6881076C25-E September 5, 2008
Theory of Operation: Power Amplifiers 3-77
3.7.3 800 MHz Band Power Amplifiers
3.7.3.1 15- and 35-Watt Amplifiers
3.7.3.1.1 Transmitter
The 15-Watt and 35-Watt ASTRO Spectra power amplifiers are discussed in the following text.
Transmit Buffer
The PA receives 18 to 23 dBm (60 to 200 mW) at the transmit injection (TX INJ) coax. The first
stage, TX BUFFER, uses adaptive biasing which varies the base voltage inversely proportional to
the input drive level. With Keyed 9.4 V (K9.4) ON and NO DRIVE, Q9800 base voltage should equal
the voltage drop across CR9800. R9801 sets the diode current, and R9802 sets the base voltage
referenced from CR9800. At the input, L9804, C9801, and C9800 are for matching while C9808 and
R9806 prevent interfacing instability. L9800 is the base feed choke and L9801 is the collector choke.
R9800 parallels L9800 for added stability. L9805, and C9803 are on the buffer's supply (K9.4) for
stability. C9802 and L9802 are for output matching. L9803 and C9804 are added as a "suckout" for
half carrier. Like the input, C9807 and R9805 were added at the output to help prevent interfacing
instability. The power output of this stage should be greater than 325 mW (25 dBm).
The TX Buffer applies the modulated RF signal to pin 1 of U9850, the Power Amplifier Module, which
is a 5-pin, 20-Watt, three-stage amplifier. The control voltage from the power control series-pass
transistor, Q9500, controls the gain of the first two amplifier stages of U9850, through pin 2 and pin
3. Battery voltage (A +), connected to pin 4, powers the third stage.
Power Module
The power module (U9850) is the major gain block for both the 15- and 35-Watt amplifiers. The
50-ohm input and output impedances connect to adjacent power stages via 50-ohm microstrip lines.
The parallel resistor, R9805, and capacitor C9807, on the input, reduce circuit response at lower
frequencies and improve stability. The 350 mW (typical) input power is increased to approximately
15 Watts. The amplifier power is monitored by the power control IC on the command board and
adjusted by controlling the voltage on U9850, Pins 2 and 3. A+ is applied directly to the final stage
inside the power module via pin 4. No repairs can be made to the module; damaged or failed units
must be replaced.
The power module leads will not tolerate undue stress; handle carefully when
repairing.
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