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Theory of Operation
Analog Section Detailed Circuit Description
2
2-89
2-146. PA Sense Current Cancellation Circuitry
During dc operation of the Power Amplifier, sense current in the 500 kΩ feedback
resistor (on the resistor network in the HR8 assembly) can cause an output error because
of the finite resistance path of the connection to the load. Op amp U1 eliminates this
error by feeding an equal and opposite current in this path. The magnitude of this current
is determined by PA OUT HI, which is connected to the non-inverting input of U1. This
circuit generates a current through R8, which is equal to current flowing through the 500
kΩ feedback resistor. Sense-current cancellation is active only in dc 220V range.
2-147. PA in Standby
The Power Amplifier schematic shows all relays and DG211 FET switches in the
standby condition. The Power Amplifier 25 kΩ input resistor and R17 are tied to OS
COM through Q39 and R118. Power Amplifier output is close to zero and the whole
loop is stabilized.
To better understand Power Amplifier configuration in the ac/dc 220V range, refer to
Figure 2-22.
2-148. PA Operation: 220V DC Range
During dc operation, Power Amplifier gain is -20, as determined by the 500 kΩ/25 kΩ
resistor network on the HR8 assembly. Control line SW3, inverted by U8, turns on Q51.
This references the +input of the precision op amp in the input stage to R COM. The
DAC assembly is set to the negative 11V range and its outputs, DAC OUT HI and DAC
SENSE HI, are connected to pin 2 of the resistor network on the HR8 assembly by relay
K2. The sense current cancellation circuit is active during dc operation. Its output, SIG1,
is connected to the resistor network feedback resistor pin 1 by relay K15A. The amplifier
has a much lower bandwidth in this mode because of the much higher Miller capacitance
in C57. Lower bandwidth results in lower amplifier noise.
The output signal, PA OUT HI, is routed to the High Voltage Control assembly (A14),
where it goes through relay K10 and becomes PA OUT DC. PA OUT DC is routed to the
Switch Matrix for connection to the OUTPUT HI binding post. The sense signal, PA
SENSE DC from the Sense Current Cancellation circuit, is routed to the Switch Matrix
assembly (A8) for connection to the OUT/SENSE HI or SENSE HI binding posts, thus
making the binding post the sense point in internal sense and allowing for external sense
through the SENSE HI binding post.