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ECE 291 - LABORATORY XI
OPERATIONAL AMPLIFIERS
INTRODUCTION
Operational amplifiers (so called Op-Amps) are wonderful
devices. General principles of their
operation are easy to understand and are usually described in introductory
circuit courses. They are also easy to use
and very handy in many applications. If you need to process
analog signals you will most likely use Op-Amps rather than discrete
transistors.
The devices are made as integrated circuits and come in a
variety of packages, often in multiple
units. One of the oldest successful designs, still in demand, is the famous mA741
which you will meet in this laboratory. The circuit
consists of over twenty bipolar transistors
but, thanks to the miracles of modern IC manufacturing technology, it sells for a fraction of a dollar a piece.
Other courses in EE curriculum will provide you with the
knowledge of applications and design with
op-amps. This laboratory exercise is just an introduction to this important field.
PRELAB
Draw schematic diagrams of an inverting amplifier and a
non-inverting amplifier utilizing operational
amplifiers. Express their gains as functions of the values of resistors used in the circuit.
References:
1. P. Horowitz and W. Hill
The
Art of Electronics, second edition,
Cambridge University Press 1989, pp.
175-179.
2. T. C. Hayes and P. Horowitz "Student Manual for The Art
of Electronics"
3. S. Franco "Design with Operational Amplifiers and Analog
Integrated Circuits", McGraw-Hill 1988.
Chapter 1.
LABORATORY
1. INVERTING AMPLIFIER
Using an op-amp in your parts kit wire an inverting amplifier.
Choose two sets of resistors in the circuit
to obtain two gain values between five and a hundred. Measure the gains
and compare them with the values calculated from the known resistances in the circuit. Measure the resistors with a digital
ohmmeter.
2. NONINVERTING AMPLIFIER AND THE GAIN BANDWIDTH
PRODUCT
(GBP)
Assemble a non-inverting amplifier with low gain, up to 10.
Check its frequency response. Op-amp
performance is limited at high frequency; it behaves as a low-pass filter.
Measure its bandwidth, which is defined by f-3db frequency. Recall your work on filters,
Experiments VII. It is not necessary to measure the
whole frequency distribution.
Repeat the bandwidth measurements
for two more resistor sets, giving the gain in tens and hundreds
range. Measure the gain and the bandwidth in each case.
There is a rule that applies to these circuits which says that
the gain bandwidth product (GBP) is a
constant. It means that, just like in life, you can not have something for nothing In this case, to get more gain you have to
pay for it with frequency you can amplify,
and the other way around. Test this rule for your three circuits. Consider also the op-amp specifications which give you open loop
gain (no feedback) as 200,000 and the
bandwidth limiting frequency of 5 Hz.
3. FOLLOWER
Build a voltage follower circuit, which is an op-amp version of
an emitter follower circuit. It is also
called a buffer, because with its high input impedance and low output impedance it is used for isolating circuits, in a sense that
they do not influence one another.
Hint: The gain of the follower is one and it does not invert the
phase. You do not even
need
resistors!
REPORT
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Draw the schematics and list the values of the resistors used.
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Compare measured and calculated gains.
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Comment on GBP rule applied to the circuits you built.
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