Theory of Operation 3-5
3.2.1.1 Driver Amplifier
The Driver Amplifier is the first amplifier within the PA, and is capable of reaching output power
levels of approximately 4 W max. The Driver Amplifier consists of three adjustable-gain amplifier
stages and supporting matching circuits and attenuators. The Driver Amplifier and supporting
circuitry is denoted by reference designators in the 2000’s (i.e. R2000, C2450, etc). The driver gain
stages are gain-controlled, by way of V_Control, in order to adjust the PA output power to the
desired level through the power control loop within the PA. Depending on the PA input level and the
desired PA output level, the control voltage will be adjusted to yield the necessary driver gain, up to
a maximum of approximately 25 dB, in order to achieve the desired PA output power.
3.2.1.2 Final Amplifier
The RF output signal delivered from the Driver Amplifier is then further amplified by the final
amplifier stage within the PA. This is the last RF amplification stage, and will yield the output power
required to deliver the requested power to the base station/repeater’s antenna port. The final
amplifier and supporting circuitry is denoted by reference designators in the 4000’s (i.e. R4000,
C4450, etc). The final amplifier is a fixed-gain stage with approximately 20 dB of gain, which will
vary depending on the frequency band of operation. The final amplifier consists of four discrete
devices operating in a parallel configuration. A multi-stage hybrid splitter/combiner distributes the
Driver Amplifier output signal to these final amplifier devices, and then later recombines the
amplified signals from these devices to deliver combined power to the next element within the PA.
3.2.1.3 Module I/Os
RF INPUT (J2000) – RF input from the Exciter to the PA
ISOL_RF_INPUT (M2050) – RF output from the final amplifier to the isolator input
28V_A (M2021) – 28 V supply input from Distribution Board for final amp
28V_B (M2020) – 28 V supply input from Distribution Board for final and driver amps
14V (M2022) – 14 V supply input from Distribution Board, for final and driver amps
V_CONTROL (M2023) – Control voltage input from Distribution Board
V_OMNI (M2024) – Metering signal output supplied from driver to A/D
PA_ENABLE (M2025) – PA enable signal input from Distribution Board
PA_ID (M2026) – PA_ID metering circuit input from Distribution Board for PA ID
3.2.2 Isolator
The PA internal isolator (which is comprised of a discrete circulator and a bolt-down isolation load)
follows the final amplifier within the PA. It maintains good loading for the final amplifier (regardless of
the PA's external load) by preventing a high VSWR at the output of the PA from damaging the final
amplifier stage. Also, by providing additional isolation to the final amplifier, the isolator improves the
intermodulation attenuation (IMA) performance of the transmitter.
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