AR 10043414, Operating And Service Manual, Page: 35 / 44

Rev A 17
3. THEORY OF OPERATION
3.1 INTRODUCTION
The Model 25A250B RF amplifier consists of a 0.01-250 MHz RF amplifier assembly. The RF amplifier
assembly consists of one RF amplifier module.
The power supply section consists of an AC input filter, a circuit breaker, +5V, +15V, and -15V power
supplies; a +28V power supply, voltage regulator circuity, an Operate/Control circuit, and an interface board.
The control system consists of a Control/Fault Board, an Interface Board and remote interfaces for IEEE-488,
RS-232, USB, and Ethernet.
3.2 RF AMPLIFIER OPERATION
3.2.1 A1 RF Module (Schematic 10040606)
The 25W module consists of RF matching circuits, four stages of RF amplification, DC current control
circuitry, and a temperature and current fault detection circuit.
The RF input is fed to a variable attenuator circuit (Q1-Q4) used for amplifier gain control and input level
overdrive protection. The signal is then fed through 3 stages of amplification (Q6, Q7, Q8) with
approximately 33 dB of gain. The final push pull stage (Q9) is coupled through balun transformers on both
input and output. The push pull stage has a gain of approximately 17 dB. Each amplifier stage has a drain
voltage of 28V and the drain current for the entire module is approximately 4A. The module has an output
compression point of 20W or greater.
The current through Q9 is monitored by U8. The output of U8 is fed to an op amp (U10). U10 has a
reference voltage on the non-inverting input which represents the maximum allowable drain current. U10
reacts when the output of U8 is surpasses this reference voltage which reduces the gate voltage of Q9 in order
to limit the drain current. Op amp, U11, is used to set the quiescent bias of the transistor.
U9 and associated circuitry provides fault detection for both Q9 drain current and module temperature. The
output of U8 is monitored by U9 and it will signal a fault if the drain current of Q9 exceeds safe operating
limits. Similarly, a thermistor (RT1) senses the temperature of the heatsink and U9 will also signal a fault if
the module temperature exceeds safe operating limits.
3.3 POWER SUPPLIES (PS1, PS2)
Power supply PS1 supplies a +5VDC housekeeping supply for the control system assemblies A4
Control/Fault Board and A5 Interface Board.
PS1 also su15 VDC at 2.5 amps and -15 VDC at 1 amp. PS1 is a switching supply that automatically
sets the AC input circuits to the correct connections for the line voltage 90-264 VAC input ranges 47-440 Hz.
The –15 V at 1 amp power supply is fed to the A1 RF module to provide gate voltage for this module.