HP 6177B Operating And Service Manual page 18

late d) is not controlled by the regulator circuit and
is used in less cit1cal circuitry of the unit.
4-2C CONSTANT CURRENT COMPARATOR
4-21 The constant current comparator is
6
differen-
tial amplifier that compares the drop across the
samplinQ resistor{s) to the guard potential and if a
difference exists, sends an error: signal proportion-
al to this difference to the series regulato;,- via the
mair'"; error amplifier. The error signal alters the
conduction of the regulator thus keeping the output
constant and at the desired programmed level.
4-22 The comparator consists
0:
a differential am-
plifier, 019. and a constan".:. current source. 021.
Q19 consists of two silicon transistors housed in a
single package. The transistors have matched char-
acteristics minimizing diiierential voltages due to
mismatched stages. Moreover. drift due to thennal
differentials is minimal since both transistors oper-
ate at essentially the same temperature. Potentiom-
eter, E56 allows further balancing of the stage
shoule: this be necessary. CR25 and CR26 prevent
excessive voltage excursiO!1s from over driving the
stage.
4-23 The constant current comparator a cts in con-
junction witb the guard supply to set and maintain
the output current at the desired level. It continu-
ously compares t."' r :te drop across the sampling resis-
tance, which is proportional to the output current,
to the potential of the guard supply, which is equal
to the programmed output current (as will be ex-
plained, subsequently). If a difference exists, the
comparator sends an error signal to the series regu-
lator, via the main error amplifier and driver, in
orde: to maintain the output current equal to the
curren: programmed by the guard supply.
4-24 The RANGE SWitch selects the proper value of
current sampling reSistance. These values of resis-
tance maintain equal voltage drops for proportional
output current in each range. For instance; in the
5181E, in the 25mA range, the sampling resistance
is 100./1,.. creating a voltage drop of 2.5 Volts across
the sampling reSistors at maximum output. In the
250m;; range, the sampling resistance is 10 . . . . . . . , again
creating a 2.5 Volt drop.
4-25 GUARD SUPPLY
L.;-26 The guard supply is a self-contained, constant
voltage supply, which compares a portion of the
guard potential to a fixdd reference voltage and en-
sures that their difference is zero.
4-27 The guard supply consists of a differential
amplifier. Q18. error amplifier and series regulator
22.
The differential amplifier consists of two sili-
con'transistors housed in a single package. The
4-3
transistors have matched characteristics, minimiz-
ing differential voltages due to mismatched stages.
Moreover, drift due to thermal differentials is mini-
mal since both transistors operate
a:
essentially the
same temperature. Q22, acting as
a
constant cur-
rent source for the differential amplifier, is used to
bia
s
the emitters of both transistors. Potentiometer,
R46. allows further balancing of the differential
amplifier should this be necessary. GR1S and CR16
form a limiting network whicb prevents excessive
voltage excursions irom over driving the stage.
4-28 The output of the guare supply is fed to a
copper guard which surrounds the positive output
te:-minal. As previously mentioned, this prevents
leakage currents across the outpt.:.t terminals of the
supply.
4-29 Stage Ql8B is connected to the common point
0:
the unit. The base of Ql8A is connected to a
summing pOint (Al) at the junction of the program-
ming resistor. R40, and the curren": pullout resistor,
R3 7.
Instantaneous changes in the guard potential
result in an increase of decrease i:'. the summing
point potential and thus, unbalances the differential
amplifier. An error signal is then produced and fed
to the series regulator of the guard supply causing
it to conduct more or less in accordance with the
5umr:1ing point change. This restores the potential of
the guard which balances the differential amplifier.
4-30 R37, connected to the +5.9 reference voltage,
acts to provide a constant programming current
through potentiometer R40, which ensures linear
programming by R40. Changing the setting of the
potentiometer changes the potential at A 1. This
creates a momentary imbalance at the input of the
differential amplifier, which then sends an error
signal to the regulator to alter the guard voltage
and correct the difference. R36, connected in series
with R40, acts to make up any difference which
might be created by the tolerance of the potentiom-
eter, by providing en addi:ional current trliout;h R40.
This then ensures full range prog:e:nming by the
CDRRENT control.
4-31 H-F ERROR AMPLIFIER
4-32 The E-F amplifier improves :he high frequen-
cy gain of the unit. This then helps to improve the
transient response of the U:1it to both changing ioad
conditions and ac modulation (see Paragraph 3-31).
4-33 Ml'.IN ERROR AMPLIFIER AND DRIVER
4-34 The main error amplifier (Zl) receives the
feedback Signal. amplifies it and feeds it to the
driver stage. Q26. 027, and 028 brm. the driving
circuit for the series regulator. This circuit further
amplifies the feedback signal. inverts it and the
resultant signal is used to bias the emitter of the
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