Synthesis of Wideband High-Quality Factor Delay- Tunable Fully Differential All-Pass Filters

In this article, all possible wideband fully differential second-order voltage-mode active all-pass filters (APFs) based on a common-source differential amplifier are systematically derived using two-port network modeling techniques and symbolic math CAD tools. The proposed APF structure is based on the generic two-transistor differential-pair and all possible combinations of the surrounding impedances. In particular, 26 transfer functions and a total of 180 APF circuits, all of which can have a pole quality factor (Q) value greater than unity, are found to be possible. Hence, the proposed topology can be used to produce APFs with dispersive delay as well as flat delay for true time delay elements. The operation of some APFs is verified with simulations and a selected circuit with tunable peak delay is designed and fabricated in a 65-nm CMOS process. The implemented filter achieved a maximum peak delay of 88.4 ps at 31 GHz, a constant delay of 11.5 ps over the frequency band 5-20 GHz, and a delay Q-factor (QD) value of 3.74, while consuming 1.74 mW from a 1-V supply voltage. The fabricated circuit occupies an area of 0.11 × 0.348 mm
²
and experimentally demonstrates an APF with the highest Q value of any published second-order APF known to the authors.