Purpose of ceramic cap across a transistors base and collector?

[eurekaiv]

What's the purpose of a ceramic cap with a small value (like just a few pf) that's tied across the base and collector of a transistor?  I've seen this a few times recently digging through schematics and was curious as to what it might be up to.  I think it was in wah schematic somewhere...  maybe a fuzz as well?

I had saved an pic of a schematic for the purpose of posting but I can't seem to find it now.  But it would look something like this...

[kvb]

negative feedback/attenuation of high frequencies

[eurekaiv]

Ahhh...  ok.   Thanks!!

[Paul Perry (Frostwave)]

For the experimenters: 10pF is right in the range of trimmer capacitors 

[raycroft]

negative feedback/attenuation of high frequencies


I've always read if you have radio interference in your wah, you put that cap on there to filter it out.  Probably the same thing he said. 

[Derringer]

hold my hand ... learning too

"negative feedback" means that the signal being returned to the base from the collector is 180 degrees from the signal driving the base right?

the 180 degree offset then cancels the correlating inverted signals causing the attenuation...right?


'/\/\/\' cancels '\/\/\/' right ?


and the signal heard  (the leftovers not canceled) is the difference between the absolute values of the signals ... right?

[eurekaiv]

I remembered where it was...  it was a wizard wah layout I saw at FSB so I'll refrain from posting it here.  Is 10pf really enough to filter out any highs or is the only answer the radio interference thing suggested above?  I do suppose I could breadboard a wah and find out for myself.   

[Paul Perry (Frostwave)]

The technical name for capacitance in this situation is "miller capacitance".
The capacitor appears - if you are using it as part of a filter - as though it has been multiplied by the gain of the stage, approximately.
The case in  a tube amp is treated here: http://www.aikenamps.com/MillerCapacitance.html
(the capitance there, is usually the cap between the plate & anode - which is why you have an actual electrostatic shield between them, in a pentode, I believe).

So, although 'tiny', it's going to have a significant effect.
Who knows, maybe this will enable the design of a filter based on air plate capacitors!