AR 10046249, Operating And Service Manual, Page: 42 / 58

Model 150/150AW1000
22 Rev C
3.2.2 A2 150W 100MHz Module Assembly (Schematic 10039874)
The 150W 100MHz Module Assembly consists of RF matching circuits, an RF transistor (Q1), a bias control
circuit, and a fault detection circuit.
The RF input is fed to a 4:1 balun transformer composed of T1 and T2. The output signal of the 4:1 transformer
is connected to the gates of Q1. The drains of Q1 are connected to a 4:1 balun transformer composed of T3 and
T4. Q1 has approxi34 VDC applied to both drains at a total current of 8.5A. The RF stage has
approximately 21 dB of gain and an output compression point of 110W or greater.
The current through Q1 is monitored by U1. The output of U1 is fed to an op amp (U3) which has a reference
voltage on the non-inverting input. U3 compares the output of U1 to the reference voltage and generates an
error signal. This error signal varies the gate voltages of Q1 which in turn controls Q1’s current draw.
U5 is a comparator that monitors the output of U1 and generates a fault signal if the drain current varies outside
normal operating conditions. It also monitors the module temperature via a thermistor (R31) and generates a
fault signal if the module heatsink exceeds approximately 80
o
C.
3.3 RF AMPLIFIER OPERATION- 80-1000 MHZ (SCHEMATIC 10046147)
The RF amplifier assembly consists of a Pre-Amplifier (Pre-Amp), a Driver Amplifier (Driver Amp), and a 20-
watt amplifier.
3.3.1 A1 Variable Gain Amplifier (Schematic 10033908, 10031972,
10032111)
The A1 variable gain amplifier consists of 3 subassemblies: the A1 Pre-Amplifier PWB Assembly, the A2 High
Isolation Switch Assembly, and the A3 Two-Watt Amplifier Assembly.
3.3.1.1 A1 Pre-Amplifier PWB Assembly (Schematic 10033908)
The Pre-Amplifier PWB Assembly consists of a stage of gain (U1), a variable attenuator (U2), a resistive splitter
(R3, R5, R4), another stage of gain (U3) an RF power detector (U9) and control circuitry. The overall gain of
the pre-amplifier assembly is approximately 17 dB at minimum attenuation. The power detector (U9) is used
to sense the input RF power and it increases the attenuation of U2 if an input overdrive condition is detected.
3.3.1.2 A2 High Isolation Switch (Schematic 10031969)
The High Isolation switch is normally in the thru mode. The switch can reduce the amplifier gain by more than
40 dB when the pulse input (E4) is pulled low.
3.3.1.3 A3 Two-Watt Amplifier
The Two-Watt Amplifier has a gain of approximately 16 to 18 dB. The Two-Watt Amplifier increases the
overall variable gain amplifier to approximately 31 to 32 dB of gain. The output RF power is approximately 30
dBm at the 1 dB compression point.