Seriously Modded Rat Layout - Few Questions.

[SeaWitch]

Hey all

Just a couple quick questions on the Rat layout in the contributions section here:

http://guitar-fx-layouts.42897.x6.nabble.com/rat-with-mods-td39191.html

1.  The 20k trimmer is for biasing the JFET drain to 1/2 supply voltage, correct?

2.  I should be using DPDT (on-on) switches, or DPDT (on-on-on)?

3.  D2 and D3 are labeled "whatever you want", but they should be different from each other, correct?

4.  What will change by altering the 15nF cap?  i'm gonna socket it and try different values, but just wondering what a  higher or lower value would yield.

Thanks!!

[ξεναγος νεκροπολης]

1. yeap!
2. on - on
3. they should be the same. personally i love mosfet clipping.
4. no idea! socket and tell us what you liked most!

[SeaWitch]

Thanks!

I was actually thinking about using MOSFETS as clippers in a few circuits...

I have some 2n7000's I was planning on using - but:

should I go (is there a preferred method):

G+S - D?

or

G+D - S

AND, do I have to be aware of orientation (anode/cathode), or does it not matter?

EDIT:  

I found this while researching:

"MOSFET: there are few different ways to wire a three legged transistor and use it as a diode. The one that’s different than a regular silicon diode uses drain and gate connected together, acting as a cathode and source acting as an anode...

Is this accurate?  any other advise on favorite MOSFETs or comments are welcome!!

[nocentelli]

rg keen wrote

G+S - D = normal silicon diode

This is the intrinsic body diode. In an N channel mosfet, the G+S must be positive with respect to the drain to get the diode to conduct. The anode is the source, the cathode is the drain, and the gate actually plays no part other than being tied to the source. It is a silicon diode with a forward voltage determined by the silicon doping of the semiconductor substrate. Typical Vf is 0.5 to 0.7V when the diode is clearly into conduction. All diodes begin conducting tiny currents well before they nominally turn on.

If you had a P channel MOSFET, the polarities are reversed and the anode of the diode is the drain and the cathode is the G+S.

D+S - G = low leakage, high res diode

This does not exist for MOSFETs. MOSFET gates are insulated by 20 volts thickness of high purity glass. This connection exists for JFETs, where you are using the gate-channel junction as a diode forward biased, instead of reverse biased as it is in normal JFET operation. The junction is typically lightly doped, low leakage, and will show typical silicon diode drops (0.5-0.7V) as above. The only reason to do this or the connection above is to get a slightly different silicon diode junction, hoping that a diode not normally intended for use as a diode is different a little bit.

G+D - S = diode conneted mosfet = very soft knee = as used in the Shaka 5

The gate shorted to drain means that the "diode" doesn't start to conduct until the gate/drain is more than the threshold voltage more positive than the source (positive for N-channels, negative for P channels that is). So the forward voltage of these diodes is usually 1.5 to 3V. The knee of conduction is quite soft, and so diodes using this don't go from full off to full on, they gradually turn on. This happens to be quite good for soft distortion. This is the only FET diode connection worthy of any special study or pursuit IMHO. The others are just too similar to ordinary diodes.

It's this connection that needs the series ordinary diode to keep the body diode (connection 1) from conducting when the signal reverses.

....slightly edited from a thread at

https://www.diystompboxes.com/smfforum/index.php?topic=38581.0