1730–Series Theory of Operation
4–10
V
IN
V
OUT
1
/2
R
L
1
/2
R
L
1
/3
R
L
+V
-V
-V
+V
CR1
CR2
CR3
CR4
Figure 4-3: Simplified illustration of the Bridge limiter circuit.
When the bridge unbalances the current through the diodes changes, with more
current flowing into the load through either CR3 (positive excursion) or CR4
(negative excursion), which turns the diode on harder. At the same time current
flowing through the complementary input diode CR1 (positive excursion) or
CR2 (negative excursion) is reduced and the diode starts to turn it off. If the
change in V
in
is large enough, the output diode takes all of the current (which
turns off the input diode) and disconnects the input from the output.
The bridge load, R378, R377, and R374, is also a voltage divider that sets the
input dc level for the Vertical Output Amplifier at approximately –2 V.
The level shifted input signal drives the base of Q383 during active video signal
time. The active video signal is disconnected while Line Select readout is
displayed (U277D). Q383 is driven by the Y-Axis signal, through U277A, when
the vertical component of the readout is displayed. The Y-Axis signal is enabled
through the switch when ROEN
goes low.
The combination of Q382 and Q383 forms a shunt-feedback amplifier. Q382
amplifies and inverts the collector current flowing in Q383 to provide most of
the signal current through R485. Because the current across Q382 is nearly
constant the input-signal voltage is applied directly across its emitter resistor,
R485, with very little distor+tion. Negative feedback is employed to improve
Vertical Output Amplifier
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