How to bias a CS mosfet

Two so far. Pretty interesting kids. Both Giants fans.

John

Practically .sig material!

Actually I can see how an OP-AMP might serve a pedant who wants to comply exactly with the OP's request to stabilise Vd at exactly 1/2 Vdd (even if Vdd has a large range).

My suggestion is a minimalist solution that develops a constant average voltage across Rd so it is possible for falling headroom to starve the MOSFET.

Not *EXACTLY* what the OP asked for, but a close approximation that works well enough in most practical applications and doesn't totally overwhelm the original MOSFET with about 8x the original board space full of bias setting components.

It worked perfectly OK in the application I designed it for and very likely would have done so for the OP too, even though it was a minimalist design that could be modified or added to and improved to fit the OP's application.

The OP was free to consider the suggestion and reject it if unsuitable and move on - something you also had the option of doing instead of going off on one.

I don't know--op amps are pretty cheap, and even a 324 will do a good enough job that you don't have to worry about it. If sloppy is good enough, you can just use a voltage divider from drain to gate, and have the drain sit at some multiple of the threshold voltage.

I do lots of discrete designs, but this one seems like a good candidate for a quarter of some cheap quad op amp, or even a massively overcompensated section of a 339. Either way, you can run it at higher impedance, which means the compensation cap will be cheaper, and it'll very likely take less board space and be more reliable.

Cheers

Phil Hobbs

An opamp could combine the AC-coupled signal and the DC feedback, all into the gate. I'm not sure why.

John

I wouldn't like to try and guess how many different ways the OP's requirement could be met, I put forward my suggestion for a minimalist approach and discovered that as far as JF is concerned the right to free speech doesn't apply to me!

I have less than zero interest in flame wars, and there aren't many personalities round here that are interesting to anyone other than their possessors. We do talk about some interesting circuits sometimes, though, which is my interest.

Frequently it's possible to confine the flame wars by throwing in a political or climate post--it works a bit like a doggie treat.

Control-k in Thunderbird gets rid of most of the noise while that's going on, which is a relief. I don't do it very often, though, because it's depressing to have to treat adults that way.

Cheers

Phil Hobbs

I was thinking along the lines of generating an error voltage by comparing Vd to a resistive 1/2 Vdd divider. With a generous helping of nfb to avoid the O/P saturating to one rail or the other.

It was one of the options I considered too complex for the application so haven't put pen to the back of an old envelope.

I think Ian Field is a Larkin wannabee. ...Jim Thompson

--- Less than zero still indicates an interest, but from a different viewpoint.

I kinda like flame wars every once in a while, since they're a little like mental escapades in logic and language and allow us to test our literary mettle against anyone who rides into Dodge for the purpose of flaunting their own mettle.

-- JF

You do not post full spice sims (no capitalization needed, idiot).

You post fool pathetic excuses.

That would decidedly make you the simple one.

I like doing that in front of a whiteboard, _designing_ something real that works. And it's only fun if I'm with other people who don't mind having their ideas demolished. I don't mind it at all myself--I'd far rather have someone show me what's wrong with my ideas in 10 minutes than have to spend a week building stuff and finding out the hard way. Prima donnas are a pain in the tuchis, and IME they're never the top-tier designers, though they always think they are. With smart, creative people around, there's never a shortage of good ideas, so why get attached to bad ones just because they're yours?

One thing that depresses me most about the SED flame wars is that the participants are so very attached to their (quite imaginary) dignity. Any person with actual dignity wouldn't be doing it.

The other main thing is that there's so much interesting electronics to talk about, and some of those folks could contribute some instead of worshiping themselves and dissing others.

Cheers

Phil Hobbs

Exactly. It IS, however, a way to examine and SIMULATE circuits.

The only thing is that simple twits like Ian and you shirk away from them because they would show your lack of understanding... of both them and the electronics rules they are based on.

If you put it into a sim, you have already created it, dumbass, even before you hit the power switch. Are you so dumb as to not see that?

The truth is that you are worse than Ian, because you supported his lack of ability to post a sim, and you are now deliberately making it appear as if someone in this group that wants a sim needs to want a complex circuit sim only, and no 'simple' circuits are allowed.

How can you not see just how stupid that behavior makes you look?

Oh... nevermind. Stupid does as stupid is. Bwuahahahahahahahaahah!

I have in a few cases, where it was interesting.

OK, post a Spice sim of your own. Then we can talk about it.

John

You haven't created it if you copy and paste it from a newsgroup.

Post a Spice net that _you_ have created. If you post an interesting circuit, I will too.

John