Current-oriented operational amplifier (OpAmp) design has been common for its orderly current-to-speed tradeoff. However, for high-precision or high-linearity applications, increasing the current does not help much, as the supply voltage (V DD ) and intrinsic gain of the MOSFETs in ultra-scaled CMOS technologies are very limited. This paper introduces voltage-oriented circuit techniques to address such limitations. Specifically, a 2xV DD -enabled recycling folded cascade (RFC) OpAmp is proposed. It features: (1) current recycling to enhance the effective trans conductance by 4x with no extra power; (2) transistor stacking to boost the output resistance by one to two orders of magnitude; and (3) V DD elevating to enlarge the linear output swing by 4x. Comparing with its 1xV DD RFC and FC counterparts, the proposed solution achieves 20-dB higher DC gain (i.e. 72.8 dB) in open loop and 20-dB lower IM3 (i.e., -76.5 dB) in closed loop, under the same power budget of 0.6 mW in a 1-V General Purpose 65-nm CMOS process. In many applications, these joint improvements in a single stage are already adequate, being more power efficient (i.e. less current paths), stable (i.e. more phase margin), and compact (i.e. no frequency compensation) than multi-stage OpAmps. Voltage-conscious biasing and node-voltage trajectory check ensure the device reliability in both transient and steady states. No specialized high-voltage device is necessary.
This paper proposes two techniques for improving the linearity and power efficiency of switched-capacitor (SC) circuits. The first is a high-speed switched-current-assisting (SCA) path that helps the main (folded-cascode) OTA to deliver most of the desired charge to the integration capacitor, leaving the final error correction to be completed by the main OTA. The second is a pre-charging (PC) path that assists the main OTA to speed up the charging of the load capacitor. Both SCA and PC paths share one auxiliary (differential-pair) OTA that features a high speedto-power efficiency. The prototype is a bandwidth-scalable 5 thorder Butterworth SC lowpass filter (LPF) for software-defined radios. Fabricated in 65-nm CMOS, the LPF exhibits a decadewide tunable bandwidth (1.5 to 15 MHz) solely defined by the clock, leading to a compact die size (0.127 mm 2 ). Under the same power (5.6 mW) and bandwidth (10 MHz) targets, the IIP3 reaches +23.5 dBm (+15.3 dBm) and the cutoff accuracy is 97% (82%) with (without) the SCA + PC paths. The achieved Figureof-Merit (0.014 fJ) compares favorably with the state-of-the-art. Index Terms-bandwidth, CMOS, linearity, lowpass filter (LPF), operational transconductance amplifier (OTA), software-defined radio, switched capacitor (SC).
This paper reports a switched-capacitor (SC)-buffer Biquad that can be recycled efficiently as an ultra-compact low-pass filter (LPF) in nanoscale CMOS. It incorporates only passive-SC networks and open-loop unity-gain buffers; both are friendlier to technology downscaling than most conventional Biquads that use high-gain amplifiers and closed-loop negative feedback. Complex-pole pairs with independent Q factors are recursively realized in one clock period, while ensuring low crosstalk effect between the formations of each pole. Nonlinearity and parasitic effects are inherently low due to no internal gain. The fabricated 65 nm CMOS prototype is a 1x-recycling SC-buffer Biquad that is equivalent to a 4th-order Butterworth LPF with 75% buffer utilization. It occupies a die size of only 0.02 mm and exhibits 20x bandwidth tunability (0.5 to 10 MHz), linear with the clock rate. At 10 MHz bandwidth, the in-band IIP3 is +17.6 dBm and input-referred noise is 19.5 nV/ Hz; they correspond to 59.2 dB SFDR and 0.013 fJ figure-of-merit which are favorably comparable with the recent art. The 1 dB compression point conforms to the out-of-band blocker profile of the LTE standard at a 20 dB front-end gain. Index Terms-1 dB compression point , Butterworth, clock generator, CMOS, channel selection, clock-rate-defined bandwidth (BW), die area, figure-of-merit (FOM), low-pass filter (LPF), long-term evolution (LTE), operational transconductance amplifier (OTA), recycling, switched capacitor (SC), wireless radios.0018-9200
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