MOS-only allpass filters modified for low operating frequency range

The objective of this paper is to present a modification technique that can be used to alleviate the low-frequency limitations of MOS-only filters. The technique is based on adding an additional pair of cross-coupled transistors to modify the time constant of the filter. In this case, lower frequency operations is attainable without having to use large transistors with higher intrinsic capacitors; hence the chip area occupied by the circuit is kept low. Two existing MOS-only filter circuits are modified to illustrate the versatility of the idea and Spectre simulation results are provided in order to show the proper operation of the circuits.