Analysis and Design Considerations of Switched-Opamp Techniques

Abstract

SC circuits consist of three types of components: capacitors, operational amplifiers (Opamp) and MOS switches. A standard CMOS process with either double-poly or linear-capacitor options provides linear integrated capacitors, which do not have any voltage supply requirements for proper operation. Though transistor stacking should be avoided in designing the opamps for maximum output swings at low-voltage operation, multi-stage operational amplifier [CHE 00a, FAN 97] (Opamp) topologies could still, in general, be designed at a very low supply voltage (sub- 1-V operation is still possible) while achieving sufficient gain and bandwidth for SC circuits. The switches’ driving capability, however, becomes the main limitation for supply voltages reduction. The minimum supply voltage required by most SC circuits is primarily determined and limited by the turn-on requirement of the switches that have to be able to switch the total signal swing. Usually this occurs for switches that are connected to the output of opamps. In this chapter, the existing solutions for improving the switches driving capabilities are briefly discussed. Then the principle of the Switched-Opamp (SO) technique [CRA 95, CRO 94] is introduced. After that, a few key modifications of SO techniques are described.