Circuit Description—Type 324
>
To
INPUT
Coupling
Switch
11) Attenuator output capacitance and Preamplifier circuit input capacitance.
(2) In combination with ( l ) f provides the AC voltage division indicated by the selected attenuation factor.
13) In parallel with the (1 ) and (2 ) series combination, provides 47 pF input capacitance.
(4) 1
input resistance.
(5) In combination with ( 4 ) , provides a 1
input resistance. Also provides voltage division as determined by the VOLTS/DIV
switch position.
R27 limits current caused by over-driving signals in .01 mV position.
Fig. 3-1. Simplified input circuit configuration for .01, .02 or .05 V O L T S /D IV switch positions.
Fig. 3-1 shows a simplified input circuit configuration
for .01, .02, or .05 V O L T S /D IV switch settings. Brief
descriptions of component functions are included.
Calibrator. Refer back to the Vertical Preamplifier
schematic. Assume that Q1 is conducting and Q9 is cut off.
In th is condition Q1 is saturated and its emitter is at
approximately +4.4 V. C5 is being charged through R9 by
the 9.4 V difference existing between the —5 V power
supply and the Q1 emitter voltage. Initially, the current
through R9 is sufficient to keep the Q9 emitter more posi
tive than —0.6 V, preventing Q9 from conducting. When C5
charges to approximately 5 volts, the current through R9 is
decreased sufficiently to lower the Q9 emitter voltage to
approximately —0.6 V. Q9 goes into saturation. The Q9
collector and the base of Q1 fall 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 collec
tor 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 800 Hz rate.
The multivibrator square-wave output is taken from the
collector of Q9 and applied to the D11-D12 switching cir
cuit. When Q9 is cut off, D l l 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
tops of R17, R18 and R19 respectively. When Q9 conducts,
D11 also goes into conduction and the voltage at the
bottom of R12 drops below +0.6 V . 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 operating
temperature range.
Source Followers
Input signals are developed across R101 and applied
through C101 and R102 to the gate of Field Effect Tran
sistor (FET) Q11A. No signal current flows through the
gate of Q11A, and therefore no signal loss occurs across
R102. The operation of N-channel FET's such as Q11 is
comparable to that of a triode vacuum tube, with the
source, gate and drain comparing to the cathode, control
grid and plate respectively. In typical cathode-follower
fashion, most of the signal at the gate of Q 1 1A is developed
across R105 and applied to the base of Q 21A. R104 and
R107 permit adjustment for offset differences between
Q 1 1A and B, and between Q 21A and B.
Q11A and Q11B 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 in
cancellation of effects from + and — 5 V power supply
variations and FET thermal variations.
R103 and C103 are decoupling components. D14, D15,
D16 and D17 provide protection to Q 11A by limiting
Q 11A gate signals to approximately ± 1 .2 V . R102 limits
the overload current to a safe value during conduction of
the diodes. C101 permits high frequency components of
signals to bypass R102 to provide optimum transient
response.
3-3
