| A current goal in electronic system design is to develop low-voltage, low-power, low-cost, small-size, and high-dynamic range circuits. Analog filtering is a key signal processing application, and filtering approaches have been developed to fulfill the concurrent requirements of electronic systems. Log-domain filtering is a useful filtering approach, which exploits the nonlinear exponential relation between the current and voltage in the bipolar junction transistor (BJT) or the metal-oxide-semiconductor (MOS) field-effect transistor (FET) in the subthreshold region. Log-domain filtering provides electronically tunable, low-voltage, low-power, continuous-time current-mode, externally linear, and internally nonlinear filters. Methodologies for designing log-domain filters have been developed, and many transistor-level circuits have been designed. However, there has been little exploration of their internal operation.; In this dissertation, the nonlinear internal operation of log-domain filters is explored theoretically and systematically, with single-ended and differential log-domain filters designed by interconnecting the basic building blocks E cells. Large-signal analysis of log-domain filters conforms with the small-signal analysis of the current-mode transconductance-C (Gm -C) equivalent circuit at the operating point. Methods for adding common-mode feedback (CMFB) circuits to differential log-domain filters without degrading external linearity are presented. Simple but practical CMFB circuits are demonstrated. The CMFB allows derivation of closed-form expressions for the nonlinear internal signals. Differential second-order log-domain filters with CMFB circuits are built in the MOSIS 1.5 μm AMI ABN complementary MOS (CMOS) process. The theoretical analyses are supported by HSPICE simulations and experimental measurements. Finally, a design methodology based on E cells is generalized for implementing higher-order log-domain filters, presenting a systematic design procedure and possible ways to manipulate the circuits for a given transfer function. |
