A summer amplifier with extended bandwidth is proposed. Compensation of the frequency characteristics is achieved by employing two operational amplifiers instead of external reactive components

Huertas Díaz, José Luis

Rodríguez Vázquez, Ángel Benito

English

idUS. Depósito de Investigación Universidad de Sevilla

Conclusions: Experimental investigations revealed that hygro-scopic water increases both the real and imaginary parts of thedielectric constant of dust. The increase in the imaginary partis more remarkable. Previously published results on micro-wave attenuation due to dust storms are therefore valid onlyfor dust with low water content, e.g. in desert and semidesertregions. For dust storms in tropical areas, microwave attenua-tion can be worse.S. I. GHOBRIAL 27th March 1980Electrical Engineering DepartmentFaculty of EngineeringUniversity of KhartoumPO Box 321, Khartoum, SudanReferences1 AHMED, i. Y.: The effects of sand and dust storms on microwavepropagation'. ASBU Tech. Rev., 1977, 1, pp. 23-292 GHOBRIAL, s. I., et al.: 'Microwave attenuation in sand storms'.International symposium on antennas and propagation, Sendai,Japan, August 19783 CHU, T. s.: 'Effects of sandstorms on microwave propagation', BellSyst. Tech. J., 1978, pp. 549-5554 SUCHER, M., and FOX, J.: 'Microwave measurements: vol. II' (Poly-technic Press of the Polytechnic Institute of Brooklyn, 1963)5 MANDEL, M.: Physica, 1961, 27, p. 8290013-5194/80/100393-02$U0/0The low-frequency magnitude and phase errors are given, re-spectively, by. _ ,0)arg \e{ja))\ a - —whereGB(4a)(4b)(5)Proposed summer: The new circuit is shown in Fig. 1. Routineanalysis allows us to write the summer output voltage, by eqn.1 whenever the following restriction applies:(6)wherej+ = Gj+/GF; (7)Assuming identical amplifiers, the error function for this circuitmay be given bys(s) = \GB )+ 1(8)NOVEL ACTIVE-COMPENSATED WEIGHTEDSUMMERIndexing terms: Active networks, AmplifiersA summer amplifier with extended bandwidth is proposed.Compensation of the frequency characteristics is achieved byemploying two operational amplifiers instead of externalreactive components.Introduction: In recent years, many authors have focused theirattention on the development of active compensation schemesfor the frequency characteristics of operational amplifiers. Theavailability of dual integrated amplifiers renders the approachvery attractive: However, little effort has been directed towardsthe realisation of active-compensated summers which combinepositive and negative weights. This is a serious limitation onthe usefulness of active-compensated amplifiers, which are inpractice reduced to the implementation of V.C.V.S.S.1"3 The pur-pose of this letter is to report a new circuit configuration whichcan act as a compensated summer for both positive and nega-tive gains.Preliminaries: The output voltage of a conventional one-amplifier summer4 is given byK(s) = kJ+VJ+- t-Vt- \e(s) (1)The term e(s) is an error function5 which represents the effectof the operational amplifier's dominant pole. It may be ex-pressed bye(s) = 1 (2)SGB+lwherek = max (3)withIt is worth noting the difference between eqns. 3 and 9.(9)Vo(compensated output)|9A0/1|Fig. 1 Active-compensated summerTo reduce the phase error, the best choice of the auxiliaryparameter a isa = k-2 (10)The low-frequency magnitude and phase errors are then ex-pressed by\e(jco)\ ^l+co2/co20 ( l la )arg [£(>)] ^ -<o*l<o% (nb)where eqn. 5 is still valid for the value of k given by eqn. 9.Design considerations: The values of GF and R can be arbi-trarily fixed by the designer. Then G,_ and Gj+ are determinedvia eqns. 7. Since the value of a is fixed by eqn. 10 the onlyresistor values which remain unknown are Ro+ and Ro- • Their394 ELECTRONICS LETTERS 8th May 1980 Vol. 16 No. 10Authorized licensed use limited to: Universidad de Sevilla. Downloaded on March 26,2020 at 16:50:54 UTC from IEEE Xplore.  Restrictions apply. values can be obtained by the following closed formulas:G 0 + = ^ [ l + s g n M ] £ k,.Go- = y [1 + sgn -M] (l + £ kJ+\(12)where3 SOLIMAN, A. M.: 'A generalized active compensated noninvertingVCVS with reduced phase error and wide bandwidth', Proc. IEEE,1979, 67, pp. 963-9654 GRAEME, J. G.: 'Designing with operational amplifiers' (McGraw-Hill, New York, 1977)5 WILSON, G.: 'Compensation of some operational-amplifier basedKC-active networks', IEEE Trans., 1976, CAS-23, pp. 443-4460013-5194/80/100394-02$! .50/01; = 1and sgn x = 1 for x > 0 and sgn x = — 1 otherwise.It must be remarked that either Go + or Go- (or both) mustbe zero.Discussion: An active-compensated summer has been reportedwhich can be used for positive and negative gains. Theproposed summer exhibits a second-order magnitude error asfor an uncompensated one. However, the phase error, which isa significant factor in most applications, has been reduced to anegligible level, as can be seen by comparing eqns. 4b and lib.Experimental data reveals good agreement between theoreticalprediction and practical behaviour when using ^A747 dualamplifiers. As an illustration, Fig. 2 shows the Bode plot corre-sponding to a summer with n — m = 1, k+ = 20, /c_ = 10 andthe same input voltage for positive and negative inputs.3 u 5 6 78 10 20f.kHzb30 40 5060 80100Fig. 2 Bode plot for compensated summer with n = m = I, k + = 20,k. = 10andVl+ = Vl_ = Vta Magnitude characteristicsb Phase characteristics* compensated summer+ uncompensated summerContinuous trace corresponds to theoretical Bode plots25th February 1980A. RODRIGUEZ-VAZQUEZJ. L. HUERTASDepartamento de Electricidad y ElectronicaFacultad de FisicaUniversidad de Sevilla, SpainReferences1 GEIGER, R. L.: 'Amplifiers with maximum bandwidth', IEEE Trans.,1977, CAS-24, pp. 510-5122 SOLIMAN, A. M., and ISMAIL, M.: 'Active compensation of opamps',ibid., 1979, CAS-26, pp. 112-117NOVEL STRAY-INSENSITIVE SWITCHED-CAPACITOR INTEGRATORIndexing terms: Switched-capacitor networks, FiltersA stray-insensitive switched-capacitor integrator is in-troduced which enables the design of leapfrog filters suitablefor high clock/signal frequency ratios without causing largecapacitor spreads. A design example is given.The well-known active-filter leapfrog technique1 has been usedfor the design of switched-capacitor filters using Bruton's loss-less discrete integrators (l.d.i.)2 or bilinear integrators.3 Theadvantage of the leapfrog technique is that it simulates LCladder filter structures which are known to have low sensitivityto component variations. Problems arise, however, when forsome reason the ratio of the clock frequency and the signalfrequency must be made high, because the required capacitorspread may then increase to an unrealisable value. It can beshown,2 for example, that the 3 dB bandwidth of a single-poles.c. filter is given by(1)where fc is the sampling frequency and C, C" are the twocapacitors associated with the pole. Thus for a given cutofffrequency, the ratio C'/C" increases with increasing clock freq-uency. A high clock frequency is often required because itsimplifies the design of the antialiasing filter. It will be shownin this letter that by realising C" as the difference between twocapacitors, a high capacitive ratio can be obtained without theneed for a large capacitor spread.Consider the circuit shown in Fig. 1, which is an extension ofthe integrator introduced in Reference 4. Note that this circuitis completely insensitive to stray capacitances. The output vol-tages are readily obtained as(2a)(2b)Co 1- Co 1 - zyo- 2 V 4where z = exp (s/2fc) and the open-loop gain AQ of theamplifier is assumed to approach infinity.New integrators can be derived using the morphologicalbox5 shown in Table 1. A solution Sik designates an integratorobtained by connecting the input terminals j and k in Fig. 1,where i,k= 1,2, 3,4. Note that S^ = Ski. The superscript e or odenotes the time slot during which the output voltage issampled.Each solution in Table 1 must now be examined to seewhether it provides the desired integration. Useful solutionsELECTRONICS LETTERS 8th May 1980 Vol. 16 No. 10 395Authorized licensed use limited to: Universidad de Sevilla. Downloaded on March 26,2020 at 16:50:54 UTC from IEEE Xplore.  Restrictions apply. 

Electronics Letters

Novel active-compensated weighted summer

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Novel active-compensated weighted summer

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