# how to fix output voltage of an NMOS switch with changing load characteristic?

In an NMOS low side switch the output voltage at drain depends on the operating point decided from the Id-Vds curve and load line.

According to my understanding, for a low side switch with Vds around 0, we need to keep the operating point in triode region. This can be achieved by keeping Vgs-Vt>0 and relatively high load resistance to keep the NMOS in triode region. I hope this is correct?

Further, for my application, I am using NMOS switch for providing ground voltage to a memristor which is connected between Vdd and Drain terminal whose resistance changes during the ON cycle. This leads to a shift in Drain voltage as the memristor is being programmed for desired resistance. The shift is undesired as i need a constant voltage across memristor during this ON cycle.

The shift is seen from the following SPICE simulation(Vx1-).

So my final question is, How can i make a low side switch for this application which will provide a constant Vds with changing load resistance?

• How "constant" do you need the voltage to be, and over what range of $I_{DS}$ values? – Elliot Alderson Apr 11 '19 at 22:44
• The voltage must be within 0.2 volts of programming voltage at all times. However, i just found out that this changing voltage might be due to bad NMOS model selection in LTSpice, As detailed in this question electronics.stackexchange.com/questions/419953/… However, I am still confused as to how much does effect does changing the load have on output voltage and how to counter that. – SHIVAM RISHI Apr 11 '19 at 23:21
• Again, over what range of current values? This is critical information. Essentially, you choose a MOSFET that has an $R_{DS}$ low enough that your maximum current doesn't cause too much of a voltage change. – Elliot Alderson Apr 11 '19 at 23:45
• The Ids range is 30uA to 9mA, so i should find a NMOS model where 9mA*$R_{DS}$ is less than 0.2 volts? – SHIVAM RISHI Apr 12 '19 at 0:07
• Yes, and there should be many that fit that requirement. – Elliot Alderson Apr 12 '19 at 11:32