# Voltage subtractor using MOSFET

I am new to this site and maybe I won't ask a good question but anyway.

I have come up with a voltage subtractor using two MOSFETs and a voltage divider well sort of.

simulate this circuit – Schematic created using CircuitLab

Vout = VCC - VM2

If Vu < Vtm2 -> Vout = 0

If Vu > Vtm2 and Vd < Vtm1 since R1 >> Rm2 -> Vout = VCC

If Vu > Vtm2 and Vd > Vtm1 since Rm1 || R1 and Rm1 << R1 the resistance of the loop is Rm1.

The resistance Rm1 depends on gate voltage (Vd) as well as Rm2 depends on gate voltage (Vu).

We want when Vd = Vu Vout to be 0 (substractor Vu-Vd). However if M1 is M2 then when Vd = Vu -> Rm1 = Rm2 -> Vout = VCC/2.

So the rate at which resistance changes according to gate voltage in the M1 must be bigger than M2 so that Rm1 << Rm2 << R1 -> Vout = 0.

MSF1's resistance changes more rapidly with increase in gate voltage than MSF2.

However currents using MOSFETs are very big some amps while I would like this to work for mA. How can I achieve this?

• Welcome to EE.SE. The circuit diagram is quite blurry. Guidelines for a good photo of a schematic. you can also use the schematic editor to add a good diagram.
– AJN
Aug 15, 2020 at 9:59
• "MSF1's resistance changes more rapidly with increase in gate voltage than MSF2." Why is this restriction necessary ? Can you write down the equations for $V_{out}$ as a function of the input voltage ? "However currents using MOSFETs are very big some Amps" Where does this restriction come from ?
– AJN
Aug 15, 2020 at 10:04
• AJN the VI curve of VI of a MOSFET ( VG and ID) is on amps. Aug 15, 2020 at 10:09
• What is VM2 meant to be? Ditto Rm1 and Rm2? Aug 15, 2020 at 10:47
• Vm2 is the voltage drop on M2 mosfet , Rm1 and Rm2 are the resistances of M1 and M2 mosfets. Aug 15, 2020 at 11:02