[Elecraft] Re: Decoupling circuit -- why is it like it is?

[Wayne Burdick]

Vic Rosenthal wrote:
> 
> In the KPA100 the positive line to the transistors is bypassed by numerous
> decoupling capacitors:  C52-C57, each of which is 0.047 uf, and a 470 uf
> electrolytic.  My questions are:
> 
> 1) Why six capacitors instead of one 0.3 uf capacitor?  Obviously these are to
> remove RF from the +12 line, by why use more than one?  Is it due to RF current
> ratings?

Yes. The more capacitors, the lower the RF impedance to ground, and the less likely that
there will be feedback into the power supply from the PA transistors.


> 2) Why a large capacitor like 470 uf?  What does this one do?  Certainly it's
> not for RF!

The large electrolytic holds the instantaneous DC collector voltage relatively constant as
the load from the PA transistors changes rapidly. It also prevents very low-frequency
instability. Any high-power amplifier needs both high and low-frequency decoupling at the
power supply.


> Why am I asking?  Well, there's nothing wrong with my KPA100.  But it is more
> intelligenly designed than my high-power amp.  This unit has four boards like
> the one in the KPA and puts out from 550w to a KW on various bands when it's
> working.  It has only a 100 uf electrolytic in this location -- and it blows
> these electrolytics like firecrackers on 15 meters!  I have tried 0.1 uf discs
> across the electrolytics to no avail, but looking at Wayne and Eric's design I
> see why!  I'm hoping someone can explain this aspect of the design to me so I
> can understand how to fix it right (the manufacturer of the amp thinks I'm an
> idiot and won't talk to me).

The fried electrolytic may have something to do with the capacitor's surge voltage rating
in addition to it being of an inadequate size. Try 1,000 uF with about a factor of two
larger voltage rating. 

Wayne