HP 740B Operating And Service Manual page 29

Section IV
4-34. MAIN LOOP VOLTAGE FEEDBACK AMPLIFIER
(Figure 7 -3).
4-35. The input circuits feed a dc voltage to the Main
Loop. The dc voltage is the unknown dc input in Volt-
meter and Differential Voltmeter modes, or the Re-
ference Loop output in Standard Mode. The purpose
of the Main Loop is to furnish extremely high input re-
sistance, precise voltage gain and high current gain.
Extremely high input resistance allows high impedance
voltage sources to be measured without loading errors.
It also allows the Reference Loop to be well isolated
from a load connected to the Output terminals in Stan-
dard mode. Precise voltage gain is necessary for
accurate measurements in Voltmeter and Differential
Voltmeter modes and for accurate outputs in Standard
mode. High current gain allows the Main Loop output
to drive a load in Standard mode without drawing
significant current from the Reference Loop.
It
also
allows the Model 740B to beused as a dc power amp-
lifier in Voltmeter and Differential Voltmeter modes.
Significant loads can be driven by the output without
loading the signal source applied to the input.
4-36. The Main Loop consists of a Low Voltage
Section and a High Voltage Section. The two sections
operate together as a voltage feedback am plifier. De-
generative feedback voltage from the output of the
High Voltage Section is fed back to the input of the
Low Voltage Section and determines the closed loop
(effective) gain. The feedback controls the closed
loop gain in the following manner: Assume 1 Vdc is
applied as an input to the Low Voltage Section of the
Main Loop (Voltmeter mode, 1 V range). The open
loop (actual) gain of the Main Loop is 105 (Table 4-1).
The voltage at the Main Loop output starts at zero
and increases in a positive direction. On the 1 V range,
the feedback factor,
{3,
is 1 (unity) which means all of
the Main Loop output is fed back to the input. After a
very short time, the value of the feedback voltage
approaches the value of the input voltage (1 V). The
feedback voltage is compared to the input voltage in
such a manner as to subtract from it and the Main Loop
amplifies only the error difference between the two
voltages. Since the feedback voltage is rapidly ap-
proaching the input voltage, the error voltage rapidly
approaches zero. When the error voltage reaches ap-
proximately 10 /.lV, the Main Loop output stabilizes at
O. 999990 V. 10
/.l
V represents the smallest error volt-
age that will produce a feedback voltage nearly
e~al
to
the 1 V input voltage when amplified by the 10 open
loop gain.
4-37. The 10 /.l V error causes outputs to be slightly
low in Standard mode and indications to be slightly
high in Differential Voltmeter mode. Generally,
greatest instrument accuracy is desired in Standard
mode. The effects of the 10
/.lV
error in Standard
mode are compensated by adjusting the 1 V Reference
Supply until the Main Loop produces an accurate 1 V
at the Output terminals (Paragraph 5-62 steps m
through p). This procedure provides best instrument
accuracy in Standard mode with a slight loss of ac-
curacy in Differential Voltmeter mode. If optimum
accuracy is desired in Differential Voltmeter mode
(with some loss of accuracy in Standard mode), the
4-4
Model 740B
Reference Supply calibration can be performed in Dif-
erential Voltmeter mode. Paragraph 5-62, step p
describes this procedure. The effects of the small
gain error are not great enough to affect instrument
performance in Voltmeter mode.
4-38. LOW VOLTAGE SECTION.
4-39. The Low Voltage Section consists of the chopper
stabilized Low Level Amplifier (A3) and the Operational
Filter (part of A4).
4-40. Low Level Amplifier. A3 (Figure 7-3 and 7-7).
In the Voltmeter and Differential Voltmeter modes of
operation, the unknown dc input is applied throu~h the
input circuits to the Modulator, A17. In Standard mode,
the Reference Loop output (0 to + 1 V) is applied
throu~h
a section of the Input Filter to the Modulator.
4-41. The Modulator consists of two photocells, A17Vl
and A17V2, and two neon lamps, A17DS1 and AI7DS2.
The photocells are in a low resistance state when
lighted by the neons, and a high resistance state when
not lighted. The neons are driven by pulses from the
Neon Driver,
A8.
The neons light alternately at a
rate of 162 Hz. A17Vl is in series with the volta~e
from the Input Filter. A17V2 is in series with the
Main Loop feedback. The resultant output of the
Modulator is a 162 Hz ac error signal proportional
in peak-to-peak amplitude to the difference between
input voltage and feedback voltage. The ac error
signal rides on a dc level equal to the input voltage.
4-42. The 162 Hz error signal isfed through the Range
and Function Switch to the Low Level Amplifier, A3.
An ac attenuator on the Range Switch, consisting of
SICl, SIR1, S1C2, SIR2, S1R3 and S1R4, reduces
the amplitude of the signal by a factor of ten on each
increasing range above 1 V in the Voltmeter and Dif-
ferential Voltmeter modes. In Standard mode, the ac
error signal bypasses the attenuator and goes directly
to A3 pin 1.
4-43. A3Cl removes the dc component from the ac
error signal. CRI limits large negative spikes that
occur when the selected range is exceeded or when the
input voltage changes faster than the Main Loop can
respond. A3Ql through A3Q3 comprise a cascade-
coupled, three-stage amplifier. Degenerative ac
feedback from the emitter of A3Q3 to the emitter of
A3Ql ensures gain stability. DC feedback from the
collector of A3Q3 to the base of A3Q1 controls the
bias on Ql and prOVides some gain correction for
varying input voltages. The signal from the collector
of A3R9 * (typically 22 kQ) is factory selected to set
the gain of the Low Level Amplifier to properly match
the gain characteristics of the other Main Loop com-
ponents. Paragraph 5-91 explains the A3R9* selection
procedure. A3Q3 is capacitively coupled to A3Q4.
A3Q4 and A3Q5 comprise a variable gain differential
amplifier. The output of the amplifier, at the col-
lector of A3Q4 is proportional to the difference in
signal levels between the base of A3Q4 and the base
of A3Q5. The signal applied to the base of A3Q5 is
degenerative feedback from the output stage of the
Low Level Amplifier. The feedback is divided by the
A3R19/A3Rll divider before being applied to the dll-
01794-1