An ICMR-enhanced three-opamp instrumentation amplifier

Abstract

The three-operational amplifiers (three-opamp) structure is a widely used topology to design precision instrumentation amplifiers (IAs). However, the input common-mode range (ICMR) of the classical three-opamp IA is limited to the output voltage range of the internal operational amplifiers, resulting in the output voltage range being constrained by the input common-mode voltage. This article proposes an ICMR-enhanced three-opamp topology, which constructs a common-mode feedback (CMFB) loop at the first stage of the IA, enabling the first stage has the capability of common-mode rejection. Hence, the proposed ICMR-enhanced three-opamp IA overcomes the limitation of ICMR, eliminates the constraint of the output voltage range and improves the common-mode rejection ratio (CMRR). The proposed circuit was designed and simulated using complementary bipolar process. The simulation results showed that the output voltage range remains constant regardless of the input common-mode voltages, the CMRR is greater than 150 dB, and the Gain Bandwidth Product (GBW) was 4.1 MHz. The advantages of the proposed ICMR-enhanced three-opamp IA will enable its use in more environments.