SRK FMA300 User Manual - page 15
FMA 300 Manual
2003 SRK Electronics Pty Ltd
6
CIRCUIT DESCRIPTION
6.1
Equipment Overview
The FMA300 consists of the following sub-assemblies:
•
50V Switch mode PSU
•
300W RF amplifier
•
Control board
•
Output directional coupler
•
Auxiliary PSU
These assemblies are interconnected as per “FMA300 wiring diagram”.
The 50V SMPSU generates 50V at up to 10A for the RF amplifier. It incorporates inrush
current limiting and active power factor correction for excellent power factor. This is an OEM
unit. No attempt should be made to open, modify or repair this unit.
The 300W amplifier assembly is the heart of the amplifier and is centered around a double
MOSFET driven in push-pull class AB.
The RF input to this module is passed through the input directional coupler, thus allowing
monitoring of input power.
The RF output from the amplifier assembly is passed through a filter to remove all harmonics,
and then onto the output directional coupler which allows monitoring of both forward and
reverse output power.
The whole amplifier is controlled and metered by the control board assembly.
The auxiliary PSU generates +5V for the control board.
6.2
Amplifier
Please refer to the appropriate diagram.
RF enters on BNC J2 and is passed to a directional coupler formed by T5. This takes a small
sample of the input power and passes it to R6/9. This voltage is rectified by D1 and then
passed to the control board to allow the input power to be measured.
The RF is then converted to a balanced signal by the action of T12.
The balanced output is passed through 9:1 transmission line transformer formed by T3 and 4.
The RF is then applied differentially to the dual MOSFET, Q1. Bias is also applied at this
point.
The drain loads of Q1 are made up the 4:1 transmission line transformer formed by T1 and 2.
The drains are supplied with DC via this transformer and the decoupling network formed by
C20, FB1, R11, C54 and C31.
The RF output is turned from push-pull to single sided by balun T9 and passed to the output
filter.
Drain current sampling is performed by R13 and fed to the control board for metering.