HP (Hewlett-Packard) 6205B ユーザーズマニュアル

series regulator so that the output current is limited
to the proper value.

4-6 The reference circuit provides stable refer-

ence voltages used in the constant voltage compar-
ator and current limit circuits. The bias circuit
provides the less critical bias voltages used in the

supply.

4-7 The meter circuit provides a continuous indi-
cation of output voltage or current in both ranges.

4-9 FEEDBACK LOOP

4-10 The feedback loop functions continuously to

keep the output voltage constant during normal op-

eration of the supply. For purposes of this discus-
sion, assume that the output voltage instantane-

ously rises (goes positive) due to a variation in the

external load circuit. Note that the change may be
in the form of a slow rise in the output voltage or a
positive going ac signal. An ac signal is coupled

to summing point A6 through capacitor Cl and a dc

voltage is coupled to A6 through R 10.

4-11 The rise in output voltage causes the voltage

at A6 and thus the base of Q1A to decrease (go neg-
ative). Q1A now decreases its conduction and its

collector voltage rises. The positive going error
voltage is amplified and inverted by Q3 and fed to

the base of the series transistor(s) via emitter fol-
lower Q4. The negative going input causes the

series transistor(s) to decrease its conduction so

that it drops more of the line voltage, reducing the

output voltage to its original level.

4-12 If the external load resistance decreases to

a certain crossover point, the supply will operate
in the current limiting mode. In the current limit
mode, Q1O conducts sending a negative going,
turn-down signal to the series regulator via driver
Q4 .

4-14 The series regulator consists of transistor
stage Q7 (and Q6 on Model 6206 B). The regulator
serves as a series control element by altering its
conduction so that the output voltage is kept con-
stant and the current limit is never exceeded, The
conduction of the transistor(s) is controlled by the
feedback voltage obtained from driver Q4. Diode
CR11, connected across the regulator circuit, pro-
tects the series transistor(s) against reverse volt-
ages that could develop across it during parallel or
auto-parallel operation if one supply is turned on
before the other.

4-16 The circuit consists of the coarse and fine
programming resistors (Rl0A and R 10 B), and a dif-

ferential amplifier stage (Ql and associated com-

ponents).

Transistor Q1 consists of two transistors

housed in a single package. The transistors have
matched characteristics minimizing differential
voltages due to mismatched stages.

Moreover,

drift due to thermal differentials is minimized,

since both transistors operate at essentially the
same temperature.

4-17 The constant voltage comparator continuous-
ly compares a fixed reference voltage with a por-
tion of the output voltage and, if a difference ex-
ists, produces an error voltage whose amplitude

and phase is proportional to the difference.

The

error output is fed back to the series regulator,
through the (mixer) error and driver amplifiers. The
error voltage changes the conduction of the series
regulator which, in turn, alters the output voltage

so that the difference between the two input volt-

ages applied to the differential amplifier is reduced
to zero.

The above action maintains the output

voltage constant.

4-18 Stage Q1B of the differential amplifier is

connected to a common (+S) potential through im-
pedance equalizing resistor R5. Resistors R6 and
R8 are used to zero bias the input stage, offset-
ting minor base-to-emitter voltage differences in
Q1. The base of Q1A is connected to a summing
point at the junction of the programming resistors
and the current pullout resistors, R12 and R13.
Instantaneous changes in output voltage result in
an increase or decrease in the summing point po-
tential. Q1A is made to conduct more or less, in
accordance with summing point voltage change.
The resultant output error voltage is fed back to
the series regulator via the remaining components
of the feedback loop.

Resistor Rl, in series with

the base Q1A, limits the current through the pro-
gramming resistors during rapid voltage turn-down.
Diodes CR1 and CR2 form a limiting network which
prevent excessive voltage excursions from over
driving stage Q1A.

Capacitor Cl, shunting the

programming resistors, increases the high frequen-
cy gain of the input amplifier. Resistor R1 3, shunt-
ing pullout resistor R12, is factory selected so
that all of the + 6.2 Volt reference is dropped across
R12 and R13. Linear constant voltage programming
is assured with a constant current flowing through
R1O. C20 stabilizes the feedback loop and may be
removed to avoid current surges and increase the
programming speed.

4-19 ERROR AMPLIFIER AND DRIVER

4-20 The error and driver amplifiers amplify the

error signal from the constant voltage comparator
circuit to a leve1 sufficient to drive the series
regulator transistor(s).

Driver Q4 also receives a

current limiting input if Q10, the current limiting
transistor, conducts.

4-2