Mosfet frequency boost

[jcknowles89]

Hello all,

Been using mosfets for alot of my new projects but I was curious.

With JFets you can do a Bypass cap from the source to ground in parallel with the Source resistor and it will boost Gain in that frequency range of the capacitor. Same with Tubes, you use a cap to bypass the Cathode and boosts that frequency range of the cap.

Anyway to do this with Mosfets? Any suggestions would be great! thanks guys.

[GibsonGM]

Yes!

The same rules apply.  The larger the cap, the more the entire signal is boosted...smaller cap = top end boost.

[jcknowles89]

yeah, I thought the same thing but couldn't really tell any difference between a 680nf Cap and a 1.2nF cap. I have mine set up in a kind of Big muff set up. With the drain and the Gate bridged with a resistor. could the configuration be the issue? Thanks

[jcknowles89]

Ah figured it out! Turns out I had the wrong configuration for this to apply. I isolated the gate voltage to its own VCC voltage instead of feeding the drain voltage through the bridges resistor. Now works great!

[Mark Hammer]

One thing I've done in past, and suggest to try out is a bidirectional boost control.

Say one has a fixed resistance to ground from the source.  Tie the wiper of a pot to the source and tie each outside lug of the pot to ground through a different cap value.  Since the midpoint of the pot ought to be where no boost is applied, you'll want half the resistance value of the pot to be large than the fixed resistor value. 

For instance, Jack Orman's MOSFet booster has a 2k7 fixed resistor to ground from source.  The "boost" path is large-value electro (100uf) with a 5k variable resistance to ground from the source.  As the pot value/resistance gets smaller, the gain is increased.

Okay, imagine we have a 10k pot,with the wiper to source.  At one end is a 100uf cap for full range boost, and at the other is a 1uf cap for boosting mids and highs only.  One could also have,say, a 470nf cap to ground and a 2u2 with a 470R resistor in parallel, to provide more boost for highs than for mids.  Set to the 
midpoint, there's 5k on each side of the wiper, such that very little  boost is provided in addition to what the fixed 2k7 provides.

I'll just note that the more restricted bandwidth the boost is applied to, the less noticeable a volume boost.  So boosting content above 1kz would be just as much, but boosting the entire audible spectrum would sound louder, so one might wish to tinker with resistance values to make gain increases in each direction sound roughly as loud..