mosfet circuit - need feedback?

Yes, DC feedback is required to control the bias point. Un-buffered CMOS logic gates can be biased as linear amplifiers by connecting a feedback resistor from the output to the input. You don't need a VCVS. Just connect the output to the input.

The 100 mA is a huge current; is that really 0.1 amps? If the current mirror works, node 4 will being pulled up very hard, far too hard for the wimpy n-ch fet to pull it down with just +1 on its gate.

But the Vgs on the topside mirror fets is unreasonable, too, so something else is wrong. What's the voltage on n1?

Logic gates do not commonly have feedback, unless they're Schmitts.

John

Hi John,

My apologies, you are right - it should have read 100uA, not milli-amps. I've made an updated schematic:

Cheers,

Ted

Well, it ought to invert; a real circuit built with real parts sure would. But +1 is awfully low to turn on the n-fet, depending on the process of course.

Any idea of the transfer curve of these p and n-channel fets? You might pull them out of this circuit and simulate each alone, just plotting Id as a function of Vg, with 3.3 from source to drain. That's a basic sanity check on the part models. The input logic threshold will then be the point where the n-ch fet conducts about 100 uA. The overall circuit should have a huge voltage gain from the n-fet gate to its drain, once you find the active region.

Plot V4 versus applied V5, over the full V5 range of 0 to +3.3. Should be an s-curve, pretty steep in the middle.

For extra credit, get some real fets and build the circuit and test it. Simulation ain't life.

John

Hi John,

I think there's a misunderstanding here (maybe it's me not understanding your reply?), I'm not working with logic fets, I'm using cmos fets for analog design (well, analog design lectures, at least!). So the +1V DC biases the nmos in saturation, then I can apply my AC (small-signal) to the input and invert it at the output.

It matches what you describe:

Cheers,

Ted

It sure looks like it's working fine. But +1 volt isn't quite enough to turn on the n-fet decently; you need just a tad more to get it into the nice steep part of the curve where it has incremental gain.

OK, if you want to use this in its linear region, as an analog amp, you will need feedback in this stage, or overall feedback in some bigger external loop, to keep it reliably biased in the good part of the transfer curve. The threshold voltage isn't predictable enough that, in real life, you can just jam a dc bias into the fet and expect it to be in a usable part of the transfer curve.

I mean, you can tweak the bias up to 1.07 or whatever and demonstrate a large AC gain, but you could never manufacture something like that... it usually wouldn't work. Hence the need for some sort of feedback. Or a diffamp or something.

John