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43
B. OUTPUT CIRCUIT AND D.C. OFFSET (Fig. XIV-8)
3.1.
Differential amplifiers
The pulses produced by the timing circuit pass through phase inverter switch
NORM./INV., S8, one section of switch AMPLITUDE IN 50 Q, S4, and zeners
Z301 and Z305 to the bases of differential amplifier Z302-Z303. This amplifier
is the first link in a chain of six similar ones.
In the operating mode illustrated in the circuit diagram (Fig. XIV-8), i.e. switch $8
set to NORM. and switch S4 to —3 V, the positive-going pulse at connector 53 on
printed-wiring board 2 is applied to the base of Z303.
The signal path to the last amplifier stage goes through Z303, Z306, Z310, Z312,
Z313, Z320, Z319 and Z321.
The pulse is negative-going at the base of Z322 and this transistor is cut off.
Simultaneously and similarly, the negative-going pulse at connector 54 on printed-
wiring board 2 goes through the second branch of the differential amplifiers to the
base of 2323. Here, this pulse is positive-going and can pass the transistor to the
input of the driver Z329 via zener diode Z325.
B.2.
Driver and output stage
Refer to the simplified diagram Fig. VII-4 and the detailed circuit diagram Fig. XIV-8
The driver consists of differential amplifier Z2329-Z330 and constant current source
Z326-Z2327.
Before moment tj, Z329 is off and Z330 is on. The current source Z326-Z327
supplies current I7 through Z330. A small portion i of I is diverted to clipping
diode 2333, but the major portion I7 — i flows through Z334 — Z335.
Output transistor Z332 is connected in the common-base configuration, and its
base voltage is held at a constant level by transistor Z336.
Before moment t}, Z332 is off, which means 0 volts across load resistor Ry.
At moment tj, the negative step at the base of Z329 will switch the differential
amplifier so that current I2 flows through Z329. Z330 is cut off so that output
transistor Z332 turns on and current [1 flows through Z334-Z335. The voltage
across load resistor RL, drops to a value determined by current I}.
At moment t2, the state before t1 re-appears.
+20V
2326
~ 2327
3V
s
Jo RE301
and
P3
Ltiel
-~-Current path during interval ty-t
—— Current path before ty
2334-2335
MA6498
-20V
Fig. VH-4. Simplified diagram of the driver and output stage
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