Is the IP3=d3IDS/dVGS3 equation correct for Linearity IP3?

[M.B.]

Hello Friends,

Can any person tell me if this information is correct: The third intercept point IP3 is equal to the third drift of the drain to source current compared to the third drift of the gate to source voltage i.e. IP3=d3IDS/dVGS3.
If it is correct, please send me the reference of this information.


Best regards and good work.

 

[imar]

ip3 polynomial

why don't you try to take a look to razavi books,maybe it could help:|

 

[PoojanWagh]

intercept point ip3

If you mean derivative by drift (d^3 IDS / dVGS^3), then no, it is not.

IP3 is found by approximating a nonlinearity as a third-order polynomial:

y = A*x + C*x^3

(I omit the B*X^2 term for brevity).

As signal increases, A*X increases 1 dB for each dB x increases.
At the same time, C*X^3 increase 3 dB for each dB x increases.

Generally, A*x starts out much smaller than C*x^3. However, at some (fictious) point, C*x^3 catches up to A*x. This point, where |A*x| = |c*X^3| is the third-order intercept point. You can specify either x (input IP3 = IIP3) or A*x (output IP3 = OIP3) at this intercept point.

This point may be fictitious because the equation y= A*x + C*x^3 is a small-signal (but nonlinear) approximation. In reality, the circuit will clip. You must ensure you are putting small signals into the circuit to ensure it does not clip. In doing so (inputting small signals), you will find that the polynomial approximation is pretty good.