HP 3478A Technical Manual page 100

3478A
Figure 7-F-3. Simplified Schematic Of The DC/Ohms
Input Amplifier
switches. The following paragraphs explain how the
ohms current is generated and how the ohms circuitry
operates. Refer to Schematic I and Figure 7-F-4 for the
explanation.
7-F-20. Current Generation. The explanation on how
current is generated is as follows:
a. The Voltage Splitter's (U201) output is + 12V (or 8.4V
for the 30M ohm Range only) which is applied to
the positive terminal of Buffer U202. The reference out-
put, which is applied to the positive terminal of the Out-
put FET Control Amp U203, is divided down to + 8V,
irrespective of range, by feedback resistors RR3 and
RR4 in conjunction with R206.
b. The output of the Buffer is + 12V (or + 8.4V for the
30M ohm Range only) which is applied to one side of
either Range Resistor RRS, RR6, RR7, or RR8
(dependent on the selected range). The other side of the
Range Resistors is applied to the Output FET Control
Amp.
c. Since the Output FET Control Amp is a high gain
operational amplifier with its non-inverting terminal at
Figure 7-F-4. Ohms Circuitry Configuration
7-F-6
TM 11-6625-3071-14