A High Gain Multi-Stage Operational Amplifier using Compound Transconductance Element

The rapid increase in integrated circuit complexity attributed to the advancements in fabrication techniques combined with the increasing demand for consumer applications has created an immense demand for high performance analog systems. These analog systems include high-resolution (≥ 14-bit) and/or high-speed (≥ 1 GHz) ADCs, high- linearity filters and power management circuits. The fundamental limitations in these systems are tied to the constraints imposed by the basic design elements that tend to include amplifiers and reference generators (voltage/current). With continued technology scaling, innovative circuits and design techniques are necessary in achieving high performance analog/mixed-signal systems.

The goal of this research is to develop an operational amplifier that readily provides high open-loop gain and gain-bandwidth product for implementing high-performance analog systems. As part of this research, a ‘Compound Transconductance element’ has been developed that offers high transconductance with low power consumption. Furthermore, this element can function as a key element in the implementation of a multi-stage amplifier. Thus, a compound transconductance element based operational amplifier has been developed that achieves an open-loop gain in excess of 100 dB. This high-gain amplifier is applicable to continuous-time and switched-capacitor applications that require high linearity. The unique characteristics of this circuit element not only create new avenues in multi-stage amplifier design but also foster the development of a unique compensation topology for achieving feedback stability. Fabricated in a 5-V/0.5-µm bulk CMOS process, this amplifier achieves an open-loop gain of nearly 120 dB, with a unity- gain bandwidth of 15 MHz.