A Rail-to-Rail Amplifier Input Stage - Analog and Mixed Signal ...

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280 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 52, NO. 2, FEBRUARY 2005Fig. 18. A 2.3-V , 100-kHz unity gain step response.TABLE ITHIS WORK TO PREVIOUS WORKprimary applications of a rail-to-rail amplifier is a buffer, theamplifier was placed in the unity-gain feedback configuration.Fig. 17 shows the response to a rail-to-rail step input. Fig. 18shows the response to a 2.3-V step input. The overshoot was10% and the 2% settling time was 1 . Since linearity is a concernin buffer design, the total harmonic distortion was measured,and plotted against different amplitudes and frequenciesin Fig. 19. The spectrum of one such measurement is given inFig. 20. This spectrum corresponds to the time-domain outputsignal provided in Fig. 21. Table I provides a comparison ofthis work to previous work on this topic. Please note that thearea could have been reduced to 21 mm at the cost of 300 Wof power using the techniques described in Sections IV-A andB. Circuit simulations display comparable performance parametersto those experimentally verified in this paper.VI. CONCLUSIONFig. 19.THD versus input amplitude and frequency.A new method for achieving constant in a rail-to-rail amplifierwas introduced. It uses only one input differential pair bymaking use of programmable level shifters via MIFG transistorsin the source follower configuration. The common mode is
FISCHER et al.: RAIL-TO-RAIL AMPLIFIER INPUT STAGE 281Fig. 20. Output spectrum of the amplifier in the voltage follower configuration for V =1:4sin21000t. SFDR = 70 dB.Fig. 21. Time-domain waveform corresponding to the spectrum in Fig. 20.shifted to a constant value before the signal is input to the differentialpair. Since the common-mode level of the differentialpair is fixed, consistent operation for rail-to-rail common-modeinputs is achieved. Furthermore, since only one differential pairwas used, there is no degradation in the CMRR for any inputcommon-mode levels, which is a problem for rail-to-rail architecturesthat use complimentary input differential pairs. Experimentalmeasurements of this amplifier showed only 0.35deviation in the input stage transconductance, whereas the bestpreviously reported was .REFERENCES[1] F. You, S. H. K. Embabi, and E. Sánchez-Sinencio, “Low-voltage classAB buffers with quiescent current control,” IEEE J. Solid-State Circuits,vol. 33, no. 6, pp. 915–920, Jun. 1998.[2] T. W. Fischer and A. I. Karsilayan, “Rail-to-rail amplifier input stagewith constant g and common-mode elimination,” Electron. Lett., vol.38, no. 24, pp. 1491–1492, Nov. 2002.[3] M. Wang, T. L. Mayhugh, S. H. K. Embabi, and E. Sánchez-Sinencio,“Constant- g rail-to-rail CMOS Op-amp input stage with overlappedtransition regions,” IEEE J. Solid-State Circuits, vol. 34, no. 2, pp.148–156, Feb. 1999.
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