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When an input voltage E.
is applied to an attenuator as shown in Figure 6, the
In
R
output voltage Eo is
K
times E
in
, where
K
=
l/~.
If the attenuator is unloaded,
the mechanical ratio of Rl :R
T
is the same as the electrical ratio E
:E.
and the
o
In
attenuator could be set to the exact ratio by means of the vernier dial attached to
the wiper shaft.
Normally, however, the attenuator is loaded and the two ratios
are not equal.
The most common use of the attenuator is to feed an amplifier; thus
the wiper is loaded by the amplifier input resistor.
In order to account for loading,
it is more convenient to set the attenuator under loaded conditions by monitoring the
wiper voltage and adjusting the wiper until the desired output is obtained.
Figure 7a shows the circuit used in the TR-20 to permit setting grounded attenuators
under load.
A pushbutton switch is located next to each attenuator; depressing the
switch connects the wiper to the pot bus, removes the voltage applied at the patch
panel, and applies +10 volts to the top of the attenuator.
The wiper voltage, on
the pot bus, can be measured by the null-comparison method by using the
NULL
POT
on the control panel.
The ungrounded attenuators are connected in the circuit configuration shown in
Figure 7b.
Depressing the pushbutton switch applies +10 volts to the top of the
potentiometer and connects the loaded wiper to the pot bus for measuring purposes.
Note that the lower end of the attenuator is not grounded and voltages may be patched
to both ends of the attenuator.
The patching block has a ground termination near
the low end of the ungrounded potentiometer to make it convenient to ground the low
end if so desired.
The Type 42.183 Attenuator Group does not include the pushbutton switches.
The wiper
of each attenuator is brought out to a termination next to the attenuator to facil-
itate readout under loaded conditions.
Figure 7 contains schematics and symbols for
two potentiometer types.
The address or number qf the potentiometer (i.e., 1 or 2)
is placed within the circle; the coefficient setting (K) is written near the symbol.
The high and/or low end of ungrounded potentiometers is also indicated.
5.
THE
OPERATIONAL AMPLIFIER
a.
General Considerations
When a high-gain dc amplifier is used in conjunction with input and feedback networks
to perform mathematical operations, the resulting system is generally referred to as
an operational amplifier.
The operational amplifier is the basic and most versatile
unit in the analog computer.
It can be used for inversion, summation, multiplication
by a constant, integration, and used in conjunction with special networks for squar-
ing,extracting square root, generating logarithmic functions, etc.
17
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