Relationship between large and small signal linearity

Hi,

As far as I know, small signal non-linearity in MOSFET is mainly due to non-linear behavior of transconductance and output conductance. It is generally measured in IIP3 value. On the other hand, large signal non-linearity of MOSFET in an amplifier is related to its maximum linear swing range. If the swing range is higher, large signal non-linearity will also be higher. Large signal non-linearity is generally measured in 1dB compression point. I want to know what is the relationship between small and large signal non-linearity? Generally its said that P1dB is approximately 10dB lesser than IIP3. Why is this the case because as far as i can understand, large and small signal non-linearity originate from different sources. How can improving one help to improve other?

Thanks

I'm not aware of "small signal linearity" being a commonly understood technical term. Anyway, rather than being directly measured like 1dB compression point, IIP3 is calculated based on an assumed polynominal transfer characteristic. For a brief explanation, see https://en.wikipedia.org/wiki/Third-order_intercept_point

Strictly speaking, IIP3 and compression point are not directly related.

IIP3 term is not a Small Signal Non-linearity metric,it's just measured approx. 20dB below p1dB Compression Point and it's not a real point, it's only an extrapolated point.
Small signal does not create a harmonic distortion because the deviation from linear region with this applied small signal is negligibly too small.( see Taylor Series Expansion of an Active Device )

Small signal linearity is often an -assumption- used to
simplify (or make at all tractable) circuit analysis. You
will find very few dead-linear active devices; linearity
is an ideal. Many devices have regions of "good enough".

If a device is large signal linear then it will usually be
small signal linear. However a small signal linear (as
tested) device may become large signal nonlinear due
to things like clipping, or being moved out of the small
signal bias conditions, by larger signals. I have run
into some instances of a device that looks large signal
linear, being in fact small signal nonlinear - for example,
MOSFETs in development with "channel hookup" problems
that appear high impedance with Vds~0V, Vgs>>VT and
need significant Vds to connect the channel to the drain.

In "RF land" the nonlinearity of interest may be the Cdb,
Csb, Cgb capacitances' C-V swing more than ohmic
nonlinearity - especially for well-driven CMOS switches.
This is why FDSOI wins - bottom plate and sidewall Cxx
are now non-semiconductor, so linear (constant) terms.

IP3 can be measured with two tones at low power outputs, and "extrapolated" to higher power outputs. but the extrapolation is ... shaky at best.

Also, like the OP said, sometimes only large signal effect can generate what you want to . For instance, only under actual large signals will you see charge storage in traps, and their random release some time later, that contribute to shot noise (or phase noise/time jitter).