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Recall that the amplifier
is designed to have three essential characteristics (see
Figure 8):
(1)
The amplifier output (eo) is related to the summing junction voltage
(lb) by the gain of the amplifier: eo
=
-Ae
b
(2)
(3)
order of
3 X
The
input stage of -the amplifier draws negligible current:
ib~
0
Thg
open loop gain of the amplifier is extremely high: A»l (on the
la'
at dc).
Therefore, the amplifier output voltage is related to the input voltage by the
equation:
e
=
- -
e
(
Zf )
o
Zinl
in
CEQ.
2-1)
Equation 2-1 '::'::lustrates one of the most important characteristics of the oper-
ational
amp1i=~:2r:
the input-output relationship of the operational amplifier is
solely
depenc~~=
on the ratio of the feedback to the input impedance.
Using
Equatio~
2-1 as the basis of discussion, the following sub-paragraphs describe
the -various u:s.:2:.:S of the opera tiona 1 amplifier.
When the same
~(C".2.1ue
resis tor is used for both the feedb
9
ck
the amplifier ":::'utput voltage has the same amplitude as the
opposite in
p~~rity.
In the TR-20 the value of R
f
and R.
ln
norma 11y 100,
C-
:\:1
ohms (lOOK), there fore:
e
o
e.
ln
lOOK
lOOK e in
e.
ln
and the input impedance,
input voltage but is
used for the inverter is
Thus a +10 vo=-::..- input results in a -10 volt output, and the amplifier is said to
have a gain
0=
~inus
one.
The accuracy of the output to input ratio depends solely
on the accurac
of the ratio Rf/R . .
ln
.£.
Mu1tip_---------=--cation by a Constant
A change in
th':::::~
examp 1e, wi th
ratio of the resistors results in multiplication by a constant.
~
equal to lOOK and R.
equal to 10K, the amplifier output is:
ln
e
o
lOOK
10K
e.
ln
10e.
ln
For
19
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