# NMOS with gate source shorted and negative threshold voltage First of all is it really possible to have a nmos with negative threshold voltage? If threshold voltage for M1 was positive, then M1 would be in cutoff. How do I analyze this circuit?

• I see two solutions: 1) M1 is a depletion type NMOS. 2) M1 is drawn incorrectly and should have been a PMOS. But think what happens when V1 = 5 V, what will M2 do? – Bimpelrekkie Sep 9 '19 at 10:02
• You mean that enhancement nmos cannot have negative threshold voltage? If it is depletion type, then how do I solve it? – Parth Sep 9 '19 at 10:05
• Since V1 = Vgs2 = 5 V > 1V, I assume M2 is in saturation. I find Id which comes out to be 0.8 mA. Now I need to verify that Vds2 > 4V or else Id I have found is wrong. How do I get Vds2 now? – Parth Sep 9 '19 at 10:17
• Since V1 = Vgs2 = 5 V > 1V, I assume M2 is in saturation If M2 is in saturation, its Vds > Vds,sat. How much is Vds,sat when Vgs = 5 V? Is that going to happen in this circuit? – Bimpelrekkie Sep 9 '19 at 10:51
• Vds,sat will be 4v for Vgs = 5V and Vth=1v. To get Vds > 4 (Vds,sat), I would need M1 to be shorted? – Parth Sep 9 '19 at 11:20

First of all is it really possible to have a nmos with negative threshold voltage?

Yes, it's called a depletion mode MOSFET and, in your diagram, the MOSFET symbol is a depletion mode MOSFET: - As wired in your schematic, M1 (being a depletion type MOSFET) is acting as a constant current source.

Picture from this document entitled "An introduction to Depletion-mode MOSFETs".

An example of a depletion mode MOSFET is the BSS126.

• Current source? Isnt this an NMOS inverter with depletion type load. M2 is in saturation region and M1 is in linear region? – Parth Sep 10 '19 at 2:37
• No, M1 will be in saturation region i.e. acting like a current source. For a MOSFET, the linear region is equivalent to the saturation region of a BJT so you may be getting your terms back-to-front? M2 may be in its saturation region but, your basic circuit doesn't have enough detail to definitively state this. – Andy aka Sep 10 '19 at 7:04