How to calculate the gain of an amplifier?

[Question]

The simulation result of an amplifier is different from the result that I calculated by formula. I design an amplifier. Using spectre simulator, the gain is more than 80dB. But using the formula Av=Gm*(Ro||Rl), I can not get the same result. It is much less than the simulation result.

I get the parameter from the DC operating point of spectre. The Gm of input

transistor is near 150u, and the Ro of active load is 300K. So the gain is only 33dB.

I can not explain it, who can help me?

I use folded cascode diff-amp with active load.

Thanks a lot!!

 

[wholx]

hello Question,
i'm a little puzzle by the formula you used to calculate the voltage gain for a cascode.
normally, should it roughly equal to gm1*gm2*ro1*ro2?

 

[elbadry]

I think the formula is correct if you are using a simple folded cascode (only a single PMOS and a single NMOS in the output part).

But I think that typically, hand calculations are not quite accurate. you ignore many effects (vth variations due to substrate effect for example). Besides ,the square law formulas are far from accurate specially for modern sub-micron technologies.

 

[eng_Semi]

Try to check that all transistors are in saturation, coz if u have any transistor in the triode it will heavily decrease the gain.

I suffer from the same problem before, and when I adjust the biasing the gain jumps.

I hope that this is useful