What does this cap do?

[MicFarlow77]

Hi All,

http://gaussmarkov.net/layouts/drboo/drboo-schem.png

What is the purpose of C16 in the above schematic? It's across the trim for Q3 and the other trims do not have this cap.. should they?

Thanks,

Mick

[kurtlives]

I believe it is a lo pass filer. Maybe? To cut excess highs.

I thought it was bad design to add a cap in parallel with a FETs drain though. Not all FETs bias up the same so the drain resistance is different. A 10nF cap will then not have the same effect on all FETs.

[johngreene]

Hi All,

http://gaussmarkov.net/layouts/drboo/drboo-schem.png

What is the purpose of C16 in the above schematic? It's across the trim for Q3 and the other trims do not have this cap.. should they?

Thanks,

Mick

It's basically the same as it would be if it was connected to ground. Just filtering the highs.

[MicFarlow77]

Thanks for the replies!

It's basically the same as it would be if it was connected to ground. Just filtering the highs.

hhhmmm... isn't that an unusual place to filter highs? Seems like it would be a little more effective from the gate to ground... but I honestly have no clue...

[johngreene]

The difference is that if it is on the gate your are filtering before gain, across that resistor and your are filtering -and- reducing gain at higher frequencies.

[MicFarlow77]

The difference is that if it is on the gate your are filtering before gain, across that resistor and your are filtering -and- reducing gain at higher frequencies.

Hey John,

Thanks for the info.... so to follow-up.. Does C2 in the following circuit snippet solve the issue with frequency variation based on where the jfet biases and yet filter and roll-off the desired frequencies?



edit: Or does changing C1's value accomplish the same task?

[ayayay!]

http://www.diystompboxes.com/smfforum/index.php?topic=70514.0

[Lurco]

Thanks for the replies!

It's basically the same as it would be if it was connected to ground. Just filtering the highs.

hhhmmm... isn't that an unusual place to filter highs? Seems like it would be a little more effective from the gate to ground... but I honestly have no clue...

while I don`t mean to disagree with johngreene`s explanation, I tend to look at C16 as
"making the drainresistor look smaller for higher frequencies"

[MicFarlow77]

while I don`t mean to disagree with johngreene`s explanation, I tend to look at C16 as
"making the drainresistor look smaller for higher frequencies"

No problem.... what I am trying to figure out is why there would be high frequencies up there anyway....  that drain resistor is fed with 9V DC...

In Jono's post, he asked about the 5n hanging off the power supply and John Lyons mentioned that it was probably power supply filtering that was transferred from the amp schematic. In a DC circuits like these stompboxes I'm not sure they are needed at all. I'm sure they might come in handy in big voltage amps though... again, I'm just making a minimally educated guess though.

(I think a light bulb just came on.... but someone will have to verify... :-) 

Thanks,

Mick

[petemoore]

1.  A good cap will introduce increasing resistance to frequencies below 'X'.
 
  If those higher frequencies are passed along to ground [0.0v] they're taken out of the signal path.
  If those highs are passed along through the signal path, anything below 'x' frequency is reduced in amplitude.
  So the same capacitor can be used to reduce bass or treble, depending on where/how in the circuit it is placed.
  Right at the very end of the supply line...a capacitor may smooth any ripple that...develops in the line [in addition to the filtering at the PS end], especially needed or useful for something that 'switches' or quickly sweeps between low and high current draw.
  Electrolytics sometimes benefit from having a small film capacitor assist it when it'd have trouble with the HF filtering, sometimes you'll see this across the supply rails.

[George Giblet]

 - The purpose of the drain cap is two fold: It reduces high frequencies and, because the overall gain at high frequencies is reduced, it prevents oscillation problems due to feedback cause by stray capacitances and non-ideal layout.

- A cap from the drain to +V and a cap from the drain to ground are technically *identical*.   For the "cap to +V" to be identical it assumes the power supply looks like a short circuit to AC signals (ie. well bypassed).  In practice this always isn't so.   In order to make the power supply look like a short circuit you need to add a cap from +V to ground,   that's why the 5n cap is present - 5n is suitably larger than 1n.  If the layout is tight and well bypassed it may not be required.  The cap to ground is a more robust configuration in that it doesn't need the supply to be so well bypassed.   The other small difference between the two methods is the cap is subject to different bias voltage in each case (one is at Vdrain the other is at +V-Vdrain).

- the 5n doesn't remove mains frequency ripple at all.