Fluke 5725A, Instruction Manual, Page: 79 / 196

Theory of Operation
Detailed Circuit Description
4
4-21
•
Error Amplifier Section
The error amplifier section contains three differential amplifiers and a precision
high-current shunt resistor. Two differential amplifiers set the gain and uncorrected
frequency response of the current amplifier. The remaining differential amplifier is
used during the internal portion of 5700A/5725A calibration and during ac current
operation.
•
Output Section
The output section contains several power transistors, a heat sink, two operational
amplifiers, a current buffer amplifier, two current sensing resistors and related
components. The output stage is driven by the error amplifier section, converting a
±
3V signal into a
±
15A signal. The output section is powered by floating supplies
that are referenced to the ICOM current common. The output stage is designed to
drive inductive loads.
•
Monitor Section
The monitor section contains several op amps and comparators, and a thermistor. A
temperature monitor circuit observes the temperature of the voltage output
transistors. Excessive output compliance voltage is detected by the drive monitor
circuit. Both monitor circuits send scaled voltages to a multiplexed comparator on
the High Voltage Sense assembly (A6), where abnormal operating conditions are
detected.
•
Control Section
The control section contains nine relays, a latched relay driver, several transistors,
and a quad comparator. The relays and transistors switch the Current Amplifier into
ac current and dc current operating states, a standby state, and several self calibration
and diagnostics states during which 5725A operation is monitored and characterized
by measurement circuitry and software residing in the 5700A.
•
Power Supply Section
The power supply section contains two bipolar power supplies. Regulated
±
20V
supplies and unregulated
±
12V supplies are referenced to ICOM and power only the
output section. The error amplifier section and control section are powered from the
±
15V and +5V 5725A system supplies.
Error Amplifier Section 4-35.
Refer to page 1 of the Current Amplifier schematic. Components U201, Z201, R235,
C230, and Q211 make up the "outer loop error amplifier," a differential amplifier with
high common-mode rejection ratio. The outer loop error amplifier controls a first-order
feedback loop that determines the dc accuracy and uncorrected ac frequency response of
the current amplifier. Q211 shuts off the outer loop error amplifier during standby and,
through modulation of its channel resistance by R255 and C210, yields a soft-start
transition into the operate modes.
Components U202, R232, R241, R251, R252, R234, C213, and Q213 comprise the inner
loop error amplifier. The inner loop error amplifier works with the outer loop amplifier
in a second-order feedback loop that significantly diminishes the effects of output stage
non-linearities. Q213 reduces the inner loop gain during standby.
Refer to page 2 of the Current Amplifier schematic. Resistor R272 is a specially
constructed 0.02
Ω
shunt resistor that senses the Current Amplifier output. U208 and
Z203 comprise a gain of ten differential amplifier called the calibration amplifier (Cal
Amp). The calibration amplifier scales the voltage dropped across R272, creating a signal