A novel input differential pair for improved-linearity buffer and S/H amplifier design

As the speed and linearity requirements become harder to meet for analog circuits, it is no longer enough to rely on feedback to enhance linearity. Negative feedback suppresses nonlinearities by a factor equal to the loop gain. This works well at low signal frequencies, but as the signal frequency increases the loop gain drops. Thus, with a limited bandwidth available, nonlinearities should be suppressed at their origin. In this paper, we consider a major source of nonlinearity, namely the input differential pair (IDP) with a cascode current source, commonly used in op-amps. Then a new and simple IDP with a matched channel-length single-device current source is introduced. We show that the modified IDP is more linear and thus creates less harmonics than conventional ones, and that by satisfying a nonrestrictive condition all even harmonics can be eliminated. Experimental results are described which verify the expected reduction of harmonic distortion