# zero AC gain of CMOS inverter

1. ## zero AC gain of CMOS inverter

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

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2. ## Re: zero AC gain of CMOS inverter

The feedback resistor has a value of 1E20 ??? Typing error?

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3. ## Re: zero AC gain of CMOS inverter

vout/vin isn't zero, it's about -42 at 1 Hz starting frequency according to the plot. At GHz frequencies, vout is shorted by 1 nF Cout. Unreasonable setup, inappropriate plot parameters. You may want to plot with log frequency scale to better see what's happening.

The feedback resistor has a value of 1E20?
The resistor has no effect during first part of AC analysis, the inverter is effectively operated open loop. By chance, it's biased almost to maximum gain, see DC analysis plots.

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4. ## Re: zero AC gain of CMOS inverter

See the alter section which basically trying to switch on and off certain branches of the circuit.

By the way, removing cout solves the zero AC gain issue. Note that the RC time constant = 100G*1n = 100s , or 0.01Hz filtering.

5. ## Re: zero AC gain of CMOS inverter

Note that the RC time constant = 100G*1n = 100s , or 0.01Hz filtering.
Wrong, the gain is limited by the time constant of inverter output impedance and 1 nF Cout, I guess µs range respectively some 100 kHz cut off. You still didn't plot the gain with reasonable frequency scale?

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6. ## Re: zero AC gain of CMOS inverter

You still didn't plot the gain with reasonable frequency scale?
I do not understand what you mean ? You mean in logarithmic scale ?

Or you mean something else ?

7. ## Re: zero AC gain of CMOS inverter

Yes, as suggested in post #3.

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