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8.2.4 RF Quarter Modules
The RF amplifier subassemblies within a driver or PA module are called "quarter
modules." The quarter modules use n-channel Field Effect Transistors, or FETs, as their
active devices. FETs offer several advantages over bipolar junction transistors (BJTs),
including improved ruggedness, better linearity, and less susceptibility to thermal
runaway.
N-channel FETs operate similarly to NPN Bipolar Junction Transistors. In a common
emitter bipolar amplifier, a small change in base-emitter voltage results in a small
change in base current. The base current modulates the collector current, and the output
is taken at the collector. Similarly, in a common-source FET amplifier, a small change
in gate-source voltage modulates the drain current, and the amplifier output is taken at
the drain.
Each quarter module uses four RF FETs. The input contains a gain matching pad, a
phase matching coax line and a two-way power divider. Divider outputs each drive a
push-pull FET pair. The FET outputs are recombined in a two way combiner, whose
output is the output of the quarter module.
Temperature compensated bias voltage for each RF FET is generated from a 15 Volt
supply. The supply is part of the module control card (PCM) and switches on with
application of 50 Volts to the quarter modules. The quarter module supplies voltages
representing temperature and ISO voltage to the module PCM system.
For any given channel, class A and class AB amplifier blocks use the same quarter
module circuit. The bias voltage adjustment potentiometer controls the quiescent drain
current for each FET, which determines each quarter modules class of operation.
In cases where quarter modules are biased class AB, as in the PA module, each quarter
module is capable producing 70 Watts average output into a 50 ohm load. The excess
power is necessary to overcome losses in the combining stage.
When the quarter modules are biased class A, as in driver modules, they exhibit
improved linearity and about 1-2 dB higher gain. The trade-off, however, is lower
power output capability and reduced efficiency. Thus, class A stages are used as preamp
and driver stages, and class AB stages are used in final power amplifier stages.
Because low band and high band quarter modules utilize slightly different architectures,
the circuits are described individually below.
8.2.5 Low Band Quarter Module
(Refer to Low Band Quarter module Schematic, Tab 1000 in drawing package)
The RF input signal first passes through TL1 (Phase setting coax) and then through AT1
Which sets the gain of the quarter module to 19.25 dB. The RF input signal then passes
to T1, a two-way coaxial power divider which also performs an impedance transfor-
mation. R5 provides isolation between the two divider output ports.
