Low Pass Filter(LPF) and Application in Amplifiers

Recently I came across few comments on whether LPF is really required for Solid State Amplifiers.
I am no professional or expert in this area, however, my W6PQL amplifier built has taught me something to state here.

Any solid state design has some non linearity and thus introducing harmonics. Harmonic distortions are main reason why one hears spatter on few bands even when other parameters are conducive for HF. There are regulations in some part of the world to keep such distortions limited to lower than -40dB. It does not matter what power we are running, we need to keep this as a best practice guideline and make things better for all others on the band.

If you have an amplifier for 1 to 7Mhz, having one LPF for 7Mhz band is not sufficient for operating the amplifier on 160mtr and 80mtr. We have second and third harmonic of 160mtr falling in pass band range of 7Mhz LPF. Similarly second harmonic of 80mtr falls in 7Mhz. So essentially such LPF will only be good for operating 7Mhz band (where its harmonics are on 20 and 15Mtrs will be filtered).

Next time someone says amplifier works well because of its efficiency etc. Also pause and talk about signal purity and distortion characteristics. No modern solid state amplifier is complete without LPF sections to cover each band of its operating frequency. Else you might be splattering all over other bands even with 100 watts of power output on fundamental operating frequency!.

Spending some time on understanding these aspects will keep us serene on the bands :)

VU2XE
Kiran