Part 1: Fundamental Op Amp Properties - Courses

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EE101L Laboratory 4


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University of California at Santa Cruz

Baskin School of Engineering

EE101L Laboratory 4

Operational Amplifiers



Introduction:

In this lab of the course, you will explore the properties of operational amplifiers, one of
the most important circuit elements

in analog electronics. Make sure you read the section
on op amps in the introductory notes. First, you will use the op
-
amp in a DC circuit and
learn how to use it as an amplifier and determine its input and output impedance. In the
second part of the lab,

you will see how you can do math on sinusoidal AC signals using
a simple op
-
amp circuit. At the end of the lab, you should be able to



Understand DC and AC op
-
amp operation



Determine input and output resistance/impedance



Measure the frequency response of a
n amplifier



Build and characterize a preamplifier



Design an op
-
amp circuit that carries out a desired mathematical operation


Topics

from the lecture you need to be familiar with:



Op
-
amp circuit model



Ideal op
-
amp technique



Input and output impedance



Basic

op
-
amp circuits



Differentiators and integrators


LM741 OP AMP PIN DIAGRAM:





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Part 1: Fundamental Op Amp Properties


1. DC amplification

a)

Build the circuit shown below using a 741 op amp:


b)

Apply a DC voltage V
1

using the power supply. Vary V
1

between

5

V and +5 V in 1
V steps and record the output V
2
.

c)

Draw a graph of V
2

versus V
1

and find the relation between V
2
and V
1
from your
graph. Does this result agree with your expectations? (see prelab question 4)

d)

Now vary the input voltage (V
1)

between

V
EE

=
-
1
5and V
CC

=15 in 1V steps.
Measure V
2

and plot V
2

versus V
1
. What behavior does the circuit exhibit now?
Explain.

2. Input and output resistance

a)

For V
1

= 3 V measure I
1

and determine the input resistance of the circuit.

3. AC amplification

a)

Now use the func
tion generator as input. Use a sinusoidal signal with amplitude 1
V and frequency 100 Hz. Set the oscilloscope to the appropriate time scale to observe
a few periods and display both input V
1

and output signal V
2
. Does the output signal
agree with your exp
ectations? What is the phase difference between V
1

and V
2
?

b)

Drive the amplifier into the nonlinear regime found in part 1d) by increasing the
amplitude of V
1
. Observe V
2

on the scope and describe what you see.

c)

Build the following integrator circuit:


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NOTE:

R
DC

compensates for non
-
idealities of the op
-
amp at DC and does not
(visibly) affect the AC operation of the integrator circuit. If you’re interested in
learning more, see Horowitz and Hill, p.222f.

d)

Again, use an amplitude of 1 V for V
1

and f = 100 Hz and

observe V
2

on the
scope. What are amplitude and phase difference with respect to V
1

of the output
voltage? Does this agree with your expectation?
NOTE:

If the shape of your output
signal looks strange or is unstable, talk to your TA. You may have a DC off
set
problem and may have to increase R
DC
.

e)

Vary the frequency between 1 Hz and 10 KHz in suitable steps (1, 2, 5, 10, 20, 50,
…, 10000 Hz). Measure the amplification V
2,max
/V
1,max

and the phase difference


at
each frequency. Plot 20log(V
2,max
/V
1,max
) versu
s log(f). Do your results agree with
your expectations? Compare with theory.

f)

Using your oscilloscope traces, explain why this circuit acts as an integrator in the
time domain.



Part 2: Design of an Integrator/Differentiator


In this part of the lab, you w
ill design a circuit which is able to carry out mathematical
operations you know from calculus: differentiation and integration. Make sure you have
read and understood section 14.10 in the textbook which explains how these circuits
work.

Decide whether yo
u want to build an integrator or differentiator. Call this circuit
‘C’
.


1.Fixed frequency operation

a)

Draw a circuit diagram of the circuit
‘C’

of your choice. Identify the circuit elements
you can vary in your design.

b)

We want to carry out integration/diffe
rentiation on a sinusoidal AC signal with
frequency f=1 KHz

and
amplitude V
1,max

= 1V
. Depending on your choice for
‘C’
,
design a circuit which carries out your operation and leads to an output signal with
the same
amplitude V
2,max

= 1 V
.
NOTE:

Show your d
esign to the TA to have it

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approved before you build it. If you build a differentiator, choose C = 10 nF to avoid
running into op amp nonidealities!

c)

Build the circuit. Apply the correct input signal with the function generator. Monitor
input and output vol
tage simultaneously on channels 1 and 2, respectively. Draw a
graph of the scope images and verify the circuit does what you designed it to do.

d)

Vary the frequency of the input signal. Describe qualitatively what happens to the
output signal. Explain.


2.Tu
nable differentiator/integrator

a)

Modify your design such that you will be able to obtain an output signal with 1V
amplitude for all frequencies between 1 KHz and 10 KHz. You will need to use a
variable circuit element (potentiometer) in order to do this. Pi
ck your capacitor such
that the required resistor values at the lowest and highest frequencies are within the
potentiometer range. Draw a circuit diagram of your design and show your
calculations.

b)

Build the new circuit and try it out. Change the input sign
al frequency from 1 KHz to
10 kHz in 1 KHz steps. Adjust your pot to obtain the correct output amplitude.
Measure R
pot

for each frequency. Make a plot of R
pot

versus f. Also add the graph you
would expect theoretically. Do your measurements agree with theo
ry?