problem with differential pair

ahmad1954 said:

I want to calculate linearity for a simple differntial pair. after that increase linearity with some methods and compare results. in many papers I saw that they draw gm for linearity and in some books gm is defined d(id1)/d(vid). is there any other way for calculating linearity or this is the only one?

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LvW said:

Linearity and the slope gm are different quantities.
You have a good linearity if the slope is relatively constant over a certain range of amplitudes.
But the degree of linearity depends on the required or desired amplitude range.
For example: As can be seen in the graphic from Keith the linearity is rather good for amplitudes of (+-) 30...40 mV

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