4: Transfer Functions, Parameters, and Equivalent Circuits of Linear
Amplifiers: PART B
ECE 3200 Electronics II
updated 25 May 2012
Reference
1. A. S. Sedra and K. C. Smith, Microelectronic Circuits, 6th ed., Oxford University Press, 2009.
Objectives (cont’d)
3. To measure experimentally and confirm via simulation the dynamic open circuit voltage transfer
characteristic and the static short circuit current transfer function of a simple direct coupled
linear amplifier. DO NOT DISASSEMBLE YOUR CIRCUIT AT THE CONCLUSION OF
THIS LAB. YOU WILL NEED THIS CIRCUIT AT A LATER DATE.
4. To develop an understanding of input and output measurements in the time domain.
5. To develop an awareness of the necessity of insuring that an amplifier is operating in its linear
region when its “linear” parameters are being measured.
Pre-Laboratory Assignment
(MUST BE COMPLETED INDIVIDUALLY)
Turn in a copy of your results for items 1 and 2. Make sure that your assignment meets the
guidelines as outlined in the PARTIAL CHECK LIST FOR ANY SUBMITTED LABORATORY
ASSIGNMENT document. Paste/tape a copy of the simulation results in your notebook.
1. You will apply a sine wave, a triangular wave, and a square wave to the amplifier of LAB 4
PART A. Use your SPICE engine to simulate the amplifier response to each of these waveforms
as defined in procedure 6 using a transient analysis. Note that the input waveforms have a DC
offset. You must pick the DC offset and input voltage levels to provide symmetric 20% clipping.
In LTspice, use the SINE and PULSE functions to generate these waveforms. These functions
are available when you right click on a voltage source and select “Advanced” options.
For a sine wave: SINE(0.1 1 40)
For a triangular wave: PULSE(1 -1 0 12.5ms 12.5ms 0 25ms 0)
For a square wave: PULSE(-1 1 0 1fs 1fs 12.5ms 25ms 0)
Be sure to use a small “Maximum Timestep” in the simulation command, e.g. 10µs, otherwise
the square wave will not look “square”. You can add a DC voltage source in series with the
signal source to provide a DC offset if needed.
2. Use your SPICE engine to obtain the static short circuit transfer function as described in
procedure 7; this is a plot of the load current vs. the input current of the amplifier. Note that
we are considering RL =10 ohms to be a short (this enables measuring the load voltage and
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computing the current using Ohm’s Law). Use a DC sweep to vary and thus over a range
that insures that varies from well into negative saturation to well into positive saturation. Find
the small signal current gain, region of linear operation, and the saturation and offset currents
from your plot.
Procedures (cont’d)
5. DYNAMIC OPEN CIRCUIT VOLTAGE TRANSFER FUNCTIONMEASUREMENT. Change
to sin2 where is sufficient to drive the amplifier into bilateral
saturation (i.e. both negative and positive saturation regions are evident) and f ≈ 40Hz. Note that
is still 0. Observe vs. on the scope. Get a hardcopy. Compare to the static open circuit
voltage transfer function generated in LAB 4 PART A. Are they the same? Why?
a. While observing the oscilloscope display, slowly vary f between 0.1 Hz and 100 Hz.
Then vary both and f until you feel that you understand what you see. Keep in
the range of −1 to +1 volt.
b. Change to a triangular wave and repeat part a.
c. Change to a square wave and repeat part a.
6. Return the scope to time base mode and monitor vI and vR while triggering on vI .
a. Set to sin2 where is sufficient to produce ≈20%
“clipping” of both peaks of , creates symmetrical clipping (use the 20 dB
attenuator on the waveform generator to provide finer resolution for these
adjustments), and f ≈ 40 Hz. Either “print” or accurately sketch and vs. t for at
least one cycle of . Be sure to denote the location of the zero volt baselines for
each scope channel on your data record. Using the available instruments (DMM and
scope (use cursors)), ACCURATELY measure and record (in a table for both the
input and output) the: (1) offset voltage, (2) the amplitude, and (3) the frequency or
period. The input offset might be measured using the DMM in DC mode.
b. Set to a triangular wave and repeat part a.
c. Set to a square wave and repeat part a.
7. STATIC SHORT CIRCUIT CURRENT TRANSFER FUNCTION. Change to a short circuit
(use 10 ohms). In order to measure the static short circuit current transfer function you will need
to measure and . Do this by measuring the voltages across and using a DMM and
using Ohm’s Law. You may find it convenient to use two DMMs, one for each voltage. Use a
variable bipolar DC source for to vary over a range that insures that varies from well into
negative saturation to well into positive saturation. Use a table to present your results. PLOT
YOUR DATA ON YOUR GRAPH FROM PRELAB PART 2. Be sure to find the
a. output offset current (the output current when the input current is zero and RL ≈ 0);
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b. current gain (in the linear region of course);
c. range of linear operation; and the
d. saturation currents.
Resolve any discrepancies.
Report
Prepare a formal report for this laboratory as discussed in detailed in section 1.5. Your report must
include the exercise as described below.
Exercises
As always, use figures, graphs, circuit diagrams, theoretical analysis, etc., to support your responses.
1. Using your results from procedure 5, identify what if any effects f, , and the shape of have on the shape of the transfer function.
2. Explain the resulting output waveforms for each case of procedure 6 by using the graph of the
dynamic transfer function measured in procedure 5.
3. What property of the transfer function is readily apparent from the data in procedure 6 parts a
and b but is disguised in procedure 6 part c?
Credits, Copyright, and Use
Refer to front matter available at http://homepages.wmich.edu/~miller/ECE3200.html for material
credits, further copyright information, and use guidelines.
© 2012 Damon A. Miller
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