Fluke 5725A - Manual (Page 94)

5725A
Instruction Manual
4-24
4-40. High Voltage Amplifier (A3)
The High Voltage Amplifier assembly, operating with the step-up signal transformers,
constitutes an amplifier with an inverting gain of 100. It amplifies 2.2V to 11V signals
from the 5700A Oscillator and returns 220V to 1100V to the 5700A where they go to the
5700A OUTPUT HI/OUTPUT LO binding posts. The signals to and from the 5700A
come through the High Voltage Sense assembly (A6) where they are switched.
The High Voltage Amplifier assembly consists of an input amplifier, integrator, window
comparator, input clamp, transconductance and cascode stage, midstage, midstage -400V
filter, high voltage heat sink assemblies, autobias current source, autobias sense circuit,
high voltage amplifier feedback, and temperature monitoring circuits.
The High Voltage Amplifier drives the primary winding of the appropriate signal
transformer depending on the frequency of operation. Signal transformers are interfaced
to the High Voltage Amplifier via connector J604. Relays that select signal transformers
are mentioned under the heading "Signal Transformers" further on.
The High Voltage Amplifier assembly uses the
±
400V supplies from the Power Supply
assembly. Lethal voltage appears on the heat sink assemblies. Do not open the 5725A
cover without referring to the access procedures and warnings in Section 6.
Figure 4-4 is a block diagram of the High Voltage Amplifier assembly. Refer to the
block diagram or the schematic to better understand theory of operation.
4-41. Input Amplifier
The input amplifier, U602, is an op amp whose input comes from HV IN on J653
through R623 and R621. HV IN is the 2.2V to 11V ac signal generated by the 5700A and
routed through the High Voltage Sense assembly. The input amplifier has a high-
frequency gain of 5.5, but because feedback resistor R622 is ac-coupled through C615,
the gain is much higher at low frequencies. The inverting input of U602 is buffered with
Q601. The nominal output of the input amplifier, LVAMP TP602, is about 6V dc.