Transcribed Text
PSPICE Homework 1 – Op Amps rev7
PSPICE homework assignments provide you with an opportunity to express your creativity in
designing a circuit to meet specific specifications.
Problem 1: TwoStage Amplifier Design
1. Need to design and construct a twostage amplifier with a total voltage gain of A = A1 ∙ A2 =
+X. where X is the sum of digits of your student Z number. What is the total voltage gain of
your circuit?
2. You should do it in two different ways both leading to the same performance: a) Using two
inverting amplifiers, and b) Using two noninverting amplifiers. Let A1 be the gain of the first
amplifier and A2 be the gain of the second amplifier. The output of Stage 1 becomes the input
of Stage 2. Choose any suitable values A1> 1 and A2 >1that you wish. (Note that each of the
two gains must be strictly larger than 1. No buffers allowed in this design. Assume that the
available batteries are +15V and 15V. Use the µA741 part in the EVAL library.
3. Using the PSPICE cursor to check whether the overall gain that you get and each stage’s gain
are exactly as planned. Simulate the Transient response of the circuit and observe the output for
a VSIN input of amplitude 0.1V and frequency of 1 KHz (in Transient). Avoid resistors less
than 1kΩ. No clipping should appear on the output waveform.
Hints: a) Whenever you construct a noninverting opamp leave it at its original position  no need to
"mirror vertically". However  don't forget to double check that pin 7 receives a positive DC voltage,
and pin 4 receives a negative DC voltage,
b) In the inverting amplifier you have to mirror the opamp vertically, but again verify that correct DC
voltages reach pins 4 and 7,
c) The gain may not be exactly +X because the 741 opamp is not "ideal". It is only "almost ideal".
Checking the peaktopeak output amplitude will show you that the error is very small,
d) Use multiple plots or multiple Y axes, to see each signal at full scale,
(e) Initially when you set the simulation parameters for the Transient / Time response leave the
“Maximum Step Size” entry blank and observe how wrong the simulation results look (i.e. the signals
are supposed to look smooth, but if the step size is wrong they may have corners),
(f) Now set the maximum step size to be 1/1000 of the input signal’s period and see the correct results.
(g) An example of a twostage amplifier is shown in Figure 1.
Figure 1: Example of twostage amplifier  Transient Simulation
vout
R1
7.5k
vin U2
uA741
3
2
7 4
6
1
5
+

V+ V OUT
OS1
OS2 vcc
vcc
0
vee
0
vee
0
V1
15Vdc
R4
27k
0
U1
uA741
3
2
7 4
6
1
5
+

V+ V OUT
OS1
OS2
V6
FREQ = 1000
VAMPL = 0.1V
VOFF = 0V
R3
1k
V2
15Vdc
vee
0
vcc
R2
15k
vout1
Type your first and last name
on each schematic and
simulation plot
2  P a g e
Problem 2: Level Shifting Design
1. Create a sinusoidal signal V(in) with the following parameters –VOFF=(last digit of your Z
number), AMPLITUDE=2.5V, FREQUENCY=5 KHz. What is your VOFF?
2. Design a level shifting circuit such that the lower peak of V(in) is shifted to 8V, and the higher
peak of V(in) is shifted to 4V.
3. Show by means of PSPICE Time Response simulation that your circuit really works. [Continue
to use the two 15V batteries]. Make sure to explain every step in your design.
Hint: Follow the stepbystep design procedure in the lecture notes (Unit 2 Part 2). You may even use
the notes as a template so that you don’t have to retype the formulas. Just put in the numbers relevant
to your design.
Problem 3: LowPass Filter Design and Testing
1. Let the desired signal be an AC with amplitude of 3.2 V and frequency =(first four numerical
digits of your Z number in Hz). What is the frequency of your desired AC signal?
2. Let the “noise” signal be AC with amplitude of 2V and frequency of 15 KHz.
3. Need to design an op amp lowpass filter (as in the lecture notes).
4. You need to choose the cutoff frequency f=fc. (Hint: fc must be slightly larger than the desired
signal’s frequency [1.5 to 2 times larger may be a good choice] but sufficiently smaller than the
noise frequency. You may have to experiment with a few possible choices]. Test the filter both
in AC Sweep (asking “Is f = fc located as designed?”) and in Transient (as in the demonstrated
example in the lecture notes, asking “Is the amplitude of the output noise small enough relative
to the amplitude of the desired output signal?”). [Continue to use the two 15V batteries].
Homework Solution Format: Each simulation solution must be submitted neatly edited and should
include the following items:
1) Some calculations (in case of a design exercise) predicting approximately the expected outcome.
Always explain your design considerations.
2) Printout of the circuit diagram – As the author, print your name on each circuit diagram sheet.
3) Output printouts – Be selective and use only the most relevant output. Don’t dump on the grader
your entire collection of computer printouts. In particular, never submit graphs that you cannot
explain. Print your name on every result sheet that you submit.
4) Annotations to the results: PSPICE allows you to put comments and annotations on all output
graphs and circuit schematics. It is highly recommended (for best readability of your work) to
include notes and computations directly on the output graph pages themselves.
5) Brief conclusions – Did the circuit work as expected? If the results are far from your handcalculation prediction, where is the difference coming from?
These solutions may offer stepbystep problemsolving explanations or good writing examples that include modern styles of formatting and construction
of bibliographies out of text citations and references. Students may use these solutions for personal skillbuilding and practice.
Unethical use is strictly forbidden.