# How to design a discrete linear regulator with an opamp and an N-channel MOSFET?

So as far as I understand, the basic idea of a linear regulator is to use a series-pass element along with an opamp with negative feedback to stabilize the output. This is what I usually see as an example:

simulate this circuit – Schematic created using CircuitLab

Now, to use a MOSFET in place of the BJT, can I just do a direct replacement like this:

simulate this circuit

Or should the gate of the MOSFET be driven by another BJT like below?

simulate this circuit

Or does it not make any difference? Is one more stable than the other?

Also I remember reading somewhere on the internet that in order for the opamp to regulate correctly, a small current should flow out of the output through the negative feedback loop, but a MOSFET gate doesn't allow any current in, which is also confusing.

• Remember that a MOSFET has a minimum gate-source ON voltage, thus your op-amp and Vin should be able to source the voltage needed to saturate the MOSFET ON. For standard MOSFET's this is about +10 volts. Thus if Vin is => 12 volts it should work just fine.
– user105652
Jun 7, 2020 at 4:57
• @Sparky256 you mean just the MOSFET by itself, right? Do I not need a BJT to drive the gate? But yes, the input voltage is greater than 12. Jun 7, 2020 at 5:01
• Don't think the third one will work. The BJT will dump charge into the MOSFET, but there's no discharge path. Gate leakage (if any) likely won't be enough to discharge it either. So this will end up just turning on the MOSFET and no proper regulation. regarding your comment on the current feedback: maybe add an RC network between the output and negative terminal? In general that configuration can help improve phase margin. Jun 7, 2020 at 5:01
• @MatthewDiNardo I see. Can you please show me what that RC network looks like? Jun 7, 2020 at 5:03
• parallel resistor and capacitor Jun 7, 2020 at 5:04