This RF power amplifier design is to boost the output power of low power FM broadcast band exciters based on Motorola MRF171A MOSFET with power output 40W. To reduce the harmonics to an acceptable level, Integrated 7 pole Chebyshev low pass harmonic filter (LPF) is added at the final section.Component Choices
As the input power is only half a watt, standard ceramic capacitors and trimmers were used in the input matching circuit. L1 and L2 (refer to schematic) could have been made much smaller, but were kept big for consistency with the inductors used in the output network. On the output network, mica metal clad capacitors and mica compression trimmers were used to handle the power and keep component losses to a minimum. The wideband choke L3 provides some lossy reactance at lower RF frequencies, C8 takes care of AF (audio frequency) decoupling.
The use of an enhancement mode N-channel MOSFET (a positive voltage biases the device into conduction) means the bias circuitry is simple. A potential divider taps off the required voltage from a low voltage stabilised by a 5.6V zener diode. The second 5.6V zener, D2, is fitted as a precautionary measure to ensure excessive voltage are not applied to the gate of the FET, this would certainly result in the destruction of the device. Purists would temperature stabilise the bias current, but as the bias is not critical in this application, this was not bothered with.
A BNC socket had been used for the RF input, due to the low RF input power. I've used N type for the RF output, I don't use BNC for above about 5W and I don't like UHF style connectors. Personally, I don't recommend using UHF connectors above 30MHz.
RF Power Amplifier Schematic-Printed Circuit Board
Power Amplifier Construction
The Power amplifier was constructed in a small aluminium diecast box. RF input and output connections are made by coaxial sockets. The power supply is routed through a ceramic feedthrough capacitor bolted in the wall of the box. This constructional techniques results in excellent shielding, preventing RF radiation escaping from the amplifier. Without it, significant amounts of RF radiation could be radiated, interfering with other sensitive circuits such as VCOs and audio stages, also significant amounts of harmonic radiation could occur.
The base of the power device sits through a cut-out in the floor of the diecast box and is bolted directly onto a small extruded aluminium heatsink. An alternative would have the base of the power device sitting on the floor of the diecast box. This is not recommended for two reasons, both concerned with providing an effective path to conduct heat from the FET. Firstly the floor of the diecast box is not particularly smooth, which results in a poor thermal path. Secondly, having the floor of the diecast box in the thermal path introduces more mechanical interfaces and hence more thermal resistance. Another advantage of the chosen constructional technique is that it correctly aligns the device leads with the top face of the circuit board.
Using the specified heatsink will require the use of forced air cooling (a fan). If you plan not to use a fan, a much bigger heatsink will be required, and the amplifier should be mounted with the heatsink fins vertical to maximise cooling by natural convection. RF Power Amplifier Part List
Source: A Design for a 40W broadband VHF RF Power Amplifier for FM broadcast
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