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I'm working on a problem that requires me to change the switching threshold of a cmos inverter to a different Vout when Vin is the same. All I know is this relates to the W/L ratio of pfet and nfet. Does anyone know more detail about such kind of modification? Thanks a lot in advance.

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  • \$\begingroup\$ Vout is dictated by power supplies. The switching threshold is not related. \$\endgroup\$
    – Andy aka
    Commented Nov 8, 2021 at 15:55
  • \$\begingroup\$ If you have the "point C" situation as shown in the graph. What would happen if you would double the Width of the NMOS? First assume that the input voltage stays the same, what happens to the output voltage, will it stay Vsp or...? \$\endgroup\$ Commented Nov 8, 2021 at 16:04
  • \$\begingroup\$ @Andyaka The OP is asking about the switching threshod, which is defined as the dc point where Vin = Vout. For a given value of the supply voltage, Vout at the switching threshold is determined by the transconductance of the transistors. \$\endgroup\$ Commented Nov 8, 2021 at 16:06

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You should know how to find the drain current for a transistor with given values of k' and W/L for specific values of the gate-to-source and drain-to-source voltages.

You know that the drain current through the NMOS and PMOS must be the same.

Since Vin = Vout you can express all of the voltages in terms of Vout.

You will be left with one equation to solve. Use that to find the transistor W/L ratios for a given Vout or to find Vout for given values of transistor sizes.

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Just to add intuition to other answers, I highly recommend to take any spice program and stepping w/l parameters, with a dc sweep of the voltage input. Look at the Vout for a fixed vin and see what happens. Does Vout stay the same for a fixed vin? If not how did a different W/L effect that?

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