Fully Differential OTA for Large Bandwidth Application

Abstract

VLSI is becoming increasingly important in the fields of electronics and communications, mobile communications, computers etc. Efforts are continuously being made to reduce the power consumption of VLSI circuits and to improve their performance, all this needs to be done together with the shrinking size of the circuits, reduction in operating voltages. Designing high performance analog circuits is becoming increasingly challenging with the persistent trend towards reduced supply voltages. The fundamental block in analog circuit is the operational amplifier and its performance should continuously be improved in order to enhance the performance of analog circuits.

In applications such as optical communication and ADC’s (used in data acquisition systems) large bandwidth requirement is essential so we need large bandwidth OTA’s in such cases. At the same time differential signaling is beneficial in rejecting common mode noise which may be introduced in the communication channel or by supply voltages so in order to reject it fully differential OTA topologies are preferred for these applications.

This work involves designing a fully differential folded cascode OTA for large bandwidth applications. The OTA has been designed in 0.35um TSMC035 technology at a supply voltage of 3.3V. The load capacitance was taken to be as 1pF. The tools used are ASIC Design Kit (Mentor Graphics) for schematic simulation and IC Station (Mentor Graphics) for layout design. As being a fully differential architecture the common mode feed back circuit has been added to it. An attempt has been made to increase the high frequency response by connecting the input signals to the gate of NMOS cascode transistors. The main characteristics under consideration are Gain 50dB, UGB 700MHz, Power Dissipation 16mW.