Agilent Technologies 6621A, Service Manual

Page: 23 / 156

interface circuit, must be on (approxi 2 V) in order
to activate the voltage control circuit.
The voltage control circuit compares the output voltage to
the programmable reference voltage CV REF to produce the
CV signal. As shown in the simplified schematic of Figure 2-
6, the major components in the voltage control circuit are:
unity gain inverter buffer U352B, output sense inverting
differential amplifier U352A, and CV error amplifier U347.
The reference voltage (CV REF, 0 to -10 V) is applied to
U353 which produces a 0 to + 10 V signal feeding into the
summing junction S1. The output voltage is monitored by
U352A which produces a 0 to -10 V signal that represents
the output voltage magnitude which is also fed into S1. The
0 to -10 V signal is also sent back (V READBACK) to the
secondary interface to indicate the magnitude of the output
If the output voltage exceeds the programmed voltage, the
summing junction S1 goes negative causing U347 to
produce a positive going CV control signal. For this
condition, the base drive circuits will conduct more and pull
current away from the power m BASE DRIVE input
via the - DRIVE input line. This will cause the power
module's series regulators to conduct less and thus reduce
the output voltage.
If the output voltage is less than the programmed voltage,
S1 goes positive causing U347 to produce a negative going
CV control signal. For this condition, the base drive circuit
2-13
voltage.