HP 8112A Operating, Programming And Servicing Manual page 230

Trigger Output Amplifier
Level Check Circuit
1 0.4-6 Servicing the Shaper and Output Amplifier
Voltage gain
The signals from the input stage, generated across R524 and R527,
are applied to the bases of Q503 and Q504. These transistors operate
as emitter followers for Q505 and Q506 which provide the actual
voltage gain.
Output
The output stage consists of the emitter-follower pairs Q510, Q512
and Q51 1 , Q513. These decouple the voltage gain stage from the low
output-impedance.
The input to the trigger amplifier, derived from the period generator
(
See Chapter 10.3), is passed via a schmidt trigger input Q280 and
Q281 , to the output stage. With a logic high on the input , Q282 is
switched on and in turn switches Q283 off, to produce a low (0 V)
output.
When the circuit input goes low, Q282 is switched off allowing Q283 .
to switch on and provide an output voltage of 2.4 V into 50
4.8 V into a high impedance.
Due to the inverting action of the trigger amplifi e r, the inverted
trigger input pulse is output as a positive going pulse, synchronized
to the output of the period generator. In normal mode the TRIG
OUT signal is a 503 duty cycle square wave at the same frequency
as the main HP 81 12A output . In TRIG and GATE modes, the
TRIG OUT signal is a pulse-shaped equivalent of the trigger input.
The rising and falling edges correspond to the trigger level voltage
passing through the threshold level set by the TRIG LEVEL control.
This circuit is used during instrument self-test which is performed
at switch on, to verify the operation of the output amplifier and
pre-attenuator. For more details see the self-test routine described in
chapter 10.1.
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