Tektronix 323, Instruction Manual

Page: 41 / 170

Circuit Description— Type 323
The effect of signals on the Limiter in the upper half
of the amplifier is explained in this paragraph. (The re­
action of the lower half is identical, but is always of op­
posite polarity to that of the upper half.) A forward cur­
rent signal through D86 subtracts from the current through
D87 and the Q91 circuit. When all of the R87 current is
demanded by D86, D87 cuts off. A negative signal at the
anode of D86 causes an opposite reaction. In each case,
when D86 or D87 cuts off, larger signal changes are pre­
vented from reaching amplifier Q91. This prevents the Ver­
tical Output Amplifier from being overdriven.
Buffer Amplifier
The Buffer Amplifier consists of two identical operational
amplifiers with a gain of approximately one (from the emit­
ters of Q61 and Q71 to the collectors of Q91 and Q99).
These operational amplifiers provide low impedance drive
to the Vertical Output Amplifier. The Q91 amplifier cir­
cuit is explained here.
A simplified circuit, with approximate values of voltage
and current under quiescent conditions, is shown in Fig.
3-4. With a decrease in current through D87 and the Q91
base, the collector of Q91 goes negative. This increases
the voltage across R95 and causes most of the D87 current
change to be shunted through R95. The resultant negative
output signal is taken from the collector and routed to the
Vertical Output Amplifier.
Fig. 3-4. B uffer A m p lifie r, sim plifie d.
It is possible for the vertical amplifier to be balanced
and the trace to be at graticule vertical center through a
wide range of equal voltages at the upper and lower de­
flection plates. The DEFLECTION PLATE DC LEVEL adjust­
ments (R91 and its counterpart in the lower half of the am­
plifier) permit individual adjustment and balancing of the
deflection plate voltages, independent of the inputs from
the limiter circuit. This enables the CRT vertical deflection
plate voltages to be centered within their dynamic operat­
ing range.
VERTICAL OUTPUT AMPLIFIER <2>
General
A basic knowledge of operational amplifier theory is
helpful in understanding the Vertical Output Amplifier and
other circuits in the Type 323 Oscilloscope. The following
explanation is therefore provided. Refer to Fig. 3-5 and
assume that:
Points B, C and D are initially at OV.
Any voltage at C will produce an amplified, inverted
output at D.
Amplifier "A " has a gain of 1000 between points C
and D, which is referred to as "open loop" gain.
When a - f 1 V signal is applied to B, point C attempts
to go toward +1 V. When C arrives at -fO.l V, the gain
of Amplifier “ A " causes —100 V to appear at point D.
Current through R
f
(100.1 V -r- 100.1 kQ — 1 mA) becomes
equal to current through Ri (0.9 V ■+■ 900 Q = 1 mA). Any
further change occurring at C will cause a change at D,
driving C back to 0.1 V as long at +1 V exists at point B.
Fig. 3 -5 . O p e ra tio n a l A m p lifie r, sim plifie d.
The overall gain of the circuit (closed loop gain) there­
fore is equal to the 100 V output divided by the 1 V input,
or 100, despite the fact that the gain of amplifier "A " is
1000.
The closed loop gain of operational amplifiers is ap­
proximately equal to the ratio between feedback resistor
R
f
and input resistor
R|.
In the foregoing example the divi­
sion results in 111. The closed loop gain of 100 is within
10% of 111. It may be noted that the accuracy of this ap­
proximation decreases as the value of the closed loop gain
approaches the value of the open loop gain.
Block Diagram Description
Refer to the Vertical Output Amplifier block diagram.
The Vertical Output Amplifier consists of a pair of multi­
stage operational amplifiers quiescently supplying approxi­
mately 50 V DC to each of the vertical deflection plates
of the cathode-ray tube. Signals at the VERT INPUT con-
3-6