SECTION 3
CIRCUIT DESCRIPTION
AMPLIFIERS
Introduction
The M Unit consists of four identical input amplifiers, a
common output amplifier, and a signal-out amplifier for
channel A. Since the input amplifiers are identical, the
following description applies to all. Throughout the circuit-
description discussion, you should refer to the block and
circuit diagrams located near the back of this manual.
Input Coupling
The signal to be displayed is applied to the input cathode
follower V5323 through one section of the MODE switch
(SW5300, IF) and the VOLTS/CM switch (SW5310). In the
DC positions of the MODE switch, input coupling capacitor
C5301 is bypassed with a direct connection. In the AC posi
tions the signal must pass through C5301 so the dc com
ponent of the signal is blocked. In the OFF position the
signal is disconnected.
Input Attenuation
The M Unit requires an input signal of 0.02 volt, peak-
to-peak, to produce one centimeter of calibrated deflection
on the crt. In order to satisfy this condition, and to make
the instrument applicable to a wide range of input voltages,
precision attenuation networks can be switched into the
input circuitry by means of the VOLTS/CM switch SW5310.
The voltage-attenuation ratios of these networks are X2'/2,
X5, X I0 and X I00.
When the VOLTS/CM switch is in the .02 position, the
signal is coupled without attenuation to the Input Cathode
Follower, V5323. For the other settings of the VOLTS/CM
switch, the attenuation networks are switched into the cir
cuit, either singly or in tandem pairs, so that the input
voltage to V5323 is always 0.02 volt for each centimeter of
crt deflection when the VAR. GAIN control R5326 is set
to the CALIB. position.
The
attenuators
are
frequency-compensated
voltage
dividers.
For low-frequency signals they are resistive
dividers, and the degree of attenuation is proportional to
the ratio of the resistances. This is because the impedance
of the capacitors, at low frequencies, is high and their
effect in the circuit is negligible. As the frequency of the
input signal increases, however, the impedance of the
capacitors decreases and their effect in the circuit becomes
more pronounced.
For high-frequency signals the impedance of the capaci
tors is low, compared to the resistance of the circuit, and
the attenuators become capacitive voltage dividers.
For
these frequencies, the degree of attenuation is inversely
proportional to the ratio of the capacitances. A variable
capacitor in each attenuator (for example, C5308C in the
X2Y2 attenuator) provides a method for adjusting the capaci
tance ratios equal to the resistance ratios.
The variable capacitor at the input to each attenuator
(for example, C5308B in the X2’/ 2 attenuator) provides a
means for adjusting the input capacity of the attenuator to
a standard value of 47 picofarads. Similarly, C5317 pro
vides a method of standardizing the input capacity when
the VOLTS/CM switch is in the .02 position. In this manner,
the probe, connected to the input connector, works into the
same input capacity regardless of the setting of the VOLTS/
CM switch. In addition to providing the same input capac
ity, the resistance values in the attenuators are chosen to
provide the same input resistance (1 megohm) for each
setting of the VOLTS/CM switch.
Input Amplifier
The Input Amplifier consists of two stages: Input Cathode
Follower V5323 and the Paraphase Amplifier Q5324/Q5334.
Input Cathode Follower V5323.
This stage employs a
Nuvistor which is essentially a subminiature triode. Nuvistor
V5323 presents a high-impedance, low-capacitance load to
the input circuit and isolates the input circuit from the suc
ceeding stages. The cathode of V5323 is long-tailed through
R5323 to the —150-volt supply. With this configuration,
stable gain is obtained, and large input signals can be
handled without distortion.
C5318 and R5318 form a protection circuit in the grid
circuit of V5323. These components prevent excessive grid
current from V5323 in case a positive-going overload signal
is inadvertently applied to the input connector. Positive
going signals passing through V5323 are prevented from
damaging Q5324 by protective diode D5324. Negative
going signals cannot damage Q5324 because current flow
in the transistor is limited to about 3 ma. R5316 and R5319
in the grid circuit of V5323 are parasitic suppressors.
Paraphase Amplifier Q 5 3 2 4 /Q 5 3 3 4 .
This stage is a
transistorized, emitter-coupled amplifier.
In addition to
amplifying the signal, the stage converts the single-ended
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