Star Tech Journal

STAR* TECH JOURNAL/FEBRUARY 1983
6
tMNUFACTsU
SUMMARY OF MAJOR OP-AMP CIRCUITS (PART 2)
By Frank Becker, Atari, Inc., Somerset, NJ
We continue with Part 2 of our 3-part series on Major OP-AMP
Circuits. This series is written as a quick reference guide on
OP-AMP circuits for technicians and does not get into detailed
mathematical analysis. Such detailed analysis can be obtained
from any good electronics text book.
v,
R,
Vout
R,
VOLTAGE FOLLOWER (BUFFE;R)
I
Vino -
~
RI
Example 1: IfRF- = 47K and RI= 22K, what is Vo when
V2 = 5 volts and VI = 2 volts?
Vo= (V2 - VI) RF= (5 - 2) 47K = 6.4 volts
5v/\
-5~
Vo= (V2 - VI) RF
- - - ; ~ + -,____.___]
v
RI
A voltage follower provides unity gain; this means a voltage gain
of one, Vin= Vout. IfRF in the previous example (non-inverting
OP-AMP. See S•TJ, Jan '83) equaled zero ohms you have:
Vo= Vin (1 +RF)= Vin (1 + _Q_J
Rin
Rin
Vo= Vin (1) = Vin
Whatever you see on the input, you see on the output.
DIFFERENCE AMPLIFIER
As the name implies, an OP-AMP configured as a difference
amplifier provides an output proportional to the difference
between the two inputs.
22K
Example 2: IfRF = 47K and RI= 22K, what is Vo when V2 =
5 volts and VI = -2 volts?
Vo= (V2 -Vl)RF
RI
(5 - (-2)] 47K
22K
[5+2]2.1
Vo =7 (2.1) = 14.7 volts
If you expected the output to be -6.4 volts because the previous
example was the same except for the value of VI , which changed
to - 2, remember that a minus ( - ) sign in front of the ( - 2) makes
the value positive.
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