Low noise rail-to-rail amplifier runs fast at ultra low currents and targets energy harvesting

Abstract

An input-output rail-to-rail buffer amplifier is presented that is low noise, fast, and operates at ultra low currents. The amplifier targets energy harvesting applications that require low cost, high volume, and rugged manufacturing by avoiding use of non-standard device configurations or special processes. The main contributions of this work are: (1) The method of lowering voltage output noise by narrow-banding an amplifier, while introducing a time dependent bias current boost that is triggered by large differential input signals. Narrowbanding an amplifier to reduce its output noise, slows its dynamic response, and this method aims to restore and boost both the amplifier's slew rate and settling time. (2) Amplifier can operate at low VDD of about VGS+2VDS, while running in subthreshold, based in standard 0.18u digital CMOS. (3) PMOSFETs are utilized as inputs, compensation capacitors, and bias resistors in both the amplifier and the boost stages, which helps optimize for yield and lower noise. More importantly, the dynamic response of both the main amplifier and the boost stage being substantially dependent on PMOSFET device parameters, facilitates a smoother dynamic response in and out of boost, over process and operating variations. Based on montecarlo (MC) and worst case (WC) simulations, the following specifications are achievable: voltage output noise (VOnOiSe) of 10 uv/Hz⁁1/2 at 1KHz, supply current (Idd) ∼ 500 nA; Vdd minimum ∼0.6V; rail-to-rail voltage input (VIN) range and output voltage (VOUT) range of ∼ +/− 25mv from the rails; output resistor load (Rl) ∼2k Ohms; output capacitor load (Cl) ∼1nF; slew-rate (SR) −/+ ∼ 4/2.5 v/us; settling time (tS) ∼ 5us to 1%; power supply rejection ratio (PSRR) −/+ ∼ 80dB/85dB, common mode rejection ration (CMRR) ∼ 125dB, unity gain bandwidth (fr) ∼ 20KHz with phase margin (PM) ∼ 75 degrees; open loop gain (G) ∼ 85dB; preliminary area estimate of ∼ 180 um per side.