A supply-scalable differential amplifier with pulse-controlled common-mode feedback

This paper proposes a pulse-controlled common- mode feedback circuit for a supply-scalable fully-differential amplifier. The pulse-controlled common-mode feedback circuit overcomes the large area cost associated with a conventional R- C common-mode feedback circuit while maintaining high gain and large output signal range of the amplifier. The amplifier is implemented in low power/leakage 65nm CMOS technology with NMOS and PMOS threshold voltages of 0.36V and -0.44V and operates at a supply voltage from 0.6V to 1.2V. The prototype has an active area of 0.01mm 2 and consumes 1.21mW at 0.6V, 1.94mW at 0.9V and 3.07mW at 1.2V. The measured output noise spectrum density is 21.5nV/ p Hz at 0.6V, 26.1nV/ p Hz at 0.9V and 30.3nV/ p Hz at 1.2V. For a total harmonic distortion of 1%, the maximum signal of the amplifier with frequency 10MHz is measured as -2.3dBm at 0.6V, 3.8dBm at 0.9V and 6.1dBm at 1.2V; as expected, the linearity of the amplifier depends on its signal headroom which scales with the supply voltage. I. INTRODUCTION Supply-scalable circuits are receiving more attention in recent years (1) (2) because they are suitable for energy- harvesting systems with unstable supplies, but without extra power management. However, they meet more design chal- lenges than low voltage circuits in low power/leakage CMOS technology with transistors with large threshold voltage, espe- cially for fully-differential amplifiers. The large threshold volt- age leads to small margins on operating points of the amplifier making its bandwidth and gain easily become insufficient under a supply change. Body-input or body-bias circuits (3) can be applied to lower the threshold voltage by biasing the body- source junction for low voltage operation, but they have limited uses at a high supply. The on-resistance of analog switches associated with the threshold voltage has much variation to supplies which limits signal swing and bandwidth (4). Switch bootstrapping circuits (5) are widely used for reducing the on- resistance of switches in low voltage circuits, but they may suffer from reliability problems for flexible supply operation. At a low supply, the gate-to-source voltage of the transistors for switches is fixed by the supply voltage to minimize switch non-idealities, but it may make the gate voltage exceed the breakdown voltage of the transistors while the supply voltage becomes high. In addition to the challenges associated with differential signal path, the common-mode feedback circuit is another bottleneck for flexible supply operation. Three commonly used common-mode feedback circuits are: a passive R-C common- mode feedback circuit (6), an active differential-pair common- mode feedback circuit (7) and a switched-capacitor common- mode feedback circuit (8). The R-C common-mode feedback circuit can operate at different supplies, but requires large resistance to maintain the high DC gain and large output swing. Differential-pair common-mode feedback circuit does not need large area passive components, but it has limited linear signal range for the main amplifier and is not suitable at a low supply. The switched-capacitor common-mode feedback circuit also has smaller passive area, but its loop bandwidth and stability is very sensitive to the supply voltage because of the supply-dependent on-resistance of the switches. In addition, a big unsettled summing error leading to a wrong common- mode voltage for large differential signals can be caused by the large and nonlinear on-resistance switch resistance; at a low supply, these switches can not fully turn due to the large threshold voltage in low power/leakage processes. A novel pulse-controlled common-mode feedback circuit (PC CMFB) is proposed to significantly reduce the area of passive components and overcome the limitations of switch on-resistance for supply scaling. A supply-scalable fully-differential amplifier with the PC CMFB was implemented in a low power/leakage CMOS pro- cess. Compared with conventional common-mode feedback circuits, the proposed PC CMFB has smaller passive area without gain reduction of the main amplifier. The paper is organized as follows. Section II discusses the operation of the pulse-controlled common-mode feedback circuit. Section III describes the circuits of the supply-scalable fully-differential amplifier with the PC CMFB and section IV presents the measurement results.