Tektronix 323, Instruction Manual

Page: 38 / 170

Circuit Description— Type 3 2 3
(1 ) A tte n u a to r o u tput capacitance and P ream plifier circuit in p u t capacitance.
(2 ) In com bination w ith ( 1 ) , provides the AC volta ge division indicated by the selected attenua tion factor.
(3 ) In p a ra lle l w ith the (1 ) and (2 ) series com bination, provides 4 7 pF in p u t capacitance.
(4 ) 1 MJ2 in p u t resistance.
(5 ) In com bination w ith ( 4 ) , provides a 1 M£2 in p u t resistance. A lso provides volta ge division as determ ined by the VOLTS/DIV
switch position.
R27 lim its current caused by ove r-d rivin g signals in .01 mV position.
Fig. 3 -1 . S im plified in p u t circuit co n figura tion fo r .01, .02 or .05 VOLTS/DIV switch positions.
+4.4V. C5 is being charged through R9 by the 9.4 V dif­
ference existing between the —5 V power supply and the
Q1 emitter voltage. Initially, the current through R9 is suf­
ficient to keep the Q9 emitter more positive than —0.6V,
preventing Q9 from conducting. When C5 charges to ap­
proximately 5 volts, the current through R9 is decreased
sufficiently to lower the Q9 emitter voltage to approximately
—0.6 V and Q9 goes into saturation. The Q9 collector and
the base of Q1 decrease to about —0.6 V, causing Q1 to
cut off. C5 discharges through R4 until the Q1 emitter reaches
about —1.2 V and Q1 again conducts. C5 stops discharging
and Q9 cuts off. The voltage at the collector of Q9 goes
positive, causing Q1 base and emitter voltage to follow. C5
again charges through R9, and the cycle repeats itself at
an approximate 750 Hz rate.
Refer to the Vertical Preamplifier schematic. The multi­
vibrator square-wave output is taken from the collector of
Q9 and applied to the D11-D12 switching circuit. When Q9
is cut off, D11 is back biased by the positive potential at
the Q9 collector. Current flows through R12, D12 and R15
to provide 0.5 V, .05 V and .005 V at the top of R17, R18
and R19 respectively. When Q9 conducts. D ll also goes
into conduction and the voltage at the bottom of R12 drops
below +0.6. D12 stops conducting and the output voltages
drop to 0.
R3, R6, C3 and C6 are decoupling components. R13 and
D13 counteract temperature effects on D12 to maintain
an accurate calibration signal over the Oscilloscope’s op­
erating temperature range.
Source Followers
Input signals are developed across R30 and applied
through C31 and R31 to the gate of Field Effect Transistor
(FET) Q33A.
No signal current flows through the gate of
Q33A, and therefore no signal loss occurs across R31.
The operation o f N-channel FETs such as Q33 is compara­
ble to that of a triode vacuum tube, with the source, gate
+ 5 V
Fig. 3 -2 . C a lib ra to r m u ltivib ra to r, s im p lifie d .
and drain comparing to the cathode, control grid and plate
respectively. In typical cathode-follower fashion, most of
the signal at the gate of Q33A is developed across R36
and applied to the base of Q41A. R34 and R39 permit ad­
justing for offset differences between Q33A and B, and be­
tween Q41A and B.
Q33A and Q33B are electrically and thermally paired,
and therefore provide identical input conditions for both
halves
of the amplifier. This provides high common-mode
rejection characteristics for the two halves which results
3 -3