Service Manual

TSM 20-399 Rev 0, Feb 2014 18 10W VHF Amplifier
4 RF POWER AMPLIFIER AND HEATSINK ASSEMBLY
The complete RF amplifier chain consists of a preamplifier and a final output amplifier mounted on a convection-
cooled heatsink on the front panel of the unit.A thermal cutout switch removes the power to the drive stages
should the temperature of the heatsink exceed a safe operating level.
4.1 PREAMPLIFIER
The preamplifier stage consists of a gain-block hybrid amplifier. This simple board has a regulator circuit to supply
the hybrid as well as a PIN attenuator module which is controlled by the VSWR limiter circuitry on the
detector/metering board. Refer to figures 10 and 11. This board has no user adjustments and is broadband,
covering both VHF high and low band.
4.2 OUTPUT AMPLIFIER
The output amplifier is a dual amplifier with FET transistors in quadrature parallel operation. The input signal is
split via hybrid 3dB coupler, and the output from each transistor amplifier is recombined with a similar (higher
power handling) 3dB hybrid. These hybrid couplers are the only components that differ between the Band I and
the Band III amplifiers.
A matching network consisting of a combination of circuit traces and impedances, lumped components and
transmission line transformers converts the input 50 ohm to the low impedance required at the input of the FET. A
similar matching network is used to match the output impedance of the FET to the required 50 ohms. There are
no tuneable circuits in these matching networks.
A thermally compensated biasing network is provided to maintain constant idle current throughout the amplifiers
operating temperature range. The bias adjustments are provided by R3 and R4. Should there be a device failure,
only the bias current should be re-verified upon replacement of the FET. Note – the circuit has a bias shutdown
feature which is not used in this application. Refer to figures 12 through 14