The approximation which results from considering e = 0 w ill be used in the further discussion
of the DC am plifier but the "approxim ately equal to*' sign w ill not be used, that is, i^ w ill be
considered to be eqqal to if.
In practice, the gain of DC computer am plifiers w ill vary from approximately 1000 fo r repeti
tive com puters to as high as 10° for a large com m ercia l installation. Since the maximum output
of an am plifier is generally 100 volts, the value of eg w ill vary from about 0.1 volt to 1 m ic ro
volt, depending on the gain of the am plifier. Thus the am plifier gain is one factor in the a ccu
ra cy of an analog computer.
The equation
eo
5 l _ e .
may be written
e _ —
„
where
o -
K . .
K - —
which is, in effect, multiplication by a constant. Since, in most ca ses, the output voltage is of
the opposite sign to the input voltage, the am plifier also acts as an inverter o r sign changer.
To change the value of the constant K, it is n ecessary only to change Rf or Rf. Generally, Rf
is kept at 1 megohm and Rj is changed. This may be done by using a different value fixed r e
sistor or by using a potentiometer as shown in Figure 3.
POTENTIOMETER AS INPUT RESISTOR
Another method of obtaining odd constant values of Rj is shown in Figure 4.
POTENTIOMETER AS INPUT RESISTOR
The input voltage ef appears a cro ss a potentiometer (usually 100 Kfl), and a fraction p of it is
selected. The voltage e0 appearing a cro ss the output is then given by the expression
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