MOSFET source/drain ... is there a difference?

[bluefoxicy]

MOSFET seem to physically have a Gate, Body, and N+ called a "source" and "Drain." I see no difference between source and drain, can I just assume either pin is either and hook them up however, as long as I assume one is source and the other is drain and keep to that?

Also I am trying to liken a MOSFET with Gate-Body-Source-Drain to a triode. My understanding is that the Gate acts as the Grid in a single triode vacuum tube (i.e. a 12AX7 stage). Does the Drain then act as the Anode (plug into B+) and the Body and Source connect together to act as the Cathode (connected to the negative rail?

I don't understand the body...

In audio amplification, should I rely on a Depletion MOSFET or an Enhancement MOSFET? I would believe a Depletion MOSFET to work better when dealing with things like a cathode follower for a tone stack, because it doesn't need a sudden high-current spike on the Gate to turn it on before it starts working...

[Smack]

There is no diff between the source and the drain.


and consider the body like a reference for the gate, if the voltage at the gate is a certain value (compared to the body) it allows current to flow from the source to the drain or vice versa..

its all I got hehe

[Don Moose]

Most (enhancement mode) MOSFETs that you can actually buy have only 3 terminals - Gate, Source and Drain, the substrate (body) is tied to the source. This forms an embedded body diode between Source and Drain.

An N-channel MOSFET's body diode is Source-Anode, Drain-Cathode and the transistor requires a positive Vgs to turn on. The P-channel MOSFET's body diode is Source-Cathode, Drain-Anode and the transistor requires a NEGATIVE Vgs to turn on.

The JFET's source and drain are interchangeable as there is no body diode to worry about forward-biasing by accident.

Hope this helps!

[Steve Conner]

Yes. JFETs have interchangeable source and drain: MOSFETs don't because of the body diode.

[bluefoxicy]

Okay so basically my understanding of the physics is correct, but manufacture adds an additional element (a P/N junction diode buzzword thingy) that negates this. Got it.

[Enzo]

I think you were calling all FETs MOSFETs. There are MOSFETs and there are JFETs. You might look them up for a description on how each works. The MOSFETs we see are usually the power transistors in the output stage of amps. The small signal FETs like used as channel switchers, mutes, and gain stages in preamps are generally JFETs.

JFETs are actually ON until turned OFF by their gates. Test a little J112 or similar small JFET with your meter. You will find a relatively small resistace netween source and drain terminals - 200 ohms comes to mind - and the gate will act like a diode towards them. That is how I verify lead pinout on JFETs. (They are not all the same)

[bluefoxicy]

The MOSFETs we see are usually the power transistors in the output stage of amps. The small signal FETs like used as channel switchers, mutes, and gain stages in preamps are generally JFETs.

http://www.geofex.com/Article_Folder...osfetfolly.htm

Used for other things (source follower after a gain stage leading into a tone stack, for one), but I get your point.

JFETs are actually ON until turned OFF by their gates.

Then a JFET will invert a signal (high voltage in means low out). Now I just need to figure out how to make it 1:1 and ultra-linear, and responsive to extremely low currents and voltages.

I really need to learn some actual electronics @_@

[Don Moose]

Then a JFET will invert a signal (high voltage in means low out).

The J112 is an N-channel Depletion mode JFET - a LOW voltage turns it OFF.

It does invert, though if connected like a typical triode - Gate Low -> Rds high(er) -> Vds high(er)

Hope this helps!

[Enzo]

Don, do you mean low as in negative? I mean, little or no voltage on the gate leaves the JFET on, while sufficient voltage turns it off. Obviously polarity must follow whether it is N channel or P channel.

The JFET inverts the same way a triode inverts or even a bipolar transistor. It inverts when the circuit makes it do so. A source follower wouldn't invert, for example.

[Don Moose]

Don, do you mean low as in negative?

Exactly - negative relative to the lower of the other two terminals. Depending on the circuit, the voltage could still be positive wrt GND.

The JFET inverts the same way a triode inverts or even a bipolar transistor. It inverts when the circuit makes it do so. A source follower wouldn't invert, for example.

Hence 'typical triode' - that's a common-cathode stage, right?

[Enzo]

Yes, meaning it is not the FET-ness that inverts the signal. it is the circuit configuration that does that.

OK, understood. I would not have used the term "lower" that way myself, but your message is correct.

I recall the old magazine Popular ELectronics way back when, and they decided that they would always refer to making a voltage more negative than it was as "reducing" it, at least during one editor's era. SO they would say "reduce the -15v to -30v..." This was always a pet peeve of mine. Increasing the potential from 15 volts to 30 volts is not a reduction no matter what the polarity. At least in my book.

I realize that dropping from 30 to 15 is indeed a reduction and would affect the JFET if the whole deal were sitting at +30 just as if the whole thing was at zero and the gate went to -15. But i feel the reference of what to do with the gate should be made with respect to the transistor. But that is just me.

[Don Moose]

I learned a long time ago not to be touchy about confusing people - if I fail to make myself understood, that's something for me to overcome, not an excuse to poke at someone or get all wrapped around some semantic axle (no, you weren't poking, we were aligning terms and definitions).

All good.