Input - HP 8112A Operating, Programming And Servicing Manual

OFFSET CURRENT
CONTROL
Figure 10.4-3. Simplified Output Amplifier circuit
The circuit is effectively an inverting amplifier with a voltage gain
given by
The main HF amplifier consists of the discrete transistors Q501 -
Q513 and their related components on Figure 10.4-5.
The offset current error of the amplifier is compensated for by the
offset-current control amplifier U501. This amplifier compares the
virtual ground at the inverting-input of the main-amplifier with
actual ground, and supplies a compensating current via R, in order
to maintain zero difference.
The offset-voltage control amplifier U502 detects any offset
voltage at the main amplifier output,. via the feedback network
c x Ri, / c x ROD, and compensates for it via the main-amplifier's
non-inverting

input.

The required output-offset is created by injecting the offset-input
signal from, the offset vernier DAC and the HILC from the timing
circuit, into the summing point of the offset-voltage control amplifier
via R,ffS.
The main amplifier can be divided into three stages:
Input
The signal from the shaper IC current-mirror is applied to the
inverting input where it is amplified by Q501 and Q502 (common
base amplifiers). CR501 and CR502 provide the required bias
voltages. The offset signal is applied to the non-inverting input at
the junction of CR501, CR502 to ensure a constant reference point.
Servicing the Shaper and Output Amplifier
R f b
OFFSET VOLTRGE
CONTROL
10.4-5