A low-distortion switched-source-follower track-and-hold circuit

A novel switched-source-follower track-and-hold (T/H) circuit has been proposed, where an input source-coupled pair conventionally used preceding the source-follower switch is replaced with another source-follower stage. Suppressing signal-dependent sample-timing jitter, which is due to the channel-length modulation of the input transistor, leads to a low-distortion T/H operation. Circuit simulation assuming a 0.1-μm InP-based HEMT technology has revealed an SFDR improvement from 58dB to 70dB for input and sampling frequencies of 3 GHz and 20 GHz, respectively. Although the present result is based on the high-speed compound semiconductor technology, the idea behind this can be applied to T/H circuits using nano-scale MOSFETs with reduced output impedance.