CURRENT-MODE FILTERING TECHNIQUES AND TRENDS

Edgar Sanchez-Sinencio (1) and Antonio C. M. de Queiroz (2)

(1) Texas A&M University, College Station, Texas, USA
(2) Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil

The course will be split in two parts, the first describing
continuous-time approaches, centered around the study of OTA-C
filters, and the second describing discrete-time approaches,
centered around the study of switched-current filters.

OTA-C and Current-mode Continuous-Time Circuits

The OTA-C technique, also called Transconductor-C (Gm-C) technique,
is a minimalist technique for the realization of continuous-time 
filters and other circuits, where capacitors and tunable trans-
conductors are the basic elements. The simplicity of the cir-
cuits make this technique the most suitable for the realization 
of high-frequency circuits using the components available in a 
conventional CMOS digital process. The course will cover the 
aspects of the design of structures and components for OTA-C 
circuits and their extensions in the so called Current-Mode cir-
cuits.

1. Introduction.
2. Basic OTA architecture and linearized OTAs.
3. Gm phase compensation techniques.
4. Integrators:
   - OTA-C, single-ended and fully differential.
   - CT current-mode, single-ended and fully differential.
5. Filter architectures:
   - Biquadratic and ladder type.
   - Practical considerations.
6. Automatic tuning of frequency.
7. Current-conveyor techniques.
8. Simulated and experimental results.

Switched-Current Circuits
 
The switched-current (SI) technique is a new idea for the re-
alization of analog sampled-data filters, and other circuits, 
based on the use of MOS transistors with gates connected to 
switches as current memory elements and current mirrors. These 
structures can realize the same functions obtainable with 
switched-capacitor circuits, but without the requirement of 
linear elements, and using only MOS transistors available in a 
digital process. It is a true current-mode approach, where only
the currents in the circuit are linearly related to the input.
In this part of the course, the operation, analysis, and
synthesis of sampled-data switched-current filters will be
described.
 
General outline of the section:

1. Introduction
2. First-generation SI delays and integrators.
3. Second-generation SI cells.
4. Component simulation technique.
5. Filter synthesis by the simulation of passive prototypes:
   - Using bilinear integrators.
   - Using Euler integrators.
6. Imperfections and compensation techniques.
7. Analysis methods.
8. Simulated and experimental results. 

Course duration: 4 hours (half day).