aprogrammable reference-voltage sourcenents used in zener diode-based reference-voltage sources is,...

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A PROGRAMMABLEREFERENCE-VOLTAGE SOURCE

MUHAMMAD TAHER ABUELMA’ATTIand SA’AD MUHAMMAD AL-SHAHRANI

King Fahd University ofPetroleum and Minerals, Box 203, Dhahran 31261, SaudiArabia

(Received 24 July, 1996; Infinalform 30 August, 1996)

A simple circuit for realizing a current(voltage)-controlled reference voltage is presented.The feasibility of obtaining a temperature-insensitive reference voltage is explored. SPICEsimulation results are included.

INTRODUCTION

Reference-voltage sources are widely used in many electronic circuits,especially in analog-to-digital converters, digital-to-analog converters andhigh-accuracy measuring instruments. Available reference-voltage sourcesare built around either the base-emitter voltage of the bipolar junction tran-sistor or the zener-voltage of a zener-diode [2,3]. Since the base-emit-ter voltage of the bipolar junction transistor is small, amplification isrequired. This requires a relatively large number of active and passivecomponents, high precision operational amplifiers, and resistor arrays witha high relative thermal stability 1]. Alternatively, the zener-voltage of azener-diode suffers from temperature dependence. Low-voltagezener-diodes with zener voltages < 5V, have negative temperature coeffi-cients. High-voltage zener-diodes with zener-voltages > 8 V have positivetemperature coefficients [4]. Careful choice of active and passive compo-nents used in zener diode-based reference-voltage sources is, therefore,required to provide a temperature-stabilized output voltage [2,3]. On theother hand, programmable voltage references are attractive for many

113

114 M.T. ABUELMA’ATTI and S.M. AL-SHAHRANI

applications. Programmability can be achieved either manually or by usingsoftware [3]. No attempt, however, has been reported to exploit the pro-grammability of the operational transconductance amplifiers (OTAs) indesigning programmable reference-voltage sources. Probably this is attrib-uted to the temperature dependence of the transconduactance of the OTA.

It is the major intention of this paper to present a current(voltage)-con-trolled reference-voltage source using the zener-diode to provide the refer-ence voltage and the OTA to provide programmability. Conditions underwhich the temperature coefficient of the OTA may compensate for thetemperature coefficient of the zener-voltage, resulting in zero temperaturecoefficient reference-voltage source, will be discussed.

PROPOSED CIRCUIT

Figure 1 shows the proposed programmable reference-voltage source. Theresistor R1 and the zener-diode establish a voltage -Vz at point A. Thisvoltage is amplified by the inverting amplifier configuration formed of the

resistor R2 and the feedback resistor formed of the two OTAs. Assumingideal OTAs with o gm(V+ v_), where v/ is the noninverting input volt-

age, v_ is the inverting input voltage, and gm is the transconduct-ance of the OTA, where Iabc is the auxiliary bias current of the OTA andVr k__T is the thermal voltage where T is the temperature. Routineqabnalysis shows that the resistance of the feedback resistor formed of the

two OTAs can be expressed as

1RF (1)

gm

Using (1), the output voltage of the operational amplifier can be expressedas

2VzVTv0 (2)

G,cR2

From (2), it can be seen that the output voltage can be controlled by adjust-ing the auxiliary bias current Iabc. This auxiliary bias current can beobtained from the output of a digital-to-analog converter. Thus, it is feasi-ble to obtain a digitally programmable reference-voltage source. Moreo-

REFERENCE-VOLTAGE SOURCE 115

ver, since low-voltage zener-diodes have a negative temperaturecoefficient, then it is possible to cancel the effect of the temperature varia-tion on the zener-voltage by the positive temperature coefficient of thethermal voltage VT. Thus, obtaining a low-temperature coefficient pro-grammable reference-voltage source is feasible.

SIMULATION RESULTS

The proposed circuit of Fig. 1 was simulated using the SPICE circuit sim-ulation program. The zener-diode was simulated using the model parame-ters: IS=0.5 ktA, Rs=6 BV= 5.0 V IBV=0.5 ktA. The OTA wasmodelled using a voltage-controlled current-source with transconductancegm lmA/V-10rn,which corresponds to auxiliary bias current Iabc50ktA- 500tA, and input resistance 2MD. The operational amplifierwas modelled using a voltage-controlled voltage-source withgain =120 dB, and input resistance 1MD. The input voltage was set at-12 V. The results are shown in Fig. 2. Shown also in Fig. 2 are the resultsobtained from calculations using (1). From Fig. 2, it can be seen that thecalculated and simulated results are in excellent agreement. Thus, the pro-posed circuit provides a current-controlled reference-voltage. This currentcan be obtained from a voltage source. Thus, obtaining a voltage-control-led reference-voltage is feasible. Moreover, if this voltage is obtained fromthe the output of a digital-to-analog converter, then obtaining a digitallyprogrammable voltage-reference is feasible.

c 21abc

R1 A R2

Vi nOutputVoltage

FIGURE Proposed programmable reference-voltage source

116 M.T. ABUELMA’ATTI and S.M. AL-SHAHRANI

4.5

3,5

2.5

1.5

050 200 400

labc, uA

FIGURE 2 Variation of the output voltage with the auxiliary bias current of the OTAs, Iabc,-: calculated (1). SPICE simulation

CONCLUSION

A simple circuit for realizing a current(voltage)-controlled reference volt-age has been presented. The circuit uses a low-voltage zener-diode, twoOTAs configured as a floating resistance, one operational amplifier, andtwo resistors. Low-voltage zener-diodes with zener-voltage < 5 V havenegative temperature coefficients. The floating resistance obtained fromthe two OTAs has a positive temperature coefficient. Thus, obtaining atemperature-insensitive current(voltage)-controlled reference-voltage isfeasible.

AcknowledgementsThe authors acknowledge useful discussions with Dr. A.R. A1-Ali.

ReferencesA.V. Churbakov, Precision source of reference voltage, Instruments and ExperimentalTech- niques, Vol.27, #3, 1984, pp.695-697

[2] M. McPhillips, A digitally controlled voltage reference, Electronic Engineering, Vol.61, 1989, p.32

[3] O.I. Andronov, V.G. Brovchenko and S.T. Evdokimov, Variable reference-voltagesource, Instruments and Experimental Techniques, Vol.34, #2, Part 2, 1991, pp.375-377

[4] T.E Bogart, Jr., Electronic Devices and Circuits, Merrill Publishing Company, Colum-bus, Ohio, U.S.A., 1986

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