[BJT] Fixed Ice, what size has better linearity?

[_RFIC_]

In mixer design, the RF input transistor has more concern about its linearity. When using bipolar with fixed tail current, what kind of size has better linearity?

Is a bipolar with larger emitter area has better linearity? But:
Larger emitter area --> smaller current density --> lower ft

How about emitter width and length?
eW --?-- linearity
eL --?-- linearity

 

[elbadry]

I think for bipolar input pair you would always need a resistive degeneration at the emitter to improve linearity. I am not sure, but I think linearity in this cae has nothing to do with the area. I understand this as follows,; for a bipolar pair

I1-I2=Io*tanh(Vi/(2*Vt))...regardless of sizing.

If headroom is a concern, you can use two tail current sources (one for each transistor) and have the degenration resistance connected between the two emitters.

You can also improve linearity by the so called schmook's technique (Refer to "RF microelectronics" Behzad Razavi, the chapter on mixers)