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-). enter image description here

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?

  • \$\begingroup\$ How "constant" do you need the voltage to be, and over what range of \$I_{DS}\$ values? \$\endgroup\$ – Elliot Alderson Apr 11 '19 at 22:44
  • \$\begingroup\$ 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. \$\endgroup\$ – SHIVAM RISHI Apr 11 '19 at 23:21
  • \$\begingroup\$ 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. \$\endgroup\$ – Elliot Alderson Apr 11 '19 at 23:45
  • \$\begingroup\$ The Ids range is 30uA to 9mA, so i should find a NMOS model where 9mA*\$R_{DS}\$ is less than 0.2 volts? \$\endgroup\$ – SHIVAM RISHI Apr 12 '19 at 0:07
  • \$\begingroup\$ Yes, and there should be many that fit that requirement. \$\endgroup\$ – Elliot Alderson Apr 12 '19 at 11:32

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