As a beginner I've had little luck finding a site that clearly explains the various configurations used to employ MOSFETs (both n-channel and p-channel) as switches. I finally figured it out (I think) so now I'm considering making a short blog post on it. Before I do though, I just wanted to check with the experts here that I've got everything straight.

The simple schematic I've come up with is this:



Does it look alright?

 

Try substituting a PNP for the p-mos transistor and an NPN for the n-channel mosfet to see if the circuits look right. The only difference (in these circuits) between a bipolar transistor and a mosfet is the gate (base) current and voltage.

Why did I say that? Because they look wrong to me.

 

Don't BJT's and FET's have different biasing requirements though? And can you be a little more specific about what looks wrong with those circuits?

Correctness aside for a moment, I was thinking it might be useful to group them together like this (and throw BJT's into the mix just to round things out):



What think ye?

 

Draw a built-in diode (it always exists) to the standard symbol and you will see that these diodes are included in the conducting direction!

 

Perhaps this will help.
For normal operation:

• An N-MOSFET has the drain voltage more positive than the source.
• A P-MOSFET has the drain voltage more negative than the source.

(Note that the Source is the terminal tied to the substrate diode).

All of your circuits above violate that rule.

 

Perhaps this will help.
For normal operation:

• An N-MOSFET has the drain voltage more positive than the source.
• A P-MOSFET has the drain voltage more negative than the source.

(Note that the Source is the terminal tied to the substrate diode).

All of your circuits above violate that rule.

Right, I should have spotted that!

Draw a built-in diode (it always exists) to the standard symbol and you will see that these diodes are included in the conducting direction!

Wouldn't that just clutter things up? Anyway, everyone knows that current flows from negative to positive.

Another aside, I was thinking it might be a good idea to simply blank out the specific load resistor values, just to prevent any confusion. Also, should probably change the 1K resistor values to read simply "bias", considering that the actual values needed varies. EDIT: On second thought, all that graphics editing probably isn't necessary. A quick explanation beforehand should be sufficient...

 

PNP transistors are also not included correctly!

 

PNP transistors are also not included correctly!

Sorry, yeah I guess I mixed up those terminal connections too. Thanks for the tip. Updated graphic:

 

Are you going to include "high side" switches in this?

 

Now the MOSFETs are incorrect again.

Perhaps you should hold off on the blog until you better understand how MOSFETs work.

 

Now the MOSFETs are incorrect again.

Perhaps you should hold off on the blog until you better understand how MOSFETs work.

Actually, looking at my original wiring arrangements (load on top, switches on bottom) I had all of the gates hooked up properly. But I was tired and in my haste to post I just made a bunch of sloppy transposition errors. Still no excuse, of course.



Now that you mention it, fact of the matter is I'd rather not undertake writing a blog post about this. Perhaps someone here with some real expertise would be willing to take on the task. Otherwise I'll try to cobble something together myself, but fair warning...might not be too pretty.

 

Are you going to include "high side" switches in this?

Sure, why not? I'll look into how it's done and post back later...

EDIT: Maybe I'm just confused, but isn't the circuit marked with the "777" load resistor an example of a high-side switch?

 

Maybe I'm just confused, but isn't the circuit marked with the "777" load resistor an example of a high-side switch?

You are confused.
A high-side switch goes between power and the load, which none of your examples show.

 

You are confused.
A high-side switch goes between power and the load, which none of your examples show.

I still don't quite grok the finer points of why it matters or for what applications it's more appropriate than a low-side switch, but yeah, I see what you mean now.

 

for what applications it's more appropriate than a low-side switch

For some applications it's desired or required that the load be grounded.

 

For some applications it's desired or required that the load be grounded.

I'm confused, you mean even while the device is powered off, or what?

 

I'll look into how it's done and post back later...

How about calling the FETs by their proper name (MOSFET) and using more conventional drawing conventions? And using the P devices for high side switching?

 

An emitter-follower and a source-follower as you show are not switches. A switch has its load connected to its collector or its drain.

 

you mean even while the device is powered off

Yes, it's connected to ground all the time.

 

How about calling the FETs by their proper name (MOSFET) and using more conventional drawing conventions? And using the P devices for high side switching?

Isn't the "metal-oxide-semiconductor" (ie: MOS) part of the acronym more of an antiquated reference to their original construction though?

As to the drawing aspect, I'm not exactly sure what a conventional diagram would look like! Would you mind providing an example? Right now I'm just using a screenshot of a simulated circuit and cropping it in GIMP, although I could probably find a decent EDA package that would produce better results.

And P devices better for high-side switching then? Okay.