MM0474
(2) The heart of the reference voltage generator is another high gain
differential-input operational amplifier similar to the one used in the bridge,
it has a lower open loop gain, a higher common mode voltage limit, and larger
output current capability (refer to Figure 13).
(3) The input stage for the amplifier is Q704. Transistors Q705, Q706,
and Q707 furnish a constant emitter current to the two halves of Q704.
Transistors Q703 and Q708 isolate the input stage from the large common mode
voltages which must be accommodated in this application. Transistors Q703 and
Q708 must be matched in accordance with the technique described in Paragraph
7f(2). In this case, however, the measurements are made at source currents of
150 microamps. The post-amplifier, Q713, is coupled directly to the drains of
Q705 and Q708 without an intermediate buffer amplifier.
This is possible
because of the lower impedances present at the drains of Q703 and Q708.
(4) The input offset voltage of Q704 is cancelled by adjustment of R710
and R719.
The input bias currents required by Q704 are supplied by current
generators composed of Q701 and Q702 for one input, and Q709 and Q710 for the
other input.
(5) The reference voltage generator unit is also a servo-mechanism in
each of its three modes of operation. In the first mode to be discussed, the
NULL mode, the feedback loop is completed through the operator. In this mode,
the high gain amplifier is used as a null detector between the slider of R703,
the six place divider, and the top of the bridge.
Small differences in
potential between those points result in a very small current flowing into one
amplifier input and out the other.
This current is amplified by Q704 and
integrated by Q713 in capacitor C705. The net charge on C705 is indicated on
the front panel meter M701. A sufficient condition for a null to exist between
the slider of R703 and the top of the bridge is that the charge on C705 remain
constant. Thus, it is not necessary to bring the output voltage meter pointer
to zero in order to obtain a null in the dials of R703. It is only necessary
that the pointer be stationary at some point between plus and minus 10 volts.
In the null mode, S702 is nonfunctional in the V position. In the V position,
the bridge voltage is presented at terminals 21 and 22.
(6) In the reference generator or RVG mode, the operational amplifier
described above is modified by the addition of Q715, which serves as a power
amplifier.
In this mode, the operational amplifier is connected as a unity
gain voltage follower.
This is accomplished by returning the output of the
amplifier from Q715 to the inverting input, terminal E1.
The non-inverting
input, terminal E2, then presents a very high impedance to the divider R703.
This avoids loading errors.
With proper adjustment of the two bias current
compensating
circuits,
the
input
impedance
so
the
voltage
follower
configuration exceeds 1000 megohms.
The maximum output impedance of the
voltage divider is on the order of 3000 ohms so that the loading error due to
the voltage follower
122
Previous Page
