Tektronix 1730 Series, Instruction Manual

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1730–Series Theory of Operation
4–9
front-panel GAIN control alters the dc level on pin 10 through an analog switch,
U585B. The switch is closed only when Variable is selected.
The current flowing out of pin 6 drives the Gain Cell Amplifier, while the
current flowing out of pin 12 drives into a collector load, R480.
The Gain Cell Amplifier is a clamped inverting operational amplifier driving
both the Switchable Gain Amplifier and the 2
nd
DC Restorer. It consists of
Q673, Q674, and Q669, with Q669, an emitter follower, operating as the output
stage. Amplifier gain is approximately 4.
The DC Restorer clamps the output level of the Gain Cell Amplifier to the dc
level occurring at back porch time. DC Restorer drive is coupled through R474
into an analog switch (U277B) that is activated by the BACKPORCH signal.
U277B closes during back porch time to charge the hold cap, C484. The Error
Amplifier (U488) drives a current summing point at the input of the Gain Cell
Amplifier through U585D. For slow restorer, R484 is in the loop compensation.
However; for fast dc restorer, R484 is shorted to ground through U277C to speed
up the loop time constant. The enable signal for U277C is from the Micropro-
cessor on Diagram 5 and is only present when Fast DC Restorer is selected.
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 4-3 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.
Gain Cell Amplifier
2
nd
DC Restorer
Switchable Gain Amplifier