AnalogDE wrote on Dec 3rd, 2010, 10:08am:I
What does source degeneration buy me and how do I go about sizing the resistor? (I need to review my theory...)
Also, for a high valued resistor it'll have to be implemented as an nwell resistor -- so another source of variation in process and tempco....
You can probably get away without it, but it will add another degree of freedom if the pure NMOS does not work. An ideal resistor will make the source less sensitive to variations in Vt since its current is proportional to V, not V^2. It will also increase output impedance compared to a pure NMOS of the same length. Not sure if the Nwell will make it better or worse.
For sizing you have two extremes:
1) NMOS is weak inversion with a resistor: I =~ (Vref-Vt)/R
2) Pure NMOS: I =~ βW/L(Vref-Vt)^2
Play around with different values between these two extremes to get a decent PVT performance and then scale everything to get the proper current.