# Amplifier Linearity measurement in CADENCE (using the calculator)

1. ## Amplifier Linearity measurement in CADENCE (using the calculator)

Hi all,

[Thank you note]
This is my first post so I just wanted to thank you all for all the stuff you have put up here, it has helped me a lot and I hope I can at least give back as much as I learned...

[Serious Stuff]

So I am designing a linear low frequency amplifier and I am looking for an expression to measure the gain compression (the amount that the gain drops as the output approaches saturation) for a specified output voltage swing AS A NUMBER

Again... I already know how to see it on a plot, but I need the calculator to give me a NUMBER.

To see it on the curve, I plot Vin vs. Vout, looks something like this:

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i deriv() it to see a waveform like this:

_/``\_

the middle is where we are getting gain and it obviously drops as the output goes toward VDD or GND. I then measure how much the gain "compresses" near the voltage swing limits I am looking for.

Some notes:

1-
I am not sure that using the "compression" function in the calculator is a good idea cuz I am not working high frequency, not running PSS, just a simple vin/vout sould do the trick

2-
Some dude told me to use THD. I dont think he is correct... THD is calculated for a given frequency, right? I dont think it appropriately demonstrates the linearity for low-frequency amplifiers (no carrier). But then, I'm pretty sure that THD and compression-point are related, i'm confused...

So what do you guys think?

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