HP 8112A, Operating, Programming And Servicing Manual

Page: 227 / 406

Output Mode
For all waveforms, normal or complement output is selected by a
digital input from the bus latch to pin 15 of the shaper IC. This
input works in conjunction with a bias adjusting network and the
internal level shift, to provide normal and complement output modes.
Shaper Output
A portion of the amplitude vernier control (See Chapter 10.5) and
HILC input (See Chapter 10.3) are used by the vernier block in the
shaper IC to attenuate the output signal. The IC output amplifier
block is a differential output circuit, whose reference currents are
derived from the 6.2
V reference on the timing generator.
Current Mirror
The differential output of the shaper IC requires a "current mirror"
output stage which eliminates the effect of quiescent currents and
doubles the available output signal. The operating principle is
illustrated in Figure 10.4-2, and depends on Q lA and Q l B being a
matched pair so that Ia
=
lb.
In the HP 81 12A Q lA
=
Q400A, Q l B
=
Q400B and Q2
=
Q402.
In fixed mode, Q403 js turned on, switching the R430/C409
combination into the 'current mirror in order to improve its
performance.
Servicing the Shaper and Output Amplifier 1 0.4-3