5.1 Introduction

Current mode signal processing circuits have recently demonstrated many advantages over their voltage mode counterparts including increased bandwidth, higher dynamic range, and better suitability for operation in reduced supply environments (e.g. 3.3V). In addition, current mode processing often leads to simpler circuitry and lower power consumption. Traditionally, however, most analog signal processing has been accomplished using voltage as the signal variable. In order to maintain compatibility with voltage processing circuits, it is often necessary to convert the input and output signals of a current mode signal processor (CMSP) to voltages. Figure 5.1 shows a block diagram of a CMSP with the necessary interface circuits.

Figure 5.1: Current mode signal processing system

Either the transconductor or the transresistor (or both) may be eliminated in those applications where the appropriate variable is in current form. When present, however, the transconductor is a crucial part of the design since it may limit the linearity, frequency response, and noise performance attainable from the system. Therefore, any transconductor intended for use as the "front end" of a CMSP must meet the following criteria:

1. high linearity for large input signals,

2. low noise,

3. no dominant internal poles,

4. large transconductance,

5. low quiescent power dissipation (i.e. high efficiency).

Tunability may also be an important requirement in applications requiring a precise value of g _m (e.g. filters), independent of process and temperature variations.

The primary goal of this Chapter is to highlight the important aspects of transconductor design in the framework...