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Fully Differential Operational Amplifiers with Accurate Output Balancing

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Fully Differential Operational Amplifiers with Accurate Output Balancing 1410 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 23. NO. 6, DECEMBER 1988 TABLE I 131 A. J. M. Boomkamp and Ci. C. M. Meijer. “An accurate biomedical temperature transducer with on-chip microcomputer interface,” in Dig. Tech. Puperx. Europeun Solrd-Stute Circurts Conf.. Sept. 1985, pp. 214-217. supply voltage 2.5 - 3.1 V [41 G. C. M. Meijer, R. van Gelder, V. Nooder. J. van Drecht, and H. M. supply current 200 PA M. Kerkvliet. “A three-terminal wide-range temperature transducer with temperature range 32OC to 44OC microcomputer interfacing.” in Dig. Tech. Puper.y, Europeun Solid-Stute Circurts Conf., Sept. 1986, pp. 161-163. accuracy t O.I’C, see text M. J. S. Smith, L. Bowman, and J. D. Meindl, “Analysis, design and error due to supply voltage changes 0.04OC performance of micropower circuits for a capacitive pressure sensor IC,” [for the voltage range 2.5V-3.1VI IEEE J. Solrd-Sture Circuits, vol. SC-21, pp. 1045-1056, Dec. 1986. G. C. M. Meijer, “An IC temperature transducer with an intrinsic O.0l0C reference.” IEEE J. Solid-State Circuits, vol. SC-15, pp. 370-373, June [for the voltage range 2.6V-3.3VI 1980. reference frequency 30 kHz G. C. M. Meijer and K. Vingerling, “Measurement of the temperature dependence of the IC( VRE)characteristics of integrated bipolar transis- frequency ratio p 12(R /R )(T - TZ)/TZ 42 tors,” IEEE J. Solid-State Circuits, vol. SC-15, pp. 237-240, Apr. 1980. G. C. M. Meijer, “Thermal sensors based on transistors,” Sensor.\ und Actuutors. vol. 10. pp. 103-125. 1986. F. V. J. Sleeckx and W. G. C. Sansen, “A wide-range current controlled Analysis showed that the maximum error is within kO.l”C, oscillator using JFET-bipolar technology.” IEEE J. Solid-Stute Circurts. provided that perfect trimming is performed at a wafer tempera- vol. SC-15. pp. 875-881, Oct. 1980. ture of about 38OC. The error is mainly caused by internal power B. Gilbert. “A versatile monolithic voltage-to-frequency converter.” IEEE J. Solid-Stute Circuits, vol. SC-11, pp. 852-864, Dec. 1976. dissipation, spreading in the base-emitter voltage and finite R. C. Dobkin, ‘‘Input supply independent circuit,” U.S. Patent 3 930 current gain of the n-p-n transistors. 172. Dec. 1975. The main measurement results are listed in Table I. The factor B. L. Hart and R. W. J. Barker, “Modified current mirror with a “ voltage-following” capability.” Electron. Lett., vol. 18, pp. 970-972, of 12 in the frequency ratio p represents the current-mirror 1982. transfer ratio of Q16 and el,. The greatest sensitivity of the frequency ratio to drift in the supply voltage is found at its lowest value (2.5 v). The changes in I, and Zrer partially compensate for each other. For changes in I, and Zref separately we measured changes of about 1 percent in Fully Differential Operational Amplifiers with Accurate the nominal voltage range. Output Balancing As mentioned in Section 11, with a single-point calibration the MIHA1 BANU, MEMBER, IEEE, JOHN M. KHOURY, MEMBER, IEEE, required accuracy can only be achieved when a process with low AND YANNIS TSIVIDIS, FELLOW, IEEE spreading in VBE values is used. In other cases a two-point calibration is necessary unless the circuit is modified. A possible Abstract -Design considerations are presented for attaining accurate improvement is introduced in [4]: for both the currents I, and output balancing in fully differential operational amplifiers, over the useful I,, the same base-emitter voltage is used. By trimming the operating frequency of the differential signals. Such output balancing is emitter area both signals I, and I,, are adjusted to the right obtained by merging the common-mode feedback and the differential gain paths as close to the front end of the amplifier as possible, ensuring value. Because spreading in the PTAT voltage is much less than maximum sharing of circuit components. Two circuit designs implemented that in VBE,no special precautions are required. in a 5-V, 1.75-pm process are presented, one based on a two-stage topology and one based on a folded cascode topology. Experimental results for both designs are given. VI. CONCLUSIONS I. INTRODUCTION The temperature information of the transducer described in this paper is converted into a frequency ratio. In this way the A balanced-output operational amplifier is a special case of the need for absolute accuracy of the signal transfer is eliminated well-known differential-output (or “fully differential”) opera- and inaccurate passive on-chip components can be applied. The tional amplifier [l]. For a regular differential-output amplifier, temperature transducer is a four-terminal device with complete having output terminal potentials y,l and Kz,the output quan- on-chip microcomputer interfacing. tity of interest is Kd = y,] - yI2.No precise design requirements High accuracy and very high resolution is obtained by using are imposed with respect to the output common-mode compo- fundamental properties of bipolar transistors. Calibration is sim- nent 5, = (1/2)(tl + yJ2),although its value should not inter- ple and can be performed in a single step. fere with the proper operation of the circuit or of the load driven The transducer contains a low-voltage ultra-linear I- f con- by it. The value of V,, can even be signal dependent due to finite verter and two new accurate low-voltage current mirrors. With input common-mode rejection, circuit nonlinearities, and device these circuits accurate signal transfer at a supply voltage of only mismatches. In contrast to this situation, the balanced-output 2.5 V is achieved. operational amplifier has a 5, that is precisely set to Vha/,a predetermined signdindependent value. Then, the two outputs REFERENCES are said to be balanced with respect to Vhoc.If Vh(,/is chosen to [I] D. Ch. van Maaren, J. Klijn, and G. C. M. MeiJer. “An integrated micropower low-voltage temperature-controlled oscillator.” IEEE J. Manuscript received May 17, 1988: revised August 30, 1988. Solid-Stute Circuits, vol. SC-17, pp. 1197-1201, Dec. 1982. M. Banu and J. M. Khoury are with AT&T Bell Laboratories, Murray Hill, [2l M. J. S. Smith, M. A. Prisbe, and J. D. Meindl, “A micropower IC for a NJ 07974. biomedical pressure transducer,” in Tech. Dig., IEEE In[. Con/. Solid- Y.Tsividis is with the Department of Electrical Engineering and the Center Stute Sensors und Actuutors (Transducers ’85) (Philadelphia, PA), June for Telecommunication Research, Columbia University, New York. NY 10027. 1985, pp. 42-45. IEEE Log Number 0018-9200/88/1200-1410$01.~ 01988 IEEE IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 23, NO. 6, DECEMBER 1988 141 I be zero (as is often the case), then V,, = - y,* = Yld/2.Accurate output balancing is important for achieving maximum benefits from differential circuits in the presence of practical nonideali- BIAS1 ties. This is because such balancing ensures that the output will not contain common-mode components that could otherwise be present due to circuit nonlinearities or power supply noise. Thus, no problems will be created by these components in the case of a 1pF 1pF following stage with limited input common-mode rejection. Accu- rately balancing the outputs has the added advantage that the even-order nonlinearities of certain circuit elements are canceled. For example, such a cancellation can reduce distortion in switched-capacitor circuits and is the basic principal behind MOSFET-C continuous-time filters [2]. M7 M2A M20 Differential-output amplifiers usually contain common-mode 8/6 feedback circuitry [3].However, the outputs of certain designs are not balanced. There are two potential causes for this. First, the Fig. 1. Circuit diagram of the “ two-stage” balanced operational amplifier circuit that detects the output common-mode signal V,, may have fabricated in 1.75-pm CMOS technology. a nonlinear characteristic, as, for example, in [l].This problem can be avoided by using linear common-mode detectors such as a pair of two identical resistors, or the corresponding switched- capacitor equivalent for sampled-data circuits [4],[5]; transistor- d) If the common- and differential-mode signal paths are only nonsampled circuits are also possible [6].Second, the open- merged at the very front end of the amplifier and their small loop gain of the common-mode feedback may not be sufficiently remaining separate parts are identical or equivalent, then objec- large due to the topology used. For example, the fully differential tives a)-c) above can be achieved automatically by the regular circuit in [4]passes the differential-mode signal through two gain design of the differential-mode path [8]. In the examples to be stages and the common-mode signal through only one, resulting discussed below it will be seen that this strategy imposes no in relatively small common-mode loop gain. fundamental restrictions on the design. In this communication we discuss the design of differential e) The common-mode signal detector that generates 5,should amplifiers with output common-mode control that inherently have a linear characteristic. have a dc gain and a gain-bandwidth product of the common- f) There are practical situations when the outputs of the mode circuitry as large as the respective quantities for the operational amplifier need to be balanced even if the differ- differential-mode circuitry. We accomplish this by having the ential-mode input stage turns off. Such turn-off can occur, for differential- and common-mode paths share as much circuitry as example, in some circuits containing latching states due to posi- possible, thus treating the two respective signals as equally as tive dc feedback [9].These states, normally avoided by differen- possible.
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