Enhanced gain, low voltage, rail-to-rail buffer amplifier suitable for energy harvesting

Abstract

A CMOS subthreshold rail-to-rail input-output buffer amplifier suitable for energy harvesting applications is presented, having high gain (AV) of ∼ 130dB, consuming ultra low currents (IDD) of ∼ 150nA, and operating with low power supply voltage (VDD) > ∼ 0.8v. Contributions of this work are the synthesis of the following attributes: First, using a single transistor, the amplifier input stage's tail current is steered between the two PMOSFET input pairs, while one of the PMOSFET pairs is level shifted by a pair of NMOSFET source followers, which keeps the amplifier's input stage transconductance (gm) roughly constant while the inputs span rail-to-rail. Second, to boost folded cascode transconductance amplifier's (FCTA) AV, the proposed plurality of regulated cascode (RGC) current mirrors (CM) utilize a small size auxiliary amplifier, containing the same type and un-scaled FETs as that of the cascoded CMs employed within the FCTA. As such, the boosting of AV is less impeded by the otherwise higher impedance and high capacitance associated with scaled FETs, utilized in most prior art, in the RGC's auxiliary amplifier's signal path. Moreover, the RGC-CM utilizing the same FET, as that of the FCTA's CM, provides more consistency in FCTA's DC, AC, and dynamic response over process and operating condition variations. Third, a buffer driver containing a Minimum Current Selector (MCS) and an inverting current mirror amplifier (ICMA) controls the quiescent current of the inactive output transistors (FETs), while a complementary noninverting current mirror (CNICM) curbs the current waste attributed to monitoring the FET's (external load) currents. The output buffer driver is inherently fast and can work at low VDD, since it operates mainly in current mode. Montecarlo (MC) and worst case (WC) simulations indicate the following specifications are achievable: input voltage range rail to rail; output voltage range ∼ 10mV from the rails; resistive load (RL) 5K ohms capability; unity gain frequency (fu) ∼ 200KHz and phase margin (PM) ∼ 40 degrees; power supply rejection ratio (PSRR) ∼ −90dB; common mode rejection ratio (CMRR) ∼ −110dB; slew rate (SR) ∼ 3V/ 10uS; settling time (ts) ∼ 15uS; size ∼130um/side.