# zero AC gain of CMOS inverter

Why is the following AC analysis of CMOS inverter resulted in zero AC gain (vout/vin) ?

• Feedback resistor 1E20 ??? – LvW Jan 20 at 11:00
• @LvW see the "alter" section – kevin998x Jan 20 at 11:16
• The phrase "alter section" may mean something to you but it does not mean anything to those of use who use other simulators. Try to be more clear with your statements. Also, using a screenshot as a graphic is asking for trouble...it's low quality. Try to export graphics from the software instead. – Elliot Alderson Jan 20 at 17:00
• @ElliotAlderson alter is just to change the value of the parameter, in this case, switch to Rs2, open Rs1, turn on the Rf – kevin998x Jan 21 at 0:48

Removing Cout solves the zero AC gain issue. Remember the RC constant = 100G * 1n = 100s, so 0.01Hz filtering on the AC gain plot result.

Your source is DC coupled, with a very low source impedance, and it's pulling the input down out of the active region.

You need to put a capacitor in series with RS1, so that Rf can then bias the inverter back into the active region.

As others have said Rf is probably a bit big, normal values are 1E6 - 1E7.

• see the "alter" section. The AC analysis is done without the Rf (by making it extremely large Ohm) – kevin998x Jan 20 at 11:20

The incremental gain is change in Vout over the change in Vin is and not just d3/d4.

The 1st plot appears to have a peak gain ~ -3.34 at Vdd/2 for Vdd=1V

• I am asking about "AC" gain – kevin998x Jan 20 at 11:17
• I am referring to the vout/vin plot in ac analysis – kevin998x Jan 20 at 11:19
• Fix your program Av=ΔVo/ΔVin – Sunnyskyguy EE75 Jan 20 at 11:25
• I am looking at ac1 result window (vout/vin), how do you get slope of 4.5 ? – kevin998x Jan 20 at 11:26
• bad eye, I meant 3.3 – Sunnyskyguy EE75 Jan 20 at 11:33