FET question

[Papa_lazerous]

Ok I am looking at a schematic right now that I am getting a little confused about how a FET is being used.......

the drain and source are connected up in such a way that the FET appears to be a way of cutting the connection between the 2, meaning when its on they are connected when its off they are not.

the gate confuses me however its an N-channel FET and the only thing directly connected to the Gate is a diode with the anode towards the gate and the cathode is pointing towards the rest of the circuit, which can have +9volts on it...

Why is the diode there and what is it doing?  I am really stuck and google hasnt helped one bit

[PerroGrande]

Sounds like the FET is being used as a switch -- not unlike the switching circuit that Boss uses in many of their pedals.

[R.G.]

Welcome to FET switching.

JFETs are best thought of as bars of resistively doped silicon (i.e. "n-channel" or "p-channel"). If you can get the gate to be truly open, they are a resistor. In fact, at quite low voltages, all JFETs show this resistive nature.

The gate on JFETs is the opposite polarity from the channel. So by reverse biasing it, we can force no current to flow between it and the channel.

However: the voltage on the gate causes changes in the channel by the effect of the electric field pulling charge carriers out of the conductive part of the channel. Get it? FIELD EFFECT transistor. The conductivity of the channel resistance is modified by the voltage on the gate in the same way that the resistance to water flow is modified by pinching a garden hose.

A JFET has a minimum resistance. This is rdson in the datasheet, and is the basic resistance of the channel area. Dinking with the gate runs the resistance UP from there, all the way to nearly infinity (i.e. off).

If you want the JFET to conduct as much as possible, you make the gate be the same voltage as the channel by tying it to the channel. This results in the channel resistance being rdson. If you want the JFET to switch off, you pull the gate negative (on an n-channel device) with respect to the lowest voltage on the channel, which we usually call the source. If you pull it negative by Vgsoff, it turns the channel off entirely, and that's why that voltage is on the datasheet.

You can also get the JFET to seek its own Vgs for conduction by leaving the gate open. That's where that diode comes in. If you put a voltage on the diode higher than the source voltage, the diode is reverse biased, the gate is open circuited, and follows the channel by any stray leakage. The channel resistance is rdson. When you pull the diode negative with respect to the source, the diode turns on and pulls the gate down, putting a reverse voltage on the gate to channel, and turning the JFET off when the gate voltage gets more negative than Vgsoff.

Yes, the JFET is being used as a switch.

[Papa_lazerous]

Thanks RG much appreciated, I was in no doubt about the FET beign used as a switch I just could see how the diode came into play, now I got it.  thanks allot

[Austin73]

Hey John,

I presume your still playing with the DD3.

Couldn't get any help for your schem wise. But if its really bugging you why don't you take the fet switching out like Giga did on one of his pedals. They must all have the same style switching. The you could make it true bypass. I know its ugly but its worth considering if your struggling.

Aus

[Papa_lazerous]

Thanks for the input Aus, your a bit off the field though mate with your guess.......I got the DD3 sussed already