Specifications

transient voltages. Each of these diodes conducts if the voltage
across them exceeds 20 Volts, either positive or negative.
G.5.2
MOSFET Switches - Full Quad, Bridged
Configuration
Figure G-2 shows the four pairs of MOSFETs as switches. The
phase of the RF drive signals is such that only two combinations
are possible for the switches when the module is “ON”. During
one half of the RF cycle (Figure G-2A), Q1/Q3 and Q6/Q8 are
both driven to cutoff (open), while Q2/Q4 and Q5/Q7 are satu-
rated (closed). During the other half of the cycle (Figure G-2B),
Q1/Q3 and Q6/Q8 are saturated, and Q2/Q4 and Q5/Q7 are cut
off.
Each section produces a square wave output, but the two square
waves are 180 degrees out of phase. This switching action effec-
tively applies the full supply voltage across the combiner trans-
former primary winding with opposite polarities on each half cycle
of the RF Drive signal. Being a Bridged amplifier, this gives an
effective RF output of twice the power supply voltage. Section A
output at TP1 leaves the module through P1-1, and section B output
at TP2 leaves the module through P1-7. The combiner motherboard
connects the transformer to these two points. To prevent a direct dc
current path through the combiner transformer winding to ground
(for example if MOSFET’s should fail), a capacitor C8 is placed in
series with one section’s RF output.
Small ferrite beads have been placed on the drain leads of
MOSFETs Q5, Q6, Q7 and Q8, to prevent high frequency
currents or oscillations.
G.5.3
MOSFET Switches - Half Quad, Single Ended
The RF Amplifierwas designed to have a separate supply voltage
and RF drive inputs to allow the “A” half to operate inde-
pendently of the “B” half. This feature of the RF module is
utilized in the Regulated Driver Module D14A and D14B, and
in the Predriver module PD1. Figure G-3 shows the MOSFETs
as switches for Section “A.” Section “B” is identical in operation
except, Q2,Q4 and Q6,Q8 are used. The squarewave RF output
Waveform,at the junction of Q1,Q3 source and Q5,Q7 drain, will
be approximately from 0 Volts to the supply voltage, at the
transmitter’s RF operating frequency. In this mode the RF output
voltage is about half of the Bridged mode of operation. This
provides some degree of redundancy because it is possible that
one of the Full Quad, Bridged RF amplifiers could have a failure
of only section “A,” or only Section “B.” Since a failure of either
section would most likely be either a shorted MOSFET or an
open fuse, the other half of the module would continue to operate
normally. In this case it becomes a Half Quad module and would
operate at reduced RF output voltage.
G.5.4
Control Switches
Figure G-4 shows the control switches for section A in the “ON”
and “OFF” conditions. The same principals apply to section B
Figure G-2. RF Amplifier Full Quad Bridged Operation
888-2339-002 G-3
WARNING: Disconnect primary power prior to servicing.