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Help with Amp design!

Posted: Wed Jun 13, 2018 11:43 am
by OldskoolPirate
Just building some amps on a copper board. They work fine but only because iv used someone else’s design to make it. So I’m trying to build a design of my own for different transistors. I’m trying not to go near mosfets. But can someone tell me why there is a need to “filter” after every transistor to amplifier the next one? Iv got some ceramic caps, a ferrite bead and a wire wound inductor between the two so far.

Also I was told to put a fuck load of 33pf high voltage caps on the LPF.

Re: Help with Amp design!

Posted: Wed Jun 13, 2018 12:59 pm
by thewisepranker
You probably don't need to filter after every stage but it wouldn't hurt. It's an idea to do it after the first stage to stop the next stage amplifying any out-of-band power. For example, if the unfiltered output of the first stage has an amount of harmonic content and you don't have a filter before the second stage, the second stage will just happily amplify that as well as the bit that you want. It might not make it out of the final LPF but this filter will almost definitely not be an absorptive one. Instead, it will be a reflective filter meaning that the harmonics will be reflected back to the transistor and dissipated as heat. The result is wasted power and excessive power dissipation in your output device.

The capacitance values required will vary based on the inductance values you use in your filter. 33 pF is not necessarily correct or incorrect - you will be able to choose inductance values to make it work, but high voltage rating is a must.
There is a really good, free program available for you to use to design filters from Iowa Hills: