Design Considerations for OTA-C Integrators and Full CMOS Resistors

Abstract

In the earlier generation of continuous-time filters the basic network elements were the inductor, the capacitor and the resistor. With the evolution of the Integrated Circuits (IC), however, the inductors have been replaced by equivalent networks composed of active devices and capacitors. The resulting topologies are called Active RC filters. The active devices of the active RC filters are either OP AMPs or OT As, see Fig. 1.1 (chapter 1). For high-frequency applications the OTA based circuits have been preferred.For IC filters, the integrator is the basic block. The lossless integrator consists of a voltage-to-current converter and of the integrating capacitor. The voltage-tocurrent converter can be implemented using a resistor and the virtual ground of an OPAMP with local feedback. The other possible implementation is using linear voltage-to-current converters, OT As. In this text we are only interested in the second technique. In particular, in this chapter the lossless integrator and the lossy integrator and the full CMOS resistors are further discussed.The OT A based continuous-time 1ossless integrator is composed by an OT A and a capacitor. In the previous chapter, the voltage-to-current converter has been discussed in detail. In this chapter, in section 3.2 the noise level of the full OT A is explored. In CMOS technologies, the capacitors are implemented as poly-silicon capacitors [1-3]. However, due to the lack of virtual grounds of the OTA based continuous-time filters, the behavior of the signal processor is affected by the parasitic capacitors. The effects of those parasitic capacitors are further discussed. In that section, it is demonstrated that the non-linearities of the bulk capacitors can produce high harmonic distortions. If the junction capacitors are comparable to the poly-silicon capacitors then these harmonic distortions are even higher than the harmonic distortions due to the OT A non-linearities. Based on the analysis of both load J. Silva-Martínez et al., High-Performance CMOS Continuous-Time Filters

Two-stage unbuffered CFOA based non-inverting resistive-feedback amplifier: a study based on the description of the operational transconductance conveyor (OTC)

“…For this reason, most analog IC designers use unbuffered feedback operational amplifiers [12] instead of classical output voltage operational amplifiers. The Unbuffered VFOA (UVFOA) can be considered as an Operational Transconductance Amplifier (OTA) since its input impedances are high [13][14][15].…”