A High-Linear Rail-to-Rail Class-AB PGA with Transconductance Compensation Based on Common-Mode Detection

A low total harmonic distortion (THD) programmable gain amplifier (PGA) with dual operational amplifiers (op-amps) for electrocardiogram (ECG) signal acquisition applications is presented. The proposed op-amp comprises the rail-to-rail input stage and class-AB output stage. To solve the transcon-ductance (gm) variation caused by the supply voltage changes in rail-to-rail amplifiers with the traditional DC level-shifting technique, a common-mode-detection-based transconductance linearization scheme is proposed in this paper, associated with a 7-steps resistor-feedback digital gain control from 1 V/V to 12 V/V, which can realize less than 2.7% transconductance variations. The proposed rail-to-rail class-AB PGA is implemented in 0.18-µm CMOS technology with 2.7-V∼5-V power supply voltage. Spice simulation results show that the THD is -110 dB with an input signal of 5 mVpp and the input-referred noise spectrum density is $\mu {\mathbf{V}}/\sqrt {Hz}$ at a 10-Hz frequency point, after adopting an 8-kHz chopping. The total DC current including the bias circuit is 30µA at 5 V.