Circuit Description—Type 422 AC-DC
In the .01 position, R30 provides the emitter resistance
for Q34 and the gain of the stage is five. In the .02 position,
R32 provides the emitter resistance for Q34 to result in a
stage gain of 2.5. In the .05 and higher Channel 1
VOLTS/DIV switch positions, the emitter Resistance is
infinity (open circuit) to produce a gain for this stage of
one. As mentioned in the Input Stage discussion, the STEP
ATT BAL adjustment is set to provide a zero-volt level at
the emitter of Q34 under quiescent conditions. Since there
is no voltage difference across the emitter resistance of
Q34, changing the emitter resistance does not change the
current in the circuit. Therefore, the trace position does not
change when switching between the .01, .02 and .05
positions of the Channel 1 VOLTS/DIV switch.
Zener diode D41 along with the network C41-R41 pro
vides a stable, low-impedance voltage source for the emitter
of Q44. Diode D30 protects the base-emitter junction of
Q44 against reverse breakdown from high negative input
voltages. Capacitor C40 provides high-frequency regenera
tive feedback from the collector to base of Q44 to stabilize
this stage. Variable Balance adjustment R35 sets the output
DC level of the Feedback Amplifier stage so there is no
current flow between the emitters of Q84 and Q94 in the
Paraphase Amplifier stage under quiescent conditions. With
this configuration, the position of the zero-volt reference
trace will not shift as the Channel 1 VARIABLE control is
rotated.
Channel 1 Trigger Pickoff
The signal at the collector of Q44 in the Feedback
Amplifier stage is connected to the Channel 1 Trigger Pick
off stage through R51. This sample of the Channel 1 input
signal provides internal triggering from the Channel 1 signal
only. Q53 is connected as an emitter follower to provide
isolation between the Channel 1 Input Amplifier and the
Sweep Trigger circuits. It also provides a minimum load for
the Feedback Amplifier stage and a low output impedance
to the Sweep Trigger circuit. The CH 1 Trigger DC Level
adjustment R57 provides a zero-volt DC output level to the
Sweep Trigger circuit with no signal applied to Channel 1.
Output from the Channel 1 Trigger Pickoff stage is connec
ted to the Sweep Trigger circuit through the TRIGGERING
Source switch.
Paraphase Amplifier
The output signal from the Feedback Amplifier stage is
connected to the Paraphase Amplifier stage. Q84 and Q94
are connected as a common-emitter phase inverter (par
aphase amplifer)2 to convert the single-ended input signal
to a push-pull output signal. Gain of this stage is deter
mined by emitter degeneration. As the resistance between
the emitters of Q84 and Q94 increases, emitter degenera
tion increases also, to result in less gain through the stage.
GAIN adjustment R80 varies the resistance between the
emitters of Q84 and Q94 to set the overall gain of the
Channel 1 Input Amplifier. This control is set to provide
calibrated vertical deflection factors. Channel 1 VARI
ABLE control R90 provides variable gain to provide con
tinuously variable deflection factors between the calibrated
settings of the Channel 1 VOLTS/DIV switch. The network
C78-D78-R77-R78-R79 connected to the base of Q94 pro
vides the same DC level and circuit response in the Channel
1 Input Amplifier as the X9 Gain Stage provides in the
Channel 2 Input Amplifier circuit. Vertical position of the
trace is determined by Channel 1 POSITION control R60.
This control sets the conduction of Q64 and Q74, which
controls the quiescent emitter current of Q84 and Q94. The
output signal at the collectors of Q84 and Q94 is connected
to the Diode Gate stage in the Vertical Switching and Out
put A m p lifie r c irc u it through C84-R84-R85 and
C94-R94-R95.
CHANNEL 2 INPUT AMPLIFIER
General
The Channel 2 Input Amplifier circuit is basically the
same as the Channel 1 Input Amplifier circuit. Only the
differences between the two circuits are described here.
Portions of this circuit not described in the following
discussion operate in the same manner as for the Channel 1
Input Amplifier circuit (corresponding circuit numbers
assigned in the 100 - 199 range). Fig. 3-3 shows a detailed
block diagram of the Channel 2 Input Amplifier circuit. A
schematic of this circuit is shown on diagram 2 at the back
of the manual.
Feedback Amplifier
Basically, the Channel 2 Feedback Amplifier stage op
erates as described for Channel 1. Flowever, there are
several differences. Channel 2 does not have a trigger pick
off circuit. C l43 and R143 provide a similar load at the
collector of Q144 to that provided by the Channel 1 Trig
ger Pickoff stage at the collector of Q44. This RC network
assures similar response in each channel. Also, Channel 2
has a X 10 gain feature which is not contained in Channel 1.
Complete operation of this feature is described under X9
Gain Stage.
X9 Gain Stage
The Channel 2 Input Amplifier circuit contains an addi
tional amplifier stage comprised of Q154 and its associated
components. The signal at the collector of Q144 in the
Channel 2 Feedback Amplifier stage is connected to the
X I0 GAIN AC switch SW150. When this switch is in the XI
position (pushed in), the X9 Gain Stage establishes the DC
level at the base of Q194. The Channel 2 Paraphase Ampli
fier stage operates in the same manner as described for
Channel 1. When the X10 GAIN AC switch is pulled out to
the X I0 position, the signal from the Channel 2 Feedback
Amplifier stage is capacitively coupled to the X9 Gain Stage
as well as to the Channel 2 Paraphase Amplifier stage. Amp
lifier Q154 provides nine times gain for the signal which is
connected to its base through R151 and C151. This ampli
fied signal is connected to the base of Q194 in the Channel
2 Paraphase Amplifier stage through R156. Normal gain of
the Paraphase Amplifier is one times. The amplified signal
2Lloyd P. Hunter (ed.), “Handbook of Semiconductor Electronics”,
second edition, McGraw-Hill, New York, pp. 11-94.
Ell
3-5
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