Circuit Description— Type 1A1
SW105
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(Set to .1)
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Fig. 4 -1 . Simplified circuit diagram showing the most important components involved when calculating the X 2 attenuation factor.
Input Source Follower (Q122)
Q122 is a field-effect transistor (FET) that presents a high-
impedance, low-capacitance load to the input circuit and
isolates the input from the remaining stages. Resistor R116
is the input resistor for the VOLTS/CM positions from .005
to .05, and then is part of the input attenuation network
in all positions of the VOLTS/CM switch above .05.
A network consisting of Cl 19, R119, D l l 8, D l l 9 and R118
form a protection circuit in Q122 gate circuit. The resistive
component R119 limits the steady-state current if a negative-
or positive-going overload signal is inadvertently applied
to the Channel 1 INPUT connector. At normal signal ampli
tudes R119 will deteriorate the high-frequency AC response.
To offset this effect, Cl 19 is added to pass the high fre
quency information around R119.
If a negative-going overload signal is applied to the Chan
nel 1 INPUT connector, D l l 9 will conduct and limit the volt
age to —20.6 V at the gate element of Q122. Zener diode
D l l 8 sets the voltage of D l l 9 at —20V. If a positive-going
overload signal is applied, D122 conducts and clamps the
base of Q123 at +5.6 V.
Zener diode D121, connected between ground and the
drain elements of Q122 and Q142, clamps the drain elements
at + 10 V for all operating conditions.
A capacitive-coupled bootstrap circuit is connected be
tween the source and the capacitive elements connected to
the gate of Q122 to reduce the effective capacitance of the
input circuit. By reducing the input capacitance, sufficient
adjustment range is provided for the variable input capac
itors in the attenuators and the X I input circuit, such as C l04
for example. The input capacitance is effectively reduced
two ways: (1) By encircling Q122 gate terminal with an
etched wire and connecting this etched wire to the source
element; (2) by connecting D119 diode case lead to Q122
source element instead of ground. (Capacitance exists be
tween the diode case and the diode chip inside the case).
The capacitance that exists between the base of the mount
ing posts for Cl 19 and R119 and the etched wire that en
circles these posts improves the response of the circuit when
a fast rising square wave is applied to the Channel 1 INPUT
connector.
DC Balance Source Follower (Q142)
The DC Balance Source Follower (Q142), the Input Source
Follower (Q122), and the Emitter Follower (Q123/Q143)
stages constitutes a complete symmetrical circuit for the pur
pose of balancing out objectionable low-voltage power-sup-
ply fluctuations. Voltage variations common to the signal
circuit and the DC balance drive circuit arrive in phase at
the bases of the Input Amplifier stage (Q124/Q144) and
therefore cancel.
In the gate circuit of Q142, the .005 V/CM VAR ATTEN
BAL control (R130) is a dual concentric potentiometer with
both sections driven by the same front-panel shaft. This con
trol has built-in backlash coupling between its two sections.
Thus, the control serves as a combined coarse-fine adjust
ment to permit setting the DC balance drive accurately and
yet provide a wide range of coarse adjustment.
C138 bypasses any fast voltage fluctuations to ground.
C140 bypasses the chopped mode switching transients that
are induced in the ground-current loop when chopped mode
of operation is used.
First Emitter Follower (Q 1 2 3 /Q 1 4 3 )
In addition to providing a means for balancing out power-
supply fluctuations, as described previously, the First Emitter
4-2
