2 Watt VHF linear amplifier
Posted on Apr 19, 2012 10191
Under: RF Amplifier Circuits
Once again my collection of projects creation has been interrupted by another necessity. Patrick and other people have asked me for a circuit of a VHF power amplifier. These circuits are my `standard` building blocks that can be used to amplify RF power signals, from 50MHz to 170MHz, just a few component changes are required. The circuit diagrams shown are `normalised` for 100MHz and will tune about 80MHz through to 120MHz. Simply scale the components up or down if you want to go down or up in frequency bands. The circuits as shown will give a gain of about 14dB with an output power of about 2 Watts (input = 100mW). With an output power of 20 watts the gain falls off to just 12dB at 100MHz or just 10dB at 170MHz.
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L3 is a huge RF choke, made using typically 20 turns of 22SWG wire in a ferrite ring. The wire must be thick enough to carry the PA current. For low powers a small ferrite is more than sufficient. Do NOT scale this inductor for other frequencies. L4 is made using typically three or four turns of 22SWG enamelled wire inside a two ferrite beads side-by-side, as a pair of biniculars. For higher powers you should ideally use a larger two-hole binocular ferrite core. Tuning up the PA is quite easy, tune the two input caps for maximum supply current. Then tune the two output caps for maximum output power. The two caps are inter-related so you will just have to tune alternately until no further changes are required. Repeat the tuning again but for maximum output power on both occasions. You should be able to achieve 14dB of gain for 2 watts output power, but a lot depends upon the device you are using. You can use two of these circuits cascaded to give a 20-watt output FM amplifier with only 100mW of drive. The amplifier as shown was first built as a 2-watt amplifier but then extended to give 100 watts using two parallel PA transistors. The drive level was 2.5 watts. 100 watts is not exactly QRP but when you are running your 25mW QRP transmitter it can sometimes be very satisfying to have the power - QRPers are very often stamped on and it can be nice to do a bit of stamping on certain occasions. Please be aware that...