RF Circuits

This 30W RF amplifier circuit provides an appropriate power boost with an input of 4-6 watt operating on the VHF FM Broadcast Band with 88-108 MHz frequency. However, the circuit is very stable and provides a clean-output through 7 element Butterworth low-pass filter (LPF).

Circuit Explanation
This VHF RF power amplifier circuit uses 2SC1946A VHF RF power transistor. The transistor is specifically designed for operation in frequencies up to 175 MHz, with very good results.


As you can see, the power line is well decoupled. The amplifier current can be over 5 amps. All the coils are made from 16 gauge laminated wire (or Silver copper wire can do best) and the RFC can be of HF toroid core or 6 holes ferrite bead.C3 and R1 forms snubber circuit while R2 and C6 prevent the RF amplifier from self-oscillation at VHF, sometimes you need to add 180 ohms in parallel with L7.That will cause the amplifier to dissipate "undesirable VHF" thereby reducing spurious level.

Using multiple power design, by combining two power transistors, can lead to more responsibility compared to a single power, especially in the UHF and VHF bands. It may result to great interference because of the out of phase operation of the two amplifiers. To amplify the output at its operating frequency, it simply involves connecting the output to a good antenna with a 50 ohms transmission line. With this, the amplifier can be adjusted easily.


The photo above is 60Watts VHF power amplifier using the above circuit. Two of 2SC1946A transistors are arranged at 90 degrees to each other and their outputs are combined using "Power Combiner Network”. It is quite difficult to combine powers at VHF and UHF bands.

However, I recommend that hobbies should stick to single power design due to its complicity and large rate of Intereference. (in attempt to go for double transistors which involves power combiner network). Since the two amplifiers are operating in different phase (out of phase).

Tuning:
Tuning of the amplifier is not hard at all. You just have to connect the output to a good antenna with a transmission line (RG214) of 50 ohms. First match the output network, and then do the same to the input network for a maximum power output. By way of adjustment, you can increase the output at its operating frequency.

Note:
Operating unlicensed transmitters is illegal in some countries, including the UK. The circuit presented here is for educational purposes only.

Author: David Celestin, Ghana, West Africa
e-mail: mightycelestin@yahoo.co.uk
Website: www.zen22142.zen.co.uk