Voltase hobby: vhf

Showing posts with label vhf. Show all posts

Tuesday, October 28, 2014

250mW 16 dB VHF amplifier Circuit diagram

A high efficiency simple 2-transistor VHF amplifier electronic circuit project can be designed using this electronic circuit diagram . This VHF amplifier electronic circuit has a very high efficiency of about 16dB gain, and requires no tuning or alignment procedures. Wideband techniques have been used in the design and the circuit is equipped with a "lowpass" filter to ensure good output spectral purity. This VHF power amplifier circuit is specifically designed to amplify the output of 7mW to 10mW WBFM transmitters to a final level of 250mW to 300mW, after the filter.
The output of this stage is around 70mW of RF power.
The transistor is biased by means of R5, R6 and L6, and the residual DC current is set by R4. The input signal is coupled by C9 to the Base of the transistor.
The input signal from Q1 is coupled to the Base of Q2 via C7.The 78L08 voltage regulator is used to regulate the supply voltage to Q1 and the bias votages to both Q1 and Q2 so that the output RF power is relatively constant, even with large variations of supply voltage.
The output of the amplifier is filtered with a low-pass filter to reduce the output spurious and harmonic content.
The output filter consists of C3, C4, L1 and L2.
This RF power amplifier must be powered from a simple 12 volts DC power supply circuit.

Friday, August 29, 2014

Simple 30 Watt VHF Amplifier by using 2SC1946A

The 30 watt amplifier schematic shown below provides an appropriate power boost with an input of 4 watt up to 6 watts. The schema is designed to cover 88-108MHz FM Broadcast Band. However, the schema is very stable at my place and provides a clean-output through seven (7) element Butter-worth low-pass filter.

Simple 30 Watt VHF Amplifier Circuit Diagram:

Notes:

The heart of the schema is 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 16gauge laminated wire (or Silver copper wire can do best) and the RFC can be of HF toroid core (as shown in the picture) or 6 holes ferrite bead.C3 and R1 forms snubber schema while R2 and C6 prevent the 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.

The photo below is 60Watts VHF power amplifier using the above schema. 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 INTERFERENCE. (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.