# Transmitters

## Power amplifiers at VHF

For low powers, ie less than 25W (output) or so, the transistor is a relatively cheap and very convenient solution, particularly for mobile or portable operation when the power level desired can be obtained from a 12V supply.

The generally preferred output arrangement for a transistor PA at 144MHz is the L-pi circuit. This is a combination of the series-tuned L and the conventional pi circuit.

A typical circuit is shown in Fig 4.9. This uses a Mullard BLY83 and gives an output of 7W for a collector supply voltage of l2V, and 12W at 24V. These powers are capable of very good performance at VHF due to the much more effective antennas which are possible at these frequencies. The collector supply is decoupled by 0.01μF and 1,000pF capacitors in parallel. The 0.01μ F capacitor next to the output socket is for DC blocking. The relative simplicity of the circuit, which was built on a printed circuit board measuring 127 by 38mm, should be noted.

Fig 4.9. Low-power amplifier for 144Mhz

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