Mobility degradation effects in CMOS differential pair transconductors

Abstract

The influence of the mobility reduction factor on the dominant third-harmonic distortion and effective transconductance in CMOS differential pair transconductors is examined. Analytical expressions are developed which are suitable for hand calculation and generate realistic estimates for distortion and transconductance. The results produced have been tested against SPICE simulations over a wide range of parameter values and show excellent agreement. The analysis highlights the importance of mobility degradation and reveals that the linearity of the source-coupled differential pair is actually improved as the mobility reduction factor increases. This surprising finding suggests that where θ≥0.15, for example, acceptably low distortion levels (<60 dB for V i =1 V pp ) should be achievable with the basic long-tailed pair and that complex linearization schemes may be unnecessary.