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Bob Pease Lab Notes Part 9.Pdf 934 PROCEEDINGS OF THE IEEE, VOL. 68, NO. 7, JULY 1980 A test circuit of Fig. 1 was constructed using standard 741 type OA’s, monolithic transistorarrays and 0.1 percent resistors. Initially,with i = 0, the mirror gains were equalized by removing R1 and R4 (thus assuring I+ = I-) ad adjusting R 5 to obtain io = 0. For VCCS opera- tion, G, was measured leaving port B-E’ open (i = 0), while for CCCS operation, port A-A’ was shorted to ground making u1 = u2 = 0. To minimize scaling errordue to frnite gain Aof the OA’s, theratio (Rz + R3)/R1 was restricted to a maximum value of 100. ForA - lo5, this assures less than 0.1 percent error at dc due to finite OA gain. Using various resistance combinations, values of G, up to 0.5 mho and Ai as large as 400 were obtained with a scaling error of the order of 1 percent, which was largely due to nonunity gain of the mirrors. Indivi- dual trimmingof the mirrors to precisely unity gain (tO.1 percent) Fig. 1. Oscillator network in the above letter tuned for an oscillation resulted in scaling errorsof the orderof resistance tolerances. The frequency of 10 Hz. (x-axis; 50 ms/cm; y-axis: 2 V/cm) small signal performance of the convertor is shown in Fig. 2(a) for the case where R1 = - and Rz = R3 = 0. Here the circuit is a VCCS with G, = (l/R4). For large R4 the bandwidth is equal to the GBW of the the input of the next “G” block, will cause phase errors, causing the OA’s (-1 MHz). The observed decrease in bandwidth for smaller R4 is sine amplitude to be damped out. (These losses do, in practice, always due to the increasing output resistance of the OA’s at high frequencies. occur.) Thus the output will not have stable gain nor amplitude unless In the previous analysis, perfect matching of the input offsetvoltages positive feedback is added. It may, in fact, never even start oscillating, of the OA’s (Vosl= VOs2)was assumed. In practice, a finite mismatch or if it does start, it may gradually die out. This is bad enough in an A Vos = Vos2- Vosl is amplified by the circuit transconductance result- audio oscillator. For a subaudio (am*? 1 pHz) oscillator it would be ing in an output offset current iOs. The contribution due to frnite and disastrous to have to wait for, say, half an hour for a cycle before hnd- unequal input bias currents, however, can still be made negligible by ing that there was not one. properchoice of resistances (see (6)). Fig. 2(b)shows the measured I am not sure of exactly what form of DVCCS is intended, but if output offset as a function of the mismatch in input offset voltage distortion occurs at large levels, and noisepredominates withlow AVOS Here, AVOS was varied by adjusting the offset control of one of levels, then I fear that this circuit will not give satisfactory waveforms the OA’s in the circuit. For G, Q 10 mmho, an output offset less than at any frequency, slow or half-fast. 10 pAwas achieved without any special effort at VOsmatching. In other words, despite all the author’s discussion of realization, I The CMRR of the circuit was measured at 100 Hz for various resis- question whether the author hasactually built and tested thecircuit. tance combinations and a 20 percent mismatch in mirror gains. For values of G, frommho to 0.2 mho,the CMRR was virtually con- Reply2 by R. Nandi3 stant at 85 dB (~2dB), which is comparable to the 90dB specification of the 741. Larger CMRR would be expected with the use of higher The oscillatorcircuit described in the above letter’ was builtby quality OA’s. realizing DVCCS’s using pA741 type OA’s and a few resistors. Good The proposed convertor circuit may also be used with single-ended quality sine-wave oscillations were obtainedand the frequency was input sources, by appropriately terminating the unused input terminals. observed to be smoothly variable in a range of about 1 :6 from a typical Terminating the output terminal in a resistance Ro gives an output design value of 10 Hz in the lower side. An experimentally observed voltage u0 = ioRo and extends the application of the converter to in- waveform is given in Fig. 1 here. clude VCVS and CCVS operations. In this case, however, an additional Now, with the availability of high input impedance ohms ob- buffer OA may be required in order to provide lowenough output tainable at present with integrated voltage-follower modules [ 11) active resistance. DVCCS/DVCVS device in IC form [2] , the low-frequency oscillator realization proposed in the above letter is probably one of the simplest REFERENCES existing configurations with a grounded resistor control (often required B.L. Hart,and R. W. J. Barker, “A precision bilateral voltage. for the electronic control in VCO’s). current convertor,” ZEEE J. Solid-State Circuits, vol. SC-10, pp. 501-503. Dec. 1975. The comments of Pease on the workability of the circuit are appre- D. J. Hamilton, and K. B. Finch, “A single-ended curreni gain cell ciatedsince they give an opportunityfor clarificationof its perfor- witha c, low offset voltage, and large dynamic range, ZEEE J. mance through this discussion. Solid-&ate Circuits, vol. SC-12, pp. 321-322, June, 1977. S. Pookaiyaudom and W. Surakampontorn, “An integrable preci- sion voltage-to-current convertor with bilateral capability,” ZEEE REFERENCES J. Solid-State Circuits, vol. SC-13, pp, 411-413, June 1978. [ 1 I “A fastintegrated voltage-follower with low input current,” in B.L. Hart and R. W. J. Barker, “Universal operational-amplifier Linear ADDlications. vol. 1. National Semiconductor. D. AN5-1. convertor technique using supply current sensing,” Electron. Lett., [ 2 1 A. M. Sohman, “A bounded inductance simulation using DVCCS/ vol. 15, pp. 496-497, Aug. 1979. DVCVS,”Proc. ZEEE,vol. 66, pp. 1089-1091, Sept. 1978. -. “DC matching errors in the Wilson current source,” hlectron. Letk, vol. 12, pp. 589-390, 1976. K. G. Schlotzhauer and T. R. Viswanathan “Im oved integrated ’Manuscript received February 26, 1980. unity gain current controlled sources,’’ In;. I. Actron., vol. 36, ’The author is with the Department of Electronics & Telecommuni- pp. 97-100, 1974. cation Engineering, Jadavpur University, Calcutta 700032, India. J. G. Graeme, G. E. Tobey, and L. P. Huelsman, Operational Am- plifiers Design and Applications. New York: McGraw-Hill, 1971, pp. 206-207. Comments on “Integrable Insensitive Subaudio-Frequency Digitally Rognunmable Gain Amplifiers with Sine-Wave GeneratorUsing DVCVS/DVCCS” Arbitrary Range of Integer Values ROBERT A. PEASE BARRY B. WOO There is nothing wrong with what the author has said about the circuit Abstmct-A new method for design@ -le gain ampW1m in the subject letter.’ It is what he has not said that bothers me. Any which provide a range of selectable integer gain values is presented. lossiness in the capacitors, or any resistive loading of the capacitor by The design uses a single noninverting op-amp with a switched resistor Manuscript received November 30, 1979. Manuscript received January 21, 1980;revised March 13, 1980. The author is at 682 Miramar Ave., San Francisco, CA 941 12. The author is with Fluke Automated Systems, Inc., 630 Clyde Ave., R. Nandi,Proc. ZEEE, vol. 67, pp. 1568-69, Nov. 1979. hit. View, CA 94043. 0018-9219/80/0700-0934$00.75 0 1980 IEEE 1548 PROCEEDINGS OF THE IEEE, VOL. 68, NO. 12, DECEMBER 1980 1.2 - widths. There is also a reduction in gain at fp in the eariier circuit and this is due to the effect of the nondominant poles which was also ex- pected because of the low GB products of the OA’s. Next in order to fmd the low sensitivity nature of the proposed cir- 1.1 - cuit, the supply voltages to the OA’s were reduced to *6V dc and the magnitudecharacteristics were obtained inboth thecircuits. These characteristics are also given in the same Fig. 5 for purposes of com- parison. me maximum change inmagnitudes were obtained in the 1.0 - passband of each set of characteristics. They were found to be 3.7 per- cent in the new circuit and 41.5 percent m the case of the earlier cir- cuit. The superior performance of the newly proposed circuit is clearly indicated by the comparison of thesetwo values and also by comparing 0.9 - the set of characteristics in Fig. 5. The active sensitivity is reduced by an order of magnitude. Though overdrive does not affect the stability of the new circuit, there is an occasional oscillation during switching on the dc supplies. 'Ibis problem can, however, be avoided by connecting 0.8 - silicon diodes across the resistance “QpR” as in other circuits of this category [ 31. E .d 111. CONCLUSIONS ’ 0.7 - In this letter, a new three-porthigh gain amplifier using two OA’s has been developed. Actively compensated inverting and noninverting inte grators have been developed using such a &-port and both of them do not require matched amplifiers. me double integratorhop fiter 0.6 - employing these integrators is found to possess superior performance and this has been brought out through both theoretical as well as ex- perimental results. 0.5 REFERENCES K. R Rao et d., “An actively compensateddouble integrator filter without matched operational amplifiers,”Roc. EEE (Lett.), pp. 534-535, Apr. 1980. S Natarajan and B. B. Bhattacharyyq ‘‘Design and some applica- 0.4 I II I I I 1 tions of extended bandwidth finite gain amplifiers,” J.
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