20W Linear Power Amplifier

An amplifier primarily designed for use with Pic A Star but may well be useful anywhere only low level drive is available.

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Construction notes

Many examples of this amp have been built and I think the construction data is error free..
Please adhere to the mounting instructions for the FET devices, in particular bolt the transistors THROUGH the PCB to the heatsink using spacers between transistor tab and PCB. The reason for this is to ensure the ground path is short so enhancing stability.

Provision has been made for "floating" coax braid on input and output coaxial cables, it has been found that for best stability it is good practice to connect braid pad to the adjacent ground pad - see detail in photo below.


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Picture on left is bread-boarded circuit used for first STAR contact, right hand picture is 20W PA prototype, below is the schematic of breadboard circuit which is very similar to final design.


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Links to data sheets:

For the brave RD100HHF1

Below are various plots demonstrating the AMPs performance.


160M IMD, 16W PEP

80M IMD, 20W PEP



40M IMD, 20W PEP

20M IMD, 20W PEP


14M IMD, 16W PEP

12M IMD, 16W PEP


10M IMD, 16W PEP

80M IMD, 6W PEP - indication of IMD if used to drive a large FET PA


80M IMD, 25W PEP - Note reference level is wrong, should be 25W!

Gain flatness, now better than shown with about 1.5dB mid-band to 1.8MHz and 2.5dB at 50MHz

  Effect on received signal through loss with amplifier being placed "across" signal path.
PA causes about 1dB of extra loss at 30MHz.