Parasitic reactive effects in transconductors based on differential pairs

The linearity of transconductors based on a single differential pair and on cross-coupled differential pairs is analysed. Both electrical simulations and theoretical calculations demonstrate that due to the parasitic reactive effects associated with the common-source operation the superior linearity behaviour of the cross-coupled connection at lower frequencies is not maintained for signal frequencies in the range of 100 MHz or higher. At such frequencies, the single differential pair is a better choice for reasons of bandwidth and economy of power dissipation