Tektronix 1710J Series, Инструкция по эксплуатации

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Theory of Operation
5–8
1710J-Series Waveform Monitors
The switchable gain amplifier consists of Q476, Q477, and Q478, with Q469 as
the switching element. When the base of Q469 is pulled low through R472,
amplifier gain is –1. R
i
is R475 and R
f
is the sum of R367 and R368. When its
base is high, Q469 saturates and grounds the collector end of R470 to put an
attenuator in the feedback path and increase gain by a factor of five. The output,
at the collector of Q476, drives a bridge limiter circuit comprised of CR280 and
CR380. See Figure 5–3 for a simplified diagram of the limiter.
The purpose of the limiter is to prevent the vertical output amplifier from being
over driven. The bridge limiter circuit, encompassing CR280 and CR380, is
quiescently balanced (equal current through all arms) with no V
in
. When there is
a signal voltage (V
in
) applied to the bridge (CR1-CR2), the output signal voltage
(CR3-CR4) is approximately equal to the input. When V
in
moves away from the
quiescent state, the current in the bridge arms becomes unbalanced.
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 5–3: 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.
Switchable Gain Amplifier