MM0474
Note that both curves in this figure are the result of using a voltmeter with
the accuracy stated in Table 1 under ACCURACY.
d. Precision Power Stabilizer.
Figure 7 shows the circuit arrangement.
To obtain this, the generator function switch is placed in the STAB position.
In operation, the reference generator amplifier compares the voltage at the top
of the bridge with the reference voltage set by the 6-place divider and adjusts
the modulator attenuation to keep them equal.
The PIN modulator output
connected directly to the operational amplifier is limited in the amount of
current it can supply.
This provides a measure of protection for the PIN
The MOD output can provide a much higher current from an emitter
follower.
e. 0-10 Volt Supply.
In the circuit shown in Figure 8, the reference
generator amplifier is again used as a unity gain follower.
The function
switch is placed in the RVG position and the V-REM-V switch in the V
position. The output voltage appears between terminals 22 and 4. The maximums
available output current is given under the reference generator specifications
in Table 1.
f. Bridge Circuit Description.
(1) The operation amplifier marked X107 in Figure 9 is comprised
primarily of Q609, Q614, and Q615 on the B600 board (refer to Figures 11 and
12). The error signal, due to bridge unbalance, appears at terminals E4 and E5
and is amplified by the differential pair Q609.
The output from Q609 is
buffered by Q614, with a voltage gain of approximately 1, into operational
amplifier Q615. The output of Q615 is then returned to the base of Q601 which
drives the top of the bridge through terminal E13.
(2) Transistors Q610, Q611, and Q612 form a constant current source for
Q609.
The negative 15 volt power supply voltage is used as a reference.
Transistors Q608 and Q613 isolate Q609 from common mode voltage changes which
appear at terminals E4 and E5 as the bridge current varies. The use of this
circuit arrangement affords a common mode rejection ratio typically in excess
of 126 dB, if Q608 and Q613 are matched. Matching is accomplished by measuring
the change in the gate-source voltage at a constant source current of 30
microamps while the gate voltage is changed from 0 to 10 volts. Units which
exhibit the same change in the gate source voltage to within 1% are selected as
pairs.
(3) The output resistance of the differential input stage is 200,000
ohms at each output. This is buffered by Q614 to isolate the relatively high
bias currents drawn by the inputs of Q615. Both Q609 and Q614 are integrated
circuits which utilize a heated, temperature-controlled chip to maintain
constant offset voltage and bias currents. The differential output from Q614
passes through polarity reversing switch S601B and S601C, two sections of the
thermistor-barretter switch. Diode D605 disconnects Q615 from power amplifier
Q601 if a turn-on
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