Tektronix 422, Instruction Manual

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Circuit Description—Type 422 AC-DC
ing Switch stage switches to Q 1114, the voltage induced to
the bases o f Q 1174 and Q 1184 forward biases Q1184 and
holds Q1174 reverse biased. Operation is the same as
described fo r Q1174. Waveform 5 in Fig. 3-15 shows the
collector currents o f Q1174 and Q1184. Waveform 6 shows
the voltage across the Q1184 side of transformer T1201.
Feedback Voltage. Diodes D1232 and D1233 form a
center-tapped, full-wave rectifier. The rectified output of
the diodes is filtered by C1232-C1233-R1232 to provide a
feedback voltage o f about +12 volts to the Error A m plifier
stage. The exact output level depends upon the voltage
induced into transformer T1201 and this feedback is used
to maintain a constant output voltage level from the AC-DC
Power Supply. The center-tap o f this winding o f T1201 is
connected to chassis ground through C l231 and to the
zero-reference voltage level.
Error Amplifier and Reference. Transistors Q1134,
Q1144 and Q1154 are connected as an error amplifier to
sense changes in the output voltage level and to provide a
correction signal to the Pulse Width Control stage. Zener
diode D1135 sets the emitter voltage o f Q 1134 at about +9
volts more positve than the zero-reference voltage level
through R 1136. This voltage remains constant due to the
zener action o f D1135. The base voltage o f Q 1134 is set by
divider R 1130-R113 T R 1132 connected between the Feed­
back Voltage and the zero-reference voltage level. If the
Feedback Voltage level changes due to a difference in vo lt­
age induced into transformer T1201, a sample of this
change is applied to the base o f Q1134 through tempera­
ture-compensation diode D1132. Capacitor Cl 133, con­
nected between the base and emitter o f Q 1134, reduces the
effect of ripple on the conduction o f Q 1134. Therefore, the
conduction of Q1134 is affected only by an increase or
decrease in output voltage from T1201. The change in vo lt­
age at the base of Q 1134 changes the current through this
transistor. The output current change is amplified by tran­
sistors Q1144 and Q1154 and applied to the Pulse Width
Control stage as an error current to correct the original
output voltage error. —12 Volts adjustment R1130, in the
base divider of Q1134, is adjustable to set the bias level of
Q 1134 such that a -1 2 -v o lt output level is produced by the
-1 2 -v o lt rectifier. The other supplies w ill also be w ithin
their voltage tolerance when this adjustment is set cor­
rectly.
To understand the operation o f this circuit, assume that
the Feedback Voltage output level increases because more
voltage is being induced into the secondary of T1201. This
places more voltage across divider R1130-R1131-R1132
and the base of Q1134 rises positive. Since the emitter of
Q1134 is held constant by zener diode D1135, this positive­
going change at the base of Q 1134 results in a reduction in
forward bias to reduce the current through this transistor.
Less current from Q1134 results in a greater forward bias
on Q1144 to produce a positive-going change at its collec­
tor. Transistor Q1154 operates as a voltage regulator to
control the flo w of current to the Pulse Width Control
stage. The positive-going change at the collector o f Q1144
forward biases Q1154 to supply more current to the Pulse
Width Control stage. This increase in error current to the
Pulse Width Control stage reduces the voltage induced in­
to the secondary o f T1201 to correct the original output
voltage error to maintain a regulated output voltage (see
Pulse Width Control discussion which follows fo r more
details). The action o f the Error A m plifier stage is similar
but opposite when the Feedback Voltage level decreases.
Pulse Width Control. The Pulse Width Control stage is
the final step in maintaining a regulated output voltage
regardless o f input voltage variations. This stage consists of
tunnel diode D1155 and amplifier Q1163-Q1164. Tunnel
diode D1155 is switched to its high-voltage state by the
positive-going signal at the emitter of the Blocking Oscilla­
to r (compare waveforms 2 and 7, Fig. 3-15). The positive
going step at the anode o f D1155 when it switches to its
high-voltage state is connected to the base o f Q1164
through R1162. Q1164 is forward biased by this step, and
it couples a negative step to the base of Q 1163 to turn it on
also. When Q1163 turns on, it completes the conduction
path between the center tap o f T1171 and the zero-
reference voltage level to allow one o f the Steering Switch
transistors to conduct (compare time relationship between
waveforms 3, 4 and 8, Fig. 3-15). The conduction o f the
Steering Switch stage controls the amount o f energy
induced into T1201 from which the output voltage is pro­
duced (see Steering Switch and Power Control discussions).
Therefore, the amount o f time that the Pulse Width Control
stage allows the Steering Switch transistors to conduct
determines the output voltage. The time that tunnel diode
D1155 remains in its high-voltage state is determined by the
recharge tim e o f Cl 121 and the error current from the
Error A m plifier stage. The recharge time of C l 121 is de­
termined by Oscillator Frequency adjustment R1125. The
error current from the Error A m plifier is determined by
—12 Volts adjustment R1130 and the output level o f the
Reference Voltage stage. When D1155 drops to its low-
voltage state, both Q 1163 and Q 1164 are reverse biased to
interrupt the collector current path fo r the Steering Switch
stage. The —12 Volts adjustment is set so the error current
supplied to tunnel diode D1155 ends the Pulse Width Con­
trol stage output signal at such a time as to provide the
correct regulated output voltages from the AC-DC Supply
(within input voltage range).
To maintain regulated output voltages with changes in
input voltage level, the Feedback Voltage Error A m plifier,
Pulse Width Control, Steering Switch and Power Control
stages work together as follows: Assume that the input
voltage from the AC-DC Power Selector circuit decreases
(e.g., Battery Pack output level drops due to normal dis­
charge). This results in less voltage induced into the second­
ary o f T1201, which produces a lower output level from
the Feedback Voltage stage. The Error A m p lifie r stage
senses this decrease in output level from the Feedback V o lt­
age stage and, as a result, produces a decrease in error cur­
rent to the Pulse Width Control stage. A decrease in error
current allows D1155 to remain in its high-voltage state
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