope of the output (dB/decade) at high frequencies, and
frequencies at which the phase error reaches +10 and reaches -10.
(b) (15 pts) In a new window, make a Bode plot (in DB) of open loop gain (the ratio Vout /0.5mV) and
overlay with a plot of phase P(Vout ). (Note: It will be necessary to invoke another Y-axis in order to do so
since the scales are very different) From the Bode gain plot, determine the frequency at which gain without
feedback (open loop gain) falls to unity (0dB). From the overlaid gain and phase plots determine phase
margin for the amplifier. Phase margin (PM) is the value of (180o – P(Vout)) at unity gain. Unity gain is
|AV| = 0dB, and since you have overlaid plots, cursors of one can be aligned to the trace of the other.
Show this result as your figures 7.5(a) and figure 7.5(b) for the cases with and without feedback. Include
clear and visible cursor coordinates. Include your cursor measurement of PM in the figure title.
Also include in your figure title the phase margin that pSPICE (more readily) determines for you using
the Goal function PhaseMargin(DB(V(Vout)/0.5m),P(V(Vout))). Goal functions are invoked under the
probe Trace menu by the menu option Eval Goal Function, which will then list a multitude of goal
functions. One of these will be that of Phase Margin. Your measurement and that of pSPICE should
compare favorably.
[Note: When feedback is present C13 is absent (open circuit) and C6 is a short circuit for signals, so the
fraction of the output fed back to the input is  = R12/(R11 + R12) = 0.0268 and 20 log10(1/) = 31.4 dB
independent of frequency in the high frequency region.]
4. (10 pts) Set Cx = 1 pF so that global feedback is given to the amplifier. There is another capacitance
C8 that produces Miller compensation and pole splitting. Parameterize capacitance C8 as {Cz}. To
observe the effect of C8, overlay Bode gain and Bode phase plots for the cases Cz = 68pF and Cz = .001
pF (essentially zero). Compare the Bode plots for the two cases, point out evidence for pole splitting
(use of cursors), and discuss its benefit in this particular amplifier. Show these as your figures 7-6(a) and
7-6(b). Emplace the figures side-by-side, as has been done for previous parts. Include clearly visible
cursors coordinates.
Include your discussion below the figures.
Completion of this quest with empower your quest character with the staff of stability with which you can
bestow peace and stability on all circuits and slay a Nazgul now and then if so required..