Circuit Description—475
cies within the bandwidth of the instrument, the Input
Attenuators are designed to maintain the same input RC
characteristics for each setting of the VOLTS/DIV switch.
Each attenuator contains an adjustable series capacitor to
provide correct attenuation at high frequencies and an
adjustable shunt capacitor to provide correct input capac
itance.
NOTE
Each attenuator is a hybrid encapsulated plug-in
assembly; therefore, replacement of individual com
ponents within the attenuator is not possible. Should
defects occur, the attenuator must be replaced as a
unit.
First Cascode Amplifier
The first amplifier stage in the Channel 1 Preamplifier
circuit is hybrid circuit U120. U120 basically consists of an
integrated emitter-coupled, push-pull, cascode amplifier cell
and two discrete field-effect transistors (FET) mounted on
a ceramic substrate with the thick-film resistors. The stage
is configured as a paraphase amplifier and converts the
single-ended input signal to push-pull output signals. CR104
and CR107 provide protection for the input to U120 if
large negative-going signals or DC levels are applied to the
CH 1 OR X input connector.
This input amplifier stage contains the CH 1 VAR
VOLTS/DIV control. This control provides the ability to
obtain continuously variable (uncalibrated) vertical deflec
tion factors between the calibrated positions of the
VOLTS/DIV switch. With the VAR control in its calibrated
detent (wiper at ground), the output transistors of U120,
whose collectors connect to pins 5 and 9 are conducting;
the output transistors whose collectors connect to pins 6
and 8 are biased off. Thus, the current available to the
following amplifier stage is that flowing from pins 5 and 9.
When the VAR control is rotated out of its calibrated
detent, the transistors whose collectors connect to pins 6
and 8 begin to conduct. The current they conduct is robbed
from the other output transistors. This causes two things to
occur:
1. The current flowing in the collectors of the transis
tors connected to pins 5 and 9 is less now than when the
VAR control was in its calibrated detent.
2. The current available to the following amplifier stage
is the algebraic sum of the currents in the collectors tied
together (i.e., pins 8 and 9 and pins 5 and 6). Since the two
collectors tied together have signals of opposite polarity,
the signal available to the following amplifier stage is less
than that available when the VAR control was in its
calibrated detent. The component values selected for the
variable function provide a variable attenuation ratio of
approximately 2.5 to 1. The Channel 1 Variable Balance
adjustment R110 adjusts for no trace shift in the display
when rotating the VAR control. The Channel 1 UNCAL
light indicates when the Channel 1 VAR control is out of
its calibrated detent. The components connected between
pins 2 and 3 of U120 provide high-frequency compensation
for the stage.
Second Cascode Amplifier
The second amplifier stage in the Channel 1 Preamplifier
circuit is U140. U140 is the same integrated emitter-
coupled, push-pull, cascode amplifier as the one used in
U120. It is used as a push-pull amplifier and contains 2 mV
and 5 mV gain adjustments that determine the overall gain
of the Channel 1 Preamplifier circuit. The output signals
available to the next stage for amplification are taken from
pins 5 and 9 of U140.
The gain of U140 is determined by biasing the output
transistors connected to pins 6 and 8 to conduct more or
less current. As more current is conducted through the
transistors connected to pins 6 and 8, that much less is
conducted through the transistors connected to pins 5 and
9. The current conducted by the transistors connected to
pins 5 and 9 in the 2 mV position of the VOLTS/DIV
switch is approximately 2.5 times the current conducted by
the same transistors in all of the other VOLTS/DIV switch
positions. This results in increased gain and a DC level shift
at the signal output terminals of U140. The VOLTS/DIV
switch compensates for a shift in the DC level by shorting
out part of the common-mode resistance (R157) when
operating with 2 mV sensitivity. The Gain Switch Balance
adjustment R135 adjusts the DC balance of the stage, so
there is no baseline shift in the CRT display when switching
between the 2 mV and 5 mV positions of the CH 1
VOLTS/DIV switch. The remainder of the components
connected between pins 2 and 3 of U140 provide high-
frequency compensation for the stage. Resistor-thermistor
RT149 reduces in value with increases in temperature to
compensate for losses in gain associated with increases in
ambient temperature.
Third Cascode Amplifier
The third amplifier stage in the Channel 1 Preamplifier
circuit is a discrete component cascode amplifier made up
of Q172, Q178, Q182, Q184, and Q188. Q184 is a
relatively constant current source for Q172 and Q182,
Q172 and Q182 convert the input voltage signals into
current signals which are in turn converted back to voltage
signals by Q178 and Q188 respectively. R172 and R182
provide thermal compensation and C172 and C182 AC-
couple the signal around R172 and R182 to reduce Miller
effect. C l75 and R175 are variable high-frequency com
pensation adjustments while CR170, CR174, and RT170
®
3-4
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