What does an emitter cap do?

[gjcamann]

I've come across several schematics with a capacitor in parallel with the emitter resistor. Some examples are Jack Orman's JFET and MOSFET boosters, and many-many lovepedals. I'm sure a few people here know what this does, anyone care to share? 

I'm guessing it lower the AC impedance of the transistor circuit, resulting in more gain and applies equally to BJTs and FETs.

What are the effects on increasing or decreasing the size of this cap?

I assume putting a resistor in series with this cap reduces it's effectiveness.

Since this is all AC in the collector-emitter path, could this capacitor be put in parallel with the collector resistor to get the same effect?

Thanks guys.

[Keppy]

You're right about the function. That cap raises AC gain. The larger the cap, the further into the bass frequencies that increase extends. It wouldn't work on the collector though, as that would shunt your boosted signal straight to ground (or whatever voltage source you strapped it to).