Design of a high common-mode voltage differential amplifier

In response to the demand for localization of differential amplifier chip, this paper introduces a ±120 V common-mode input voltage integrated differential amplifier with high Common Mode Rejection Ratio (CMRR) based on the 40 V bipolar process with metal thin film resistors. The article covers the basic principles and structural components of differential amplifiers, and analyzes the difficulties in the design process of differential amplifiers, including common-mode rejection ratio, input offset current and gain error. The methods to improve the CMRR and lower input offset current are introduced which are the technologies of integrated metal thin-film resistors trimmed by laser cutting and the modified base current compensation configuration, and improved Darlington tube is used to increase the open-loop gain of the amplifier and reduce the gain nonlinearity of the amplifier. The overall chip size is 2.82 mm ×2.02 mm, the dual power is ±15 V, and the current consumption is 1.36 mA. Experimental results show that, at supply voltage of ±15 V, the CMRR is 85 dB and the input offset current is only 84 pA with 98.4% compensation precision, and the gain error is 0.02%.