HP 740B Operating And Service Manual page 30

Model 740B
ferential am plifier. A3R20 is switched in parallel
with A3R19 on the 1 V range and above to increase the
amount of degenerative feedback and reduce the Low
Level Amplifier gain on these ranges. The outputof
the differential amplifier is applied to the base of A3Q6,
amplified by A3Q6 and applied to A3Q7 and A3Q8.
A3Q7 and A3Q8 comprise a class AB push-pull amp-
lifier which provides additional gain to the error sig-
nal. A3R13, A3CR2, A3CR3 and A3Rl7 keep A3Q7
and A3Q8 slightly forward biased under no-signal
conditions, eliminating crossover distortion and pro-
viding temperature compensation for the circuit. The
output of the push-pull amplifier can be monitored at
A3TP10.
4-44. The amplified error signal from the push-pull
output circuit is monitored by the Overload Detector
circuit. A3C6 couples the ac signal to A3CR4 and
A3CR5 which form a peak detector. If the ac signal
is large enough to turn on A3CR4 and A3CR5, negative
pulses appear at the anode of A3CR4. A3C7 filters
the pulses and the resulting dc is applied through A3R21
to A3Q9, causing A3Q9 to conduct. When A3Q9 con-
ducts, A3Q10is forward biased andbegins to conduct,
turning on DS1, the front-panel OVERLOAD light.
When the error signal is small, A3CR4 and A3CR5
turn off, and A3C 7 discharges through A3R22, removing
the forward bias from A3Q9. A3Q10 then turns off
and the OVERLOAD light goes out.
4-45. The output of the Low Level Amplifier is coupled
through C 5 to the Demodulator, A17. The Demodu-
lator consists of two photocells, A17V3 and A17V4.
The photocells are lighted alternately by A17DS1 and
A17DS2, the same neons that control the modulator
photocells (A17V1 and A17V2). Demodulation takes
place and the resultant output of the demodulator is a
dc level proportional in magnitude to the ac error
signal amplitude. Magnitude of the dc level is ap-
proximately 0 to -10 mV on all ranges and functions.
Voltage g~n of the Low Level Amplifier is approxi-
mately 10 on the 1 V to 1000 V ranges; 10 4 onthemV
ranges.
4-46. The Demodulator output is fed through the Gain
Check switch, S10, to the Operational Filter, part of
A4. The Gain Check switch, when depressed, intro-
duces a dc offset into the Main Loop and provides a
way of checking Low Level Amplifier gain (Para-
graph 5-76).
4-47. Operational Filter,p/o A4 (Figure 7-8). The
Operational Filter is basically a frequency selective,
voltage feedback amplifier. Capacitively coupledde-
generativefeedbackfrom the amplifier's output stage
to the input stage greatly reduces the amplifier's ac
gain. DC voltage gain is approximately 100. The net
effect is to greatly increase the signal-to-noise ratio
without the use of large filter capacitors or inductors
which would reduce the response time of the Main Loop
when an input is applied.
4-48. The 0 to -10 mV input to the Operational Filter
is proportional to the Main Loop input voltage and re-
presents the "error" between the Main Loop input and
feedback voltages. The dc input is applied through A4R1
and A4R2 to the base of A4Q1. The quiescent voltage
01794-1
Section IV
at the base of A4Q1 is controlled by A4Q3, A4R5 and
A4R6. A4R6 (BAL ADJ) is adjusted for 0 Vat the base
of A4Q1 with no input signal applied. Generally, A4R6
does not need to be adjusted unless A4Q1 or A4Q2 is
replaced. A4TPll monitors the voltage at the base of
A4Q1.
4 -49. The input stage consists of a differential pair,
A4Q1 and A4Q2. A4Q2 establishes the zero reference
for the circuit and provides temperature compensation
for A4Q1. A4R13 (INT ZERO) sets the operating
point of A4Q2. A4CR2 and A4CR3 regulate the + and
- supply voltages for the Operational Filter to + 6.8 V
and - 6. 8 V respectively. The output of A4Q1 is further
amplified by A4Q4 and A4Q5.
4-50. The output of A4Q5, at the junction of A4R17
andA4R18, is fed back degeneratively through 2 paths
to the base of A4Q1. DC feedback is divided by the
A4R19/A4R20 divider and fed back through A4R21 to
A4Q1, setting the dc gain of the Operational Filter at
about 100. Degenerative ac feedback is fed directly
throughA4C4to the base ofA4Q1. A4C4 is avery low
impedance feedback path for ac and the ac gain of the
Operational Filter at most frequencies approaches
zero. On instruments above Serial No. 610-00376,
A4C4 is discharged when switching modes or when
switching between the 10 V, 100 V and 1000 V ranges.
The discharge path is from A4 pin 8 to A4 pin 22
through SlR20. The path is completed by momentary
shorting contacts on the Range and Function Switch,
S1. The contacts briefly complete the discharge path
when the Range or FWlCtion switch is between posi-
tions.
4-51. The dc output of A4Q5 is further filtered by two
39 J.lF electrolytic capacitors, A4C5and A4C6. A4C5
and A4C6 are connected back-to-back which results
ina19.
5J.lF,
non-polarized, low leakage filter. Shunt
limiter A4CR4 through A4CR9, limits the Operational
Filter output to values between - 1. 8 V and + 1. 8 V.
The Operational Filter output (0 to + 1 Vdc) is applied
to the Differential Amplifier (Paragraph 5-54).
4-52. HIGH VOLTAGE SECTION.
4-53. The High Voltage Section is a voltage feedback
amplifier within the Main Loop. In Standard mode,
the High Voltage Section generates the desired output
voltage and current. In Voltmeter and Differential
Voltmeter modes, 1 V range and above, the High
Voltage Section duplicates the unknown dc input volt-
age. On the' ranges below 1 V, the High Voltage Sec-
tion generates 0 to 1 V proportional to the unknown
dc input voltage. Closed loop gain of the High Voltage
Section is controlled by voltage feedback from the
output stage to the DifferentialAmplifier input stage.
The High Voltage Section has a closed loop gain of 1
(unity) on the 1 V range and below; 10 on the 10 V,
10 2 on the 100 V Range; 10 3 on the 1000
V Range.
4-54. Differential Amplifier p/o A4 (Figure 7 -8). The
Differential Amplifier is the input stage of the High
Voltage Section of the Main Loop. The differential
Amplifier compares the output of the Operational Fil-
ter to feedback voltage from the output stage of the
High Voltage Section. It then furnishes a voltage to
the Pulse Width Converter (part of A5) which is pro-
portional to the difference between the two voltages.
4-5