HP 8112A, Operating, Programming And Servicing Manual

Page: 229 / 406

S I GNAL
R i n R f b
OFFSET
OFFSET CURRENT
CONTROL
c x
R i n
Ro f f s
5 0 OHM
Figure 1 0.4-3. Simplified Output Amplifier circuit
The circuit is effectively an inverting amplifier with a voltage gain
given by
Rfb
Gain =
-
R·
in
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 U50 1 . This amplifier compares the
virtual ground at the inverting-input of the main-amplifier with
actual ground, and supplies a compensating current via Re in order
to maintain zero difference.
The offset-voltage control amplifier U502 detects any offset
voltage at the main amplifier output, vi_a the feedback network
c
x
Rin /
c
x
Rrb , 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 Roffs·
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 1 0.4-5