Circuit Description—Type 422 AC-DC
Fig. 3-2. Channel 1 Input Amplifier detailed block diagram.
ting of the Channel 1 VOLTS/DIV switch. Each attenuator
contains an adjustable series capacitor to provide correct
attenuation at high frequencies and an adjustable shunt
capacitor to provide correct input capacitance.
In the CALI BRATE 4 DIVISIONS position of the Chan
nel 1 VOLTS/DIV switch, the INPUT 1 connector and
Input Attenuator are disconnected. Instead, a 0.2-volt
square-wave signal from the Calibrator and Regulators cir
cuit is connected to the Input Stage. The basic 0.05 volt per
division deflection factor of the vertical deflection system
provides a four-division display when the Channel 1 GAIN
control is set correctly. This position of the Channel 1
VOLTS/DIV switch allows the calibration of the Channel 1
Input Amplifier to be checked without additional equip
ment.
Input Stage
The Channel 1 signal from the Input Attenuator stage is
connected to the Input Stage through the network
Cl TC12-R10-R11-R12. Resistor R10 establishes the input
resistance of this stage and is a part of the attenuation
network at all Channel 1 VOLTS/DIV switch positions.
Variable capacitor C l2 adjusts the basic input time con
stant for a nominal value of one megohm X 33 picofarads.
FET (field-effect transistor) Q14A is connected as a source
follower to provide a high input impedance for the applied
signal with a low-impedance drive to the following stage.
Diodes D14 and D15 protect the input circuit by clamping
the gate of Q14A if the signal at the gate exceeds either
+12.5 volts or —12.5 volts.
FET Q14B is a constant current source for Q14A and
also provides temperature compensation for Q14A. STEP
ATT BAL adjustment R21 varies the gate level of Q14B to
provide a zero-volt level at the emitter of Q34 with no
signal applied. With a zero-volt level at the emitter of Q34,
the trace position will not change when switching between
the .01, .02 and .05 positions of the Channel 1 VOLTS/
Dl V switch. The signal at the source of Q14A is connected
to the Feedback Amplifier stage through C14-R14 and
emitter follower Q23. Q23 serves to isolate the Input Stage
and the Feedback Amplifier stage.
Feedback Amplifier
Feedback Amplifier* Q34 and Q44 changes the overall
gain of the Channel 1 Input Amplifier in the .01 and .02
positions of the Channel 1 VOLTS/DIV switch. Gain of this
stage is determined by the ratio of R39 and the resistance
(Re ) between the emitter of Q34 and ground (R30, R32 or
open circuit). These resistors alter the negative feedback
and thus change the gain of this stage. This can be repre
sented by the formula:
RE + R39
* Jacob Millman and Herbert Taub, “Pulse, Digital and Switching
Waveforms”, McGraw-Hill, New York, 1965, pp 15-18.
3-4
ia
To Next Page
Loading...
