Section C
RF Amplifier
C.1
Introduction
This section includes a description of the RF amplifier module,
and troubleshooting information.
The transmitter uses a total of 79 “plug-in” RF amplifier mod-
ules. One module is used in the Predriver stage, fourteen are used
in the Driver stage (D1 through D14) and 64 are used in the Power
Amplifier stage (RF33 through RF96).
Any RF amplifier module can be used in the Predriver, Driver,
or Power Amplifier position. Modules can be exchanged with no
effect on transmitter performance. If a PA Module fails, Flex-
Patch™ can be used to replace it with another PA Module without
turning the transmitter OFF. Refer to “Using FlexPatch™ To
Replace A Failed PA Amplifier” in SECTION VI, Troubleshoot-
ing, for more information.
CAUTION
ALL MOSFETs MUST BE IN PLACE ON ALL MODULES IN ALL
POSITIONS (RF33-RF96, D1-D14 AND PREDRIVER), EVEN IF
SOME MODULES HAVE SHORTED MOSFETs. FAILURE TO OB-
SERVE THIS PRECAUTION WILL RESULT IN DAMAGE TO COM-
BINER TRANSFORMER TOROIDAL CORES.
All RF amplifier modules plug into combiner/motherboards, and
are accessible from the front of the transmitter. The Predriver and
Driver Modules plug in to the Driver Combiner/Motherboard.
Power Amplifier stage modules RF33 through RF96 plug in to
the Binary Combiner/Motherboard and three Main Com-
biner/Motherboards. This section describes only the RF ampli-
fier module.
Refer to SECTION V, Maintenance, for pc board maintenance
procedures. There are no adjustments on this board.
The Driver section and Power Amplifier section are described in
SECTION IV, Overall System Theory.
C.2
Principles of Operation
Refer to the simplified diagrams C-1 through C-5.
Each RF amplifier module is a class D switching amplifier, using
four pairs of N-channel power MOSFETs in a bridge configuration.
This configuration is referred to as a QUAD. The quad is made up
of two sections: Section A includes Q1/Q12 and Q3/Q10; section
B includes Q2/Q9 and Q4/Q11. Power MOSFETs are in flat plastic
packages, and are mounted on heat sinks.
C.2.1
RF Amplifier: Basic Theory Of Operation
Figure C-1 is a simplified functional diagram of an RF amplifier
module. Each section of the module consists of two pairs of
MOSFETs in series. Each pair of MOSFETs is alternately driven
into cutoff and into saturation, acting as a switch. The RF drive
signals to the two pairs of MOSFETs in a section are 180° out of
phase, so that when the upper pair is on (saturated) the lower pair
is off (cut off). When the upper pair is off the lower pair is on.
The output is switched between ground (about zero Volts) and
the positive supply voltage at an RF rate.
Amplifier efficiency is high because each MOSFET switches
between cutoff and saturation in a very short time. Dissipation is
low in both states. The devices switch quickly through their linear
operating region, where power dissipation is high, so that average
power dissipation is low.
C.2.2
RF Amplifier: Half Quad Configuration
The RF amplifier was designed to have a separate supply voltage
and RF drive inputs to allow the A half to operate independently
of the B half. This feature of the RF module is utilized in section
D8A and D8B of the Driver stage. Independent supply voltages
for D8A and D8B are supplied by the Driver Supply Regulator.
Figure C-2 shows the MOSFETs as switches, for section A.
Section B is identical in operation, except Q2/Q9 and Q4/Q11
are used. The square wave RF output waveform, at the junction
of Q1/Q12 source and Q3/Q10 drain, is the carrier frequency of
the transmitter.
C.2.3
RF Amplifier: Full Quad Configuration
All RF amplifier modules except Driver Module D8 operate in a
full-quad configuration: section A output and section B output
are connected to opposite ends of a combiner transformer pri-
mary winding. This is equivalent to the classical push-pull con-
figuration.
Figure C-3 shows the four pairs of MOSFETs as switches. The
phase of the RF drive signals is such that only two configurations
are possible for the switches (unless a MOSFET is shorted).
During one half of the RF cycle, Q1/Q12 and Q4/Q11 are both
driven to cutoff and Q2/Q9 and Q3/Q10 are saturated. During
the other half of the cycle, Q1/Q12 and Q4/Q11 are saturated,
and Q2/Q9 and Q3/Q10 are cut off.
This switching action effectively applies the full supply voltage
to the combiner transformer primary winding across C8. Each
doubled push-pull amplifier produces a square wave output, but
the two sets of amplifier square waves are 180° out of phase.
The square wave peak-to-peak amplitude across the transformer
primary is about two times the supply voltage and will have some
“ringing” because of the reactive load.
A capacitor is placed in series with the transformer winding to
prevent a direct current path to ground if a MOSFET shorts.
C.2.4
RF Amplifier Module On/Off Control Circuit
In the Predriver and the Driver stage, the RF amplifier modules
are always turned ON when the transmitter is operating. In the
Power Amplifier stage, however, modules are turned ON and
OFF to change the power and to modulate the carrier.
Figure C-4 is simplified diagram that explains the control circuit
operation. The control section on the RF amplifier module affects
RF drive to Q3/Q10 and Q4/Q11.
888-2297-002
C-1
WARNING: Disconnect primary power prior to servicing.
