ECE 3235 Electronics II
Experiment # 7
Feedback effects on Transient and
Frequency response
Note: Lab 8 is going to be continuation of the lab 7. You will re-using the circuit you
build in lab 7. So, if you want to save effort, you can keep the built circuit in the lab until
next week for lab 8.
1. Base Amplifier
The first part of the experiment is to construct and characterize an amplifier whose poles
are set accurately by capacitors and resistors. First connect the circuit shown in Figure 1
using 741 OpAmps and ±15 V power supply voltages. Measure, and record the exact
value of each component in the circuit. The OpAmps can be assumed to be ideal over the
frequency range of this experiment. The purpose of the unity gain buffer amplifier is
merely to make computations easier (why ?).
Figure 1. Base Amplifier
Measure the gain and phase characteristic of the amplifier by applying a sinusoidal
input waveform. Note that the low frequency gain is approximately –100 (why?). Since
the power supply voltages are ±15 volts, you must keep the input voltage well below 150
millivolts peak. Always observe the output voltage to insure that there is no distortion.
Measure the phase shift at each frequency with your oscilloscope. Note that as the gain
drops with increasing frequency, so you can increase the input level, but always check for
distortion when you do so. Take enough measurements to enable you to later draw
accurate gain and phase plots. You should take particular care to measure enough values
at and near the pole frequencies, which is where the response will change most rapidly
with frequency. These pole frequencies are in the vicinity of 1, 3, and 4 Kilohertz. A
suggested sequence of frequencies is every hundred Hertz from 100 Hz to 1 KHz and
every kilohertz from 1 KHz to 10 KHz.
2. Closed Loop Uncompensated Base Amplifier
Connect a voltage feedback network around the base amplifier, as shown in Figure 2
Figure 2. Uncompensated Parallel voltage Feedback Amplifier (note that this is a
parallel voltage feedback amplifier, therefore ideally this should be modeled by a
Trans-resistance amplifier defined by voltage over current, however you can
transform it to a voltage amplifier defined by voltage over voltage, in which case the
feedback coefficient will change accordingly)
The triangular block represents the base amplifier of Figure 1. Terminals x and y should
be open circuited. Note that the 200 K-ohms resistor labeled Ro in Figure 1 should not be
removed.
First, let Ra = 1K-Ohm and Rb = 3.3K-Ohm (measure their exact values). Observe the
output with your oscilloscope with Vin = 0 (short circuit the input). Does the feedback
amplifier oscillate? If so, sketch the waveform at Vout and record peak amplitudes
(positive and negative, with your scope on dc) and period. Now change Rb to 10K-Ohm
(measure its exact value). Measure the low frequency gain with a sinusoidal signal
applied to the input. Then you do frequency sweep to determine if the gain peaks.
Measure the peak gain and the frequency at which it occurs.
Change the sinusoidal input to a 100 mV 50Hz square wave and observe the output with
your oscilloscope. Sketch Vin (t) and Vout (t), being careful to note any “ringing” around
the transitions of the waveform. You might have to increase the sweep speed and/or the
beam intensity of your oscilloscope to properly see the ringing. It might also prove
convenient to use delayed sweep. Feel free to change the frequency of the input square
wave (but don’t cut off any of the ringing). Record the steady state output voltage (flat
portion of the waveform), the first and second (+ and -) peaks, and the ringing period.
Also, note the input level(s).
3. Cadence simulation
Build the circuit shown in Figure 1 and 2 in Cadence and simulate their frequency and
transient response. Compare your simulation results with your measurement results. Also,
for transient analysis, as discussed in class, it is preferred to give a pulse with a timeduration of 50 to 100 times of 0.35/B, where B denotes the bandwidth of the amplifier.
What is the bandwidth B for the amplifier in Figure 1? (B should be the lowest break
frequency or pole frequency you derived and measured before, why?).
Cadence tips
(1) Use VSIN from AnalogLib for AC/frequency analysis (note that AC amplitude is
set to 1 V)
(2) Use VSOURCE from AanlogLib for transient analysis (select PULSE for type
and set the pulse properly).
(3) Use dB20( ) calculator statement if you want to convert the gain into dB format
(to do that click Output in the analog design window, select setup, give a name for
gain, type in dB20(VF(“/Vout”) supposing you denoted your output voltage by
Vout, and finally click ok). Note the simulation results will give you the gain in
terms of dB instead of the numerical value.
(4) Similarly, use phase( ) calculator statement if you want to find phase of the
output, note that you need to subtract 180 to get the actual phase.