Tektronix 323, Instruction Manual

Page: 40 / 170

in cancellation of effects from + and — 5 V power supply
variations and FET thermal variations.
R33 and C33 are decoupling components. D30, D31, D32,
and D33 provide protection to Q33A by limiting input sig­
nals to approximately dbl.2V . R31 limits the overload cur­
rent to a safe value during conduction of the diodes. C31
permits high frequency components of signals to bypass
R31 to provide optimum transient response.
Paraphase Amplifier
Refer to the simplified schematic in Fig. 3-3 during the
following explanation.
Under no-signal (quiescent) conditions, equivalent points
on the two sides of the amplifier are at equal potentials.
Therefore, no current is flowing through either R44 or R69.
Standing current for both Q53 and Q61 flows through R65.
The R65 current is kept relatively constant by the fixed po­
tential at the R65-R75 junction with respect to that at the
base (and therefore the emitter) of Q53. If the current
through Q61 increases, the current through Q53 must de­
crease because of this R65 constant current feature. A simi­
lar explanation applies to Q71, Q55 and R75.
When a positive signal is applied to the base of Q41A,
it is coupled into the emitter, where it unbalances the volt­
age across R44. Current flows through R44, increasing
the Q41A current and thereby increasing the current drive
to Q50. The low emitter impedance of Q50 as compared
to its high collector load impedance causes an amplified
positive voltage signal to appear at the base of Q61. This
signal is coupled to the Q61 emitter where it demands an
increase in current through R62 and R63. Since the R65
current is constant, the increase must be accompanied by
an equal but opposite current change through Q53. In
effect, the Q53 current is decreased by an amount which
is almost equal to the current in R44, with the difference
between the two giving sufficient error current drive to Q50
(via Q41A) to obtain proper circuit response.
The current change which flows through R44 in response
to the positive input signal also flows through Q55. Since
the R75 current is kept relatively constant in the same man­
ner as the R65 current previously mentioned, the R44 cur­
rent decreases the current through Q41B, decreasing the
Q58 drive current. An amplified negative signal voltage is
therefore developed at the base of Q71 and coupled to its
emitter, where it decreases the current demanded through
R72 and R73. This decrease is accompanied by an equiva­
lent increase through Q55, supplying most of the current
which was demanded through R44 by the original positive
input signal.
It should be noted that the increase in Q61 current is
accompanied by a positive signal at the R62-R68 junction,
while the decrease of Q71 current causes a negative signal
at the R72-R76 junction. This unbalance causes some of the
Q61-Q71 signal current to flow through R69. If the re­
sistance of R69 is increased, less signal current will flow
through it. Consequently, less flows through R62 and R72,
causing less signal voltage output to R80 and R84. R69
is adjusted to provide calibrated gain when the effective
resistance of R76 is 0 and the X 1 0 VERT GAIN switch is
open. Circuit gain decreases to 40% or less of the cali­
brated amount when R76 is fully inserted into the circuit.
Circuit Description— Type 323
Refer once more to the Q41 emitter circuit. If R46 is
switched info the circuit, it effectively decreases the re­
sistance between Q41A and Q41B. More current then
flows between the emitters of Q41 for a given input. A
larger output must be generated at the emitters of Q61 and
Q71 to compensate for the increased current demand. Clos­
ing the X 1 0 VERT GAIN switch increases circuit gain by a
factor of 10. R46 is adjusted to provide accurate gain while
the X 10 VERT GAIN switch is closed.
VAR V/DIV BAL (R40), in the collector circuit of Q41,
permits balancing of the two sides of the Q41 circuit. C50
and C58 improve transient response through regenerative
high frequtncy feedback. Voltage dropping resistor R52
sets the current through Q50 and Q58.
The base-biasing network of Q53 and Q55 consists of
thermal compensation components D54 and R54, voltage
divider R55 and R56, and decoupler C56. Temperature
variations cause slight differences in voltage to occur at
the anode of D54, resulting in slight voltage changes at the
bases of Q53 and Q55 to offset thermal effects on the tran­
sistors.
Total circuit current can be modified slightly by LIMIT
CENTERING (R66). The current equally affects both sides
of the amplifier and thus causes identical voltage changes
at the emitters of Q61 and Q71. LIMIT CENTERING is ad­
justed to cause D86, D87, D88 and D89 to clip the signal
when the trace reaches equal distances beyond the top
and bottom of the CRT graticule area.
R61 and R71 pass the majority of the standing current
for Q61 and Q71 operation. This is added to by slight
amounts of current in the previously described feedback
path and by current to the POSITION control and the Lim­
iter circuit.
The POSITION control connects to a dual potentiometer.
Movement of the control results in application of opposite
voltage changes to R81 and R83. R81 and R83 are con­
nected through the limiter circuits to the relatively low im­
pedance nodes of operational amplifiers Q91 and Q99.
Changing the POSITION control setting therefore injects
positioning currents into these nodes in the same way as
signal current is injected through R80 and R84. Any dis­
placement of the trace in response to a signal will be in
respect to the vertical DC reference which is established
by the POSITION control.
Clamper and Limiter Circuits
Clamper. R85 and D85 form a divider which supplies
approximately —0.6 V as a clamping reference. If D86 or
D87 cuts off, one of the opposite-polarity diodes will con­
duct in each half of the amplifier, thus clamping the D86
anode to a —0.6 V ;fc0.6 V operating range. The Limiter
circuit is therefore capable of fast recovery from overdriv­
ing signals.
Limiter. With the LIMIT CENTERING (R66) control prop­
erly set and the trace at graticule vertical center, —0.9 V
exists at the D86-D87 and the D88-D89 junctions. This
causes approximately 0.2 mA to flow through R87 and R89,
dividing equally between the diodes. The anodes of D87
and D89 are set to approximately —0.6 V by the base-
emitter junctions of Q91 and Q99.
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