Why does the vin=vout takes place at the voltage of (vdd/2) only in the case cmos inverter characteristics? and how to estimate the value of (vout) in every case, so that we can establish relationship between vgs and vds.

  • \$\begingroup\$ which cmos do you want to analyze and why? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 8 '19 at 12:35
  • \$\begingroup\$ Cmos inverter which is a combination of enhancement type of pmos and cmos \$\endgroup\$ – sai sri datta Jun 8 '19 at 12:36

Well, a CMOS inverter is the only gate that has a single Vin. Life is much more complicated with multiple inputs. Inverters are also usually designed so that the PMOS and NMOS transistor have relatively equal current drive characteristics, so the switching threshold tends to be close to Vdd/2.

If you want to analyze the relationship between Vin and Vout, just write the equations for Vds vs. Vgs for both the PMOS and NMOS. Set Vgsp = Vdd - Vgsn, and set Vdsp = Vdd - Vdsn. Solve as desired.

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  • \$\begingroup\$ Thank u sir but we know that vgs=vin-dd and vds=vout-vdd , how to know vout so that we can write any sort of equation for conditions like vds<vgs-vth like that \$\endgroup\$ – sai sri datta Jun 8 '19 at 13:07
  • \$\begingroup\$ I assume you are still talking about a CMOS inverter with no load on the output. This implies that Idsn = Idsp. So, if you have equations for Ids of the PMOS and NMOS just set them equal to each other. Make the substitutions for the PMOS Vgs and Vds. You are left with an equation with two unknowns: Vgsn and Vdsn. Choose a value for Vin, which equals Vgsn, and solve for Vdsn, which equals Vout. \$\endgroup\$ – Elliot Alderson Jun 8 '19 at 16:28

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