This is a conventional FM RF power amplifier design, using bipolar transistors in a tuned class C circuit. Thanks to the use of two stages, the amplifier can be driven to full power with less than 1 watt driving power, so that a large gain margin results in this FM transmitter.
Bipolar VHF power transistors have a severe affinity for low frequency self- oscillation. To obtain stability in this amplifier, I employed several techniques, such as placing the resonances of base and collector chokes far apart, damping the chokes with resistors, using RC combinations for absorption of unwanted frequencies, using feedtrough capacitors for bypassing on the board, etc. It took some tweaking, but the amplifier ended up unconditionally stable.
The impedance matching network between the two transistors calls for such a low inductance, that it would be impractical to make it with actual wire. So I used a micro stripline etched on the PCB. Also, the power and SWR sensor at the output was made with micro striplines.
This amplifier has a low pass filter at the output, resulting in a signal clean enough to be directly connected to an antenna. The SWR meter was placed before the filter, in order to clean out the harmonics produced by its diodes. In any case, while the signal is clean enough to easily satisfy usual legal and technical requirements, this transmitter should not be used at a multi-transmitter site without further narrowband filtering! This is so because any other strong signals on nearby frequencies would be picked up by the antenna and coupled to the power transistor, which would mix it up with the own signal, creating a wide array of intermodulation products, some of which would be re-radiated! This is a common and very big problem in many multitransmitter sites. In such places, NOT EVEN ONE FM transmitter should be allowed on the air without narrowband filtering! Such filtering is easily accomplished by means of a single tuned cavity, which can be constructed from copper tubing or sheet.
RF Power Amplifier 80W parts overlay without parts identification!
Here is the PCB layout, including the microstrips. The board is 20cm long and is double-sided, with the backside being a continuous groundplane except for two small pads at the driver transistor base and collector. I cut out these pads with a knife, rather than making a whole computer drawing for that!
Source: 80 Watt FM stereo broadcast transmitter
See more: RF TV Modulator - Audio Video RF Modulator - TV Modulator
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