Video Drivers; Cascode Output Amplifier; Dc Restoration; Video Amplifer Section (Green) - Zenith Z-100 PC series Service Manual

Circuit descriptions for the three video amplifiers are
written with reference to the green video amplifier cir­
cuits. The red and blue amplifiers function identically.
Refer to the partial schematic of Figure 5-2 while
reading these circuit descriptions.
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Figure 5-2. Video Amplifer Section (Green)
Video Drivers
The gain-controlled green video signal leaving
IC5101 at pin 15 is coupled to the resistor attenuator
formed by R5117, R5134, and R5143. C5116 pro­
vides some high-frequency peaking for the video
signal.
The video signal is then applied to a two-stage,
direct-coupled, non-inverting amplifier formed by
05107 and 05108. The gain of the green channel is
fixed by resistor R5147. In the red and blue chan­
nels, potentiometers set the gain (R5156 for red and
R5139 for blue) relative to the green channel. The
red and blue channel gains can be varied ap­
proximately 25% around the green gain.
Cascode Output Amplifier
The video signal is applied to the cascode output
amplifier formed by transistors 05205 and 05206.
The cascode arrangement allows high gain and wide
bandwidth. The amplified video signal appears at the
collector of 05206 across load resistors R5212 and
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R5213. An emitter peaking network consisting of
II
C5205, C5214, R5209, and R5211 provides fre­
quency compensation for the cascode amplifier.
DC Restoration
As noted earlier, the incoming analog color signals
are AC-coupled to the inputs of IC51 01. The DC­
component of the video signal is then restored as fol­
lows. During the horizontal and vertical retrace inter­
val, the voltage at the emitter of 05205 is sampled by
the RC network formed by R5209 and C5214. At this
time, the video signal is at black level and the clamp
pulse at pin 1 of the 5R6 connector is active-high.
This sampled voltage is fed back to pin 14 of IC5101
and compared to a reference voltage developed at
pin 13 of IC51 01. This reference voltage is used by
all three channels in the same manner.
A push-pull current source at the output of the com­
parator within IC5101 charges or discharges hold
capaCitor C5105 depending on the magnitude and
polarity of the difference between the sampled vol1­
age and the reference voltage. The voltage devel­
oped across the hold capaCitor C5105 controls the
DC bias of the gain-controlled video output at pin 15
of IC5101. During the active clamp pulse time, the
DC restoration feedback loop is gated on and the DC
bias is set such that the black level emitter voltage of
05205 equals the reference voltage at pin 13 of
IC5101. During the video time, the DC restoration
feedback loop is gated off and the vol1age across the
hold capacitor supplies the required DC bias until the
next clamp pulse arrives. In this manner, the AC­
coupled video signal applied to the input of IC5101 is
DC-restored and an accurate and stable bias is sup­
plied to the cascode output amplifier.
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Page 5-4
Circuit Descriptions