MOSFET Voltage Regulator

[Jon86]

So I built up this circuit for a Low Dropout Mosfet based voltage regulator, I tested it in multisim and it works perfectly with a dropout of less than 0.5v.
After reading about N-Channel mosfets I'm beginning to think it won't work. Can anyone tell me what will happen when I hook this circuit up? 
Why would an N-Channel not work? 

[Dave]

I think it will work, but it won't be anything near low dropout. An average N-channel MOSFET has the Vgs threshold at about 1.5-2V. Your opamp output (the gate of the FET) will go close to the positive power rail, so that means the source will be 1.5-2V lower than that. That means at least 1.5-2V dropout.
Also, you will need some capacitors on the input, output and in the feedback loop, so your circuit won't oscillate.

[Jon86]

Thanks for the response, but I'm just not understanding why the the dropout will be so high? I mean, surely if the the Opamp takes the gate to the threshold voltage then the gate will turn on completely and have only a few mV of drop?

[cyr]

Yes but the opamp can't drive the gate any higher than the input supply voltage, unless you power it separately from a higher voltage supply of course.

[Jon86]

But the gate doesn't have to be higher than the source voltage, does it?

[Andy Watson]

But the gate doesn't have to be higher than the source voltage, does it?

Yes.

[Jon86]

Ah. So with a P-Channel can it be lower than the source?

[Dave]

Correct.

In fact, the gate potential HAS to be lower than the source, if you want to turn a P-channel FET on.

[Jon86]

Okay thank you. I finally understand the difference now

[Frant]

Okay thank you. I finally understand the difference now

Yes, but it's not so easy. There are integrated voltage regulators with a P-channel MOSFET and some rare examples with an N-channel MOSFET and a charge pump circuit providing the needed gate voltage higher than that at the input. A positive voltage regulator based on a P-channel MOSFET (or a PNP transistor) can have stability issues. Voltage regulators have been an ubiquitous topic for decades. All the major manufacturers design and make them all the time, so we can assume that they have already tried just about everything.