
Theory of Operation
Detailed Circuit Description
4
4-27
Transconductance and Cascode Stage
4-45.
The output voltage of the input amplifier U602, LVAMP, is transformed into a current
by Q602. The nominal current is about 8.5 mA. This implies that the nominal output
voltage of the input amplifier is about 6V; 15V - (.0085 * 1000) - .6 = 5.9V.
The current is passed through a cascode FET Q661, which can stand off the voltage
required, about 400V. A zener diode similar to VR656 protects most of the power
MOSFETs from damage to their gate oxide layer by excessive gate voltage
(approximately
±
20V). A gate resistor similar to R675 suppresses high frequency
oscillations.
Midstage
4-46.
The current from Q661 generates a voltage of about 8.5V across R698. The ac voltage
gain from TP602, the LVAMP output, to R698 is unity. Transistor Q666 mirrors the
current into the midstage with an ac gain of 5. The dc midstage current is about 28 mA.
The midstage current passes through two cascode FETs, Q662 and Q665, to bias resistor
R674. Transistor Q660, VR658, and R678 form a 21 mA current source that, along with
R674 establishes the midstage bias current at about 28 mA. FET Q653 is a current
source from the +400V supply controlled by the auto-bias circuitry. The bias for Q654
and Q662 comes from the high voltage heat sink assemblies and is maintained about
halfway between the output voltage and the +400V supply or -400V supply, respectively.
The bias for Q665 comes from zener VR665, FET Q667, and its associated parts, which
form a 3 mA current source to bias the zener.
Zener VR661 provides some additional protection for Q665 and Q666. Capacitor C660
and R612 provide the dominant pole to stabilize the midstage amplifier. Gain-bandwidth
is set to about 7 MHz.
Midstage -400V Filter
4-47.
FET Q670 is a filter to reduce the 120 Hz and output signal ripple on the -400V supply.
Zener VR663, acting as a normal diode, clamps the gate of Q670 at the positive peak of
the ripple on the -400V supply. As soon as the -400V supply moves away from the
positive peak, the gate voltage is heavily filtered by R695 and C663. The zener action of
VR663 and VR664 prevents Q670 from being damaged by excessive gate voltage as the
-400V supply is turned on.
High Voltage Heat Sink Assemblies
4-48.
The output power MOSFET devices are mounted in pairs on the four high voltage heat
sink assemblies. Each pair is driven with the same gate voltage. To ensure they equally
divide the current, the MOSFETS are matched for gate-source voltage at 0.5A and have
3.9
Ω
source resistors. The push-pull output stage consists of Q655 and Q656 on the N-
channel heat sink assembly #2 and Q663 and Q664 on the P-channel heat sink assembly
#2. Their gates are driven with the voltage on R674. Resistors R670 through R673 sense
the output current, while Q658 and Q659 limit the current to about 2.4A. Diodes CR652
and CR654 help to reduce the amount of stored charge in the power MOSFETs that must
be driven when they are turned on and off.
The power MOSFETs on the #1 heat sink assemblies are driven cascode to share the
voltage drop between the output (MVOUT) and their respective 400V supply. This also
balances their power dissipation with the #2 heat sink assemblies. Their gates are driven
from the voltage divider R654 and R661 or R684 and R692. Capacitors C654 and C661
Summary of Contents for 5725A
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Page 132: ...5725A Instruction Manual 8 6 aq24f tif Figure 8 1 Final Assembly...
Page 133: ...List of Replaceable Parts 8 8 7 aq25f tif Figure 8 1 Final Assembly cont...
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Page 139: ...List of Replaceable Parts 8 8 13 aq31f tif Figure 8 1 Final Assembly cont...
Page 141: ...List of Replaceable Parts 8 8 15 5725A 1611 aq36f eps Figure 8 2 A1 Interconnect PCA...
Page 145: ...List of Replaceable Parts 8 8 19 5725A 1650 aq37f eps Figure 8 3 A2 Current Amplifier PCA...
Page 149: ...List of Replaceable Parts 8 8 23 5725A 1661 aq38f eps Figure 8 4 A3 High Voltage Amplifier PCA...
Page 153: ...List of Replaceable Parts 8 8 27 5725A 1631 aq39f eps Figure 8 5 A4 Power Supply PCA...
Page 155: ...List of Replaceable Parts 8 8 29 5725A 1650 aq40f eps Figure 8 6 A5 Digital PCA...
Page 159: ...List of Replaceable Parts 8 8 33 5725A 1601 aq41f eps Figure 8 7 A6 High Voltage Sense PCA...
Page 161: ...List of Replaceable Parts 8 8 35 aq32f tif Figure 8 8 A12 Transformer Enclosure Assembly...
Page 162: ...5725A Instruction Manual 8 36 aq33f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
Page 163: ...List of Replaceable Parts 8 8 37 aq34f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
Page 164: ...5725A Instruction Manual 8 38 aq35f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
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Page 169: ...Schematic Diagrams 9 3 9 Figure 9 1 A1 Interconnect PCA 5725A 1611 aq48f eps...
Page 170: ...5725A Instruction Manual 9 4 Figure 9 1 A1 Interconnect PCA cont...
Page 171: ...Schematic Diagrams 9 5 9 Figure 9 1 A1 Interconnect PCA cont...
Page 172: ...5725A Instruction Manual 9 6 Figure 9 2 A2 Current Amplifier PCA 5725A 1650 aq52f eps...
Page 173: ...Schematic Diagrams 9 7 9 Figure 9 2 A2 Current Amplifier PCA cont...
Page 174: ...5725A Instruction Manual 9 8 Figure 9 2 A2 Current Amplifier PCA cont...
Page 175: ...Schematic Diagrams 9 9 9 Figure 9 2 A2 Current Amplifier PCA cont...
Page 176: ...5725A Instruction Manual 9 10 Figure 9 3 A3 High Voltage Amplifier PCA 5725A 1661 aq56f eps...
Page 177: ...Schematic Diagrams 9 11 9 Figure 9 3 A3 High Voltage Amplifier PCA cont...
Page 178: ...5725A Instruction Manual 9 12 Figure 9 3 A3 High Voltage Amplifier PCA cont...
Page 179: ...Schematic Diagrams 9 13 9 Figure 9 4 A4 Power Supply PCA 5725A 1631 aq59f eps...
Page 180: ...5725A Instruction Manual 9 14 Figure 9 4 A4 Power Supply PCA cont...
Page 181: ...Schematic Diagrams 9 15 9 Figure 9 4 A4 Power Supply PCA cont...
Page 183: ...Schematic Diagrams 9 17 9 Figure 9 5 A5 Digital PCA cont...
Page 185: ...Schematic Diagrams 9 19 9 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 186: ...5725A Instruction Manual 9 20 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 187: ...Schematic Diagrams 9 21 9 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 188: ...5725A Instruction Manual 9 22 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 189: ...Schematic Diagrams 9 9 23 aq45f eps Figure 9 7 A12 Transformer Enclosure Assembly...
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