HP 740B Operating And Service Manual page 26

Model 740B
Section IV
SECTION IV
THEORY OF OPERATION
4-1. MODES OF OPERATION.
4 -2. The Model740B has three basic modes of oper-
ation: 1) Standard, 2) Differential Voltmeter, and
3) Voltmeter. Through the use of a function switching
technique, many of the same basic circuits are used
in each mode of operation. These circuits are: 1)
a Main Loop, 2) a Meter Loop, and 3) a Reference
Loop. The Standard and Differential Voltmeter modes
utilize all of these basic circuits. The Voltmeter male
utilizes all of them except the Reference Loop. The
following paragraphs discuss the theory for each mode
of operation.
the effective voltage gain of the amplifier and can be
calculated from the following formula:
A
G = - -
1
+ (3A
where:
G
=
closed loop gain
A
=
open loop gain
{3
=
portion (fraction) of the output voltage that
is used as degenerative feedback.
Table 4-1. Main Loop Gain Characteristics
4-9. The only purpose of the Meter Loop in Standard
mode is to indicate the value of the sixth digit of the
4-7. Mathematically, it can be shown that when A
becomes very large, the formula can be essentially
reduced to:
The output of the Main Loopis applied through an in-
ternal switch to the + and - OUTPUT terminals when-
ever the Output is turned on. The SENSE terminals
provide voltage feedback to the Main Loop Feedback
Divider.
(1 V range)
(10 V range)
(100 V range)
(1000 V range)
o
to 1 V in 1 11 V steps
o
to 10 V in 10 MV steps
o
to 100 V in 100 IlV steps
o
to 1000 V in 1 mV steps
Range A
{3
G
1V
10 5
1 1
10 V
10 6
0.1 10
100 V
10 7
0.01
10 2
1000 V 10
8
0.001
10 3
G=.!.
{3
Note that closed loop gain depends only on
{3
whenever
open loop gain (A) is very high. Closed loop gain
accuracy then is dependent almost entirely on the ac-
curacy of the voltage divider which determines
{3.
The
Main Loop Feedback Divider is made from stable,
precision wire-wound resistors enabling the Main
Loop to have stable, precise closed loop gain charac-
teristics.
4 -8. Closed loop gain increases by a factor of ten on
each increasing voltage range. The input to the Main
Loop from the Reference Loop is 0 to 1 V in 1 11 V
steps on all ranges. This results in available outputs
from the Main Loop of:
4-4. In Standard mode of operation, the Model 740B
Reference Loop generates a very stable and accurate
reference voltage. The value of the reference volt-
age (0 to 1 V) is controlled by the front panel VOLT-
AGE SET and STANDARD VERNIER switches. The
voltage is then amplified by the Main Loop and applied
through the output cable to the + and - OUTPUT ter-
minals.
4-5. The Reference Loop consists of a 1 V reference
supply, a binary coded decimal divider and a step
vernier control. The reference supply generates a
very stable + 1 Vdc which is applied to the binary coded
decimal divider. Divider ratios in the binary coded
decimal divider are controlled by the five front panel
VOLTAGE SET switches. The step vernier control,
operated by the front panel STANDARD VERNIER
switch, further divides the reference supply output.
The resultant output of the Reference Loop is a dc
level adjustable in11lV steps from 0 to +1 Vdc. The
output is applied to the Main Loop.
4-3. STANDARD (Figure 7-2).
4 -6. Basically, the Main Loop is a high gain dc amp-
lifier which uses degenerative voltage feedback to
obtain precise gain characteristics, high input re-
sistance and low output resistance. The Main Loop
consists of several internal amplifiers and a feedback
network connected from the Main Loop output to the
input. The amplifiers in the Main Loop are connected
in cascade and collectively have an open loop gain
(gain without feedback) of from'10 5 on the 1 V range
to 10 8 on the 1000 V range (Table 4-1). The feedback
network monitors the actual value of the output voltage
and feeds a controlled fraction of the output back to
the input. The feedback voltage is compared to the
input in such a way that it subtracts from the input
voltage. The amplifier then amplifies only the error
voltage: the difference between the input voltage and
the feedback voltage. Since the amplifier input volt-
age is greatly reduced by the degenerative feedback,
the effective (closed loop) gain is much smaller than
open loop gain -- providing a sizable portion of the
output is used as feedback. The closed loop gain is
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