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LINEAR DEVICES
Operational Amplifiers.
The source of gain in an operational
amplifier can be characterized as an ideal, differential voltage
amplifier having low output impedance, high input impedance,
and very high differential gain. The output of an operational
amplifier is proportional to the difference in the voltages ap¬
plied to the two input terminals. In use, the amplifier output
drives the input voltage difference close to zero through a feed¬
back path.
When troubleshooting an operational amplifier circuit, measure
the voltages at the two inputs; the difference between these
voltages should be less than 10 mV. (Note: This troubleshooting
procedure will not work for operational amplifiers which are con¬
figured as comparators.) A difference voltage much greater than
10 mV indicates trouble in the amplifier or its external circuitry.
Usually, this difference will be several volts and one of the in¬
puts will be very close to one of the supply voltages (e.g., + 15
V or -15 V).
Next, check the amplifier's output voltage. It will probably also
be close to one of the supply voltages (e.g., ground, + 15 V,
or -15 V). Check to see that the output conforms to the in¬
puts. For example, if the inverting input is more positive than
the non-inverting input, the output should be negative; if the
non-inverting input is more positive than the inverting input,
the output should be positive. If the output conforms to the
inputs, check the amplifier's external circuitry, if the amplifier's
output does not conform to its inputs, it is probably defective.
Figures B-34, B-35 and B-36 show typical operational amplifier
configurations. Figure B-34 shows a non-inverting buffer
amplifier with a gain of 1. Figure B-35 is a non-inverting amplifier
with gain determined by R1 and R2. Figure B-36 is an inverting
Figure B-34. Non-Inverting Amplifier (Gain -1)
Figure B-35. Non-Inverting Amplifier (Gain--1 + R1/R2)
1
INPUT IMPEDANCE--R2
OUTPUTIMPEOANCE-VERY LOW
Figure B-36.
inverting Amplifier (Gain - - R1/R2)
Comparators.Comparators are used as level sense amplifiers,
switch drivers, pulse height discriminators, and voltage com¬
parators. A voltage reference is connected to one of the
amplifier's inputs as shown in Figure B-37 and B-38. When
the input signal voltage crosses the reference, the output goes
positive; the output remains positive until the signal re-crosses
the reference.
B 16
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