Model 8903B
Service
PRINCIPLES OF OPERATION---SERVICE
SHEET
5
~~ ~ ~ ~
SERVICE
SHEET
5---A4 Output Amplifier/Voltmeter Assembly (Output Amplifier)
PRINCIPLES
OF
OPERATION
General
This portion
of
the Output Amplifier/Voltmeter (A4) contains the output amplifier circuitry. The output from
the Notch Filter is amplified, attenuated, and filtered as necessary to obtain a proper output reading by the
Output RMS/Average Detector (see Service Sheet 6). The ac output
of
the amplifier
is
available at the rear-panel
MONITOR output connector.
Attenuators and Amplifiers
Attenuators
1
and
2
can each be set for an attenuation of
0
or
20 dB. They are switched together. The attenuator
control line,
OU
AMP1, switches in the
0
dB path
of
Attenuators 1 and 2 indirectly through U21D, which
acts as an isolating switch and a logic inverter. When the control input (F4) to U21D is high, U21D
is
OR
FI
of U21A and
F3
of U7C are pulled to ground via R11, and the respective switches are enabled. When
F4
of
U21D is low, U21D
is
on;
resistors R2, R5, and
R11
form a voltage divider which applies a logical high
to
Fl
of U21A and
F3
of U7C.
and
the respective switches are disabled.
Amplifier
1
has
a
fixed gain
of
5.
Amplifiers
3
and 4 each have a fixed gain
of
10. The Output Clamp in
Amplifiers
3
and
4
accesses the driver stage of the respective
IC
and prevents the output from exceeding
+I2
or
-12V. The output from Amplifier
1
drives the Low-Pass Filters and the Notch Filter tune and balance
circuitry (see Service Sheet
4).
Amplifier
4
drives the MONITOR output connector and is protected against the
application
of
reverse voltage by R53 (which acts as a fuse), CR8, and CR9. A12Ll and C1
form
an
RFI
filter.
Filters
The three Low-Pass Filters are 30, 80, and 750
kHz.
three-pole, active Butterworth filters. The passband gain
of each filter
is
2.
The active element
of
the filter, U8, has an Output Clamp similar to U6 and U13. The filters
are switched via U9.
The
two High-Pass Filters are respectively a
0.3
Hz,
single-pole. passive filter and a
13
Hz.
three-pole, active
filter. Switching is via
KI.
CR3 suppresses the inductive transient created
by
the
coil
of K1 when the current
is
interrupted.
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