Tektronix 323, Инструкция по эксплуатации

Страница: 42 / 170

Circuit Description— Type 323
nector cause equal and opposite deviations from this value
at the two plates, resulting in approximately 1 division of
deflection for each 18 volts of differential signal output.
Overall gain of the circuit is approximately 90, determined
principally by the quotient of R160 (Rf) divided by R101 (R[).
(Gain for one side is the same as push-pull gain.)
The top half of the amplifier is explained here. The in­
put signal passes through R101 and a comparator stage.
The output of the comparator goes through an emitter fol­
lower and then to the output amplifier, which supplies the
signal to the upper deflection plate.
The two halves of the amplifier operate in push-pull.
Current in one side decreases as it increases in the other
side. During high frequency operation, rapid signal changes
required at the CRT require that additional current be avail­
able in the output stage. The High Frequency Boost Circuit
supplies this extra current.
Detailed Description
Quiescent Conditions. Refer to the Vertical Output Am­
plifier schematic diagram. When the trace is positioned at
graticule center and no signal is applied, the voltage ap­
plied by Q91 and Q99 to the inputs of the amplifier causes
the collectors of Q163 and Q173 to be at + 5 0 V. Almost
all of the feedback current through R160 and R170 flows
through input resistors R101 and R108.
The voltage at Q103 base is equal to and controlled by
the Q l l l base voltage, which is established by the R114-
R115 voltage divider.
With base conditions of the input transistors thus estab­
lished, the current through R104 is divided between Q103
and Q l l l . The resultant drop across R ill establishes the
base voltage of Q121, and thus its emitter voltage. The
difference between the emitter voltage and the —5 V sup­
ply sets the current through R123, which is then divided
between Q121 and Q133. The determining factor in the
Q121-Q133 current division is the base voltage of Q133,
which is established by a voltage divider in its base circuit,
in conjunction with the amount of conduction of Q141. The
total closed loop action sets the Q133 current so that the
voltage drop across R131 causes Q160 to conduct enough
to supply the necessary current through Q163 and R160
to establish the 50 V CRT deflection plate potential.
The emitter of Q121 also sets the base voltage of Q163.
The resulting voltage at the emitter of Q163 (and the volt­
age at Q173 in the lower half) then determines the amount
of current through R169, which establishes the standing cur­
rent of the entire Q160, Q163, Q170, Q173 output stage.
The anode voltage of D169 is determined by the Q163-
Q173 emitter voltages and is slightly less than that required
for D169 conduction.
Low Frequency Operation. When push-pull signals
arrive at the inputs to R101 and R108, they cause equal
and opposite reactions in the two halves of the amplifier.
A negative signal voltage at R101 attempts to decrease the
DC voltage at the base of Q103. Current decreases through
Q103 and increases through Q l l l - R i l l . The negative-going
voltage change at R ill decreases the current through Q121
and diverts it to R135, Q133, and the base-emitter junction
of Q160. The resulting increased drive to Q160 causes its
collector voltage to rise, raising the upper deflection plate
voltage. The R160 feedback current changes in proportion
to the deflection plate voltage change to null out the input
error signal at the base of Q103.
High Frequency Operation. Large transient currents are
necessary in the output circuit (Q160, Q163) to charge and
discharge the deflection plate capacitance if the plates are
to respond to high frequency or fast transient waveforms.
Additional circuitry is put into operation to satisfy these
demands. A finite time is required for the loop to respond
to input signals. If the signal at the upper deflection plate
tends to lag a single very fast negative-going input wave-
front, less than the required amount of feedback is avail­
able through R160, and a larger than normal error signal
develops at the base of Q103. The resulting increased cur­
rent drive through R135 and Q133 causes Q160 to conduct
the necessary transient current to charge the deflection plate
capacity positive quickly enough to reproduce the input
waveform faithfully. The error voltage at the base of Q163
is in the direction to reduce or turn off its current, further
helping Q160 to charge the capacity.
When a single very fast positive-going input wavefront
is applied, current drive to Q160 is decreased, and a posi­
tive-going error signal is applied to the base of Q163, in­
creasing its collector current to enable the deflection plate
to respond rapidly in a negative direction. Cl 67 and Cl 69
provide bypass paths to satisfy the increased current de­
mands of Q163 during this error operation.
If these input wavefronts are repetitive rather than single
events, the unidirectional transient current flow into C l67
and Cl 69 will cause them to charge positive, thereby cut­
ting off Q163 and Q173 during the quiescent condition fol­
lowing the wavefronts. To counteract this, the increased
signal amplitude occurring at the base of Q163 during high-
frequency operation is coupled through Cl 55 to the base
of Q151. The emitter-base junction of Q151 operates as a
peak detector during the positive signal peaks, placing a
positive charge on C l43. The resulting decreased current
through Q141 raises its collector voltage, which is common
to the base of Q133. The Q133 emitter follows, raising
the voltage at the base of Q163. This raises the Q163 emit­
ter voltage, which causes D169 to go into conduction, caus­
ing the DC current through Q160 and Q163 to increase for
as long as the signal is present. This increased current dis­
charges Cl 67 and Cl 69 between wavefronts.
It may also be noted that when the collector voltage of
Q141 goes in a positive direction, feedback action through
R160 and R170 (back to the input) causes the Q121 and
Q127 base potentials to increase by the same amount. This
allows the average DC CRT plate potential to remain at
50 volts.
It must be remembered that this amplifier is a push-pull
device, with the two sides reacting in opposite directions
from each other. Therefore, when the action described dur­
ing a negative input signal is occurring at the upper half,
the action described during a positive input signal is oc­
curring at the lower half. The peak detection which occurs
in both halves (through Q151 and Q153) has a common
® 2
3-7