What do caps on emitter to ground do?

[Nick C.]

I've noticed that many circuits have a rather large cap from a transistor emitter to ground, for example the Fuzz Face, Rangemaster. The LPB doesn't. When I added a 47uf to my LPB it definitely changed, but I'm not sure what changed. So, what do those caps do?

[plankspank]

It boosts the high frequency gain of the circuit. Since the A.C. resistance of a cap  is inversely proportional to the frequency, the higher the f, the lower the A.C resistance. it is determined by the formula:
A.C. resistance= 1/2*pi*f*C

[petemoore]

  Depending on the relevant frequencies and relevant value...
  The emitter bypass capacitor blocks DC, allowing a resistor from ground to be used for setting DC bias.
  The emitter bypass capacitor 'Feeds AC' to the chip, increasing gain to 'relevant frequencies', [which depend on the cap value].
  A good cap always: Block DC, and allows higher frequencies to pass through them with increasingly less resistance than lower frequencies...the resistance/frequency curve.
  It's how you hook them in that makes them do this or that...becoming LP filters, Hp Filters etc.
  Notice the same cap can be used to attenuate high frequencies [when assigned in a Low Pass filter circuit], as a .0047uf capacitor in Series with the signal path does.
  Try the same .0047uf capacitor, except this time from the signal path to ground.
  In both cases, the capacitor is imparting increasing resistance as frequency gets lower.
  In the first example, these higher frequencies which are passed mostly unattenuated go to the amplifier, hence you will hear more HF because the LF's were attenuated by the capacitor.
  In the second example, the 'same' high frequencies which passed through the capacitor are instead shunted to ground and what's left is passed on to the amplifier, this is a LP filter, the low resistance path to ground for high frequencies 'dumps' them to ground, the amplifier never gets to see them. 
 

[DougH]

Start here: http://www.diystompboxes.com/wiki/index.php?title=LearningResources

[frank_p]

If you don't like tough maths look in ch. 14 (frequency effects) of the Malvino book.  Simple explanations with useful formulas and without big maths.

[Nick C.]

Thanks guys on the info, though most of it is over my head. I want to learn more about electronics, but I'm so excited I want answers now! Simple answers will do. I find it intersting that many circuits use a 22uf in this situation.

[Caferacernoc]

Thanks guys on the info, though most of it is over my head. I want to learn more about electronics, but I'm so excited I want answers now! Simple answers will do. I find it intersting that many circuits use a 22uf in this situation.

That 22uf is the most common size from tube preamps.