Kaulberg, Thomas

A fully differential-input differential-output current-mode operational amplifier (COA) is described. The amplifier utilizes three second generation current-conveyors (CCII) as the basic building blocks. It can be configured to provide either a constant gain-bandwidth product in a fully balanced current-mode feedback amplifier or a constant bandwidth in a transimpedance feedback amplifier. The amplifier is found to have a gain bandwidth product of 8 MHz, an offset current of 0.8 Â¿A (signal-range Â±700Â¿A) and a (theoretically) unlimited slew-rate. The amplifier is realized in a standard CMOS 2.4Â¿m process

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Downloaded from orbit.dtu.dk on: Dec 17, 2017CMOS Current-mode Operational AmplifierKaulberg, ThomasPublished in:Proceedings of the 18th European Solid-State Circuits ConferencePublication date:1992Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Kaulberg, T. (1992). CMOS Current-mode Operational Amplifier. In Proceedings of the 18th European Solid-State Circuits Conference (pp. 246-249). IEEE.CMOS Current-mode Operational Ampli¬fierThomas KaulbergElectronics Institute, Technical University of Denmark, Building 349,DK-2800 Lyngby, Denmark.Abstract A fully differential-input differential-output current-mode operational amplifier (COA) is described. The amplifier uti¬lizes three second generation current-conveyors (CCII) as the basicbuilding blocks. It can be configured to provide either a constantgain-bandwidth product in a fully balanced current-mode feedbackamplifier or a constant bandwidth in a transimpedance feedbackamplifier. The amplifier is found to have a gain bandwidth productof 8 MHz, an offset current of 0.8 /¿A (signal-range ±700/zA ) anda (theoretically) unlimited slew-rate. The amplifier is realized in astandard CMOS 2.4/xm process.I INTRODUCTIONA current mode equivalent to the conventional voltage mode operational amplifier is a currentmode operational amplifier (COA), i.e. a current amplifier with a current mode low impedanceoK.Figure 1: A current-mode op-amp246Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 06,2010 at 12:24:55 UTC from IEEE Xplore.  Restrictions apply. Figure 2: a: Voltage transimpedance amplifier, b: Current transimpedance amplifier (CTA)input1, a differential current mode high impedance output, and a high (ideally infinite) currentgain.Most current amplifiers described in the literature have only a limited gain. However, thestructure described in the present paper is suitable for true COA operations, i.e. a current gainin the order of 80 dB.The amplifier is based on the interconnection of second generation current conveyors [1].Due to a flexible interconnection structure of the conveyors both a fully differential currentmode opamp [2] and a current mode transimpedance opamp [3] can be configured from thepresent structure. Fig. 1 shows an equivalent diagram of the COA expressed in terms of secondgeneration current conveyors. With the V^-terminal connected to ground the circuit is inter-reciprocal to a conventional balanced voltage-mode operational amplifier. According to thetheory of Adjoiiit Networks [1], it has a current transfer-function equal to the voltage transfer-function of the voltage-mode operational amplifier. The transfer-function is derived from Fig.1 :7°(*) 9xRt rfíW _ 9xmW)~l + sRTCc * GBW~C-C (1)With the Vx-terminal used as a feedback-terminal (Fig. 2b2) the amplifier can be showninter-reciprocal to a voltage domain transimpedance amplifier [3] shown in Fig. 2a [5]. Inthis configuration the amplifier therefore exhibits a nearly gain-independent bandwidth. Thetransfer-function is calculated from : I0 = gx(ZTU+Vx) where Zt = Rt \\ jç- and U = /,.+./,._and is the same for both circuits :H(s) Vo(s)V¡(*)h{s)Fig.2a M«) = (1 + ^1).Î.Fig.2b ^2 1 + SR\CC (2)where it is assumed that gx[R\ || R2] > 1 . Because the available current to charge anddischarge Cc is not limited to a quiescent value of current (bias current source) but only limitedby the current signal range of the input current conveyors, the amplifier does not suffer fromslew rate limitation like the voltage mode opamp.1A differential current input is necessary if the amplifier has to be inter-reciprocal with a coventional balancedvoltage mode opamp2ln Fig. 2b. the COA has been inserted with the /¿--terminal left open and the 70--terminal grounded247Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 06,2010 at 12:24:55 UTC from IEEE Xplore.  Restrictions apply. KXh rd>i rCfch5*52 5*72%IN- ^nPWlN +SIAHi -¥- iK5B1 5/1 5*73i<3pj u^pj ug>jK±>i5*765/5[<PJKÈh rdXl553f.5535C2v.5C1rCfchSG4XK$y Kp^ 5C3' 0UT--o'OUT+Figure 3: A CMOS current-mode op-ampFigure 4: Die Photo of the current-mode operational amplifierII RealizationThe current-mode op-amp has been realized as shown in the diagram of Fig. 3. The circuit ischaracterized by Eq. (3) and (4).19x = 9m6 + 9mbs6 + 9mS + 9mbs8RT =1(3)(4)9o2 + 9oA + 9o5 + 9o6 + 9oi5 + 9oxlwhere gon represent the output-conductance of current-mirrors or current-sources denoted byCMn and Ißn. The input current offset errors are strongly dependent on the accuracy of thecurrent-mirrors [2], and therefore all current-mirrors (and current-sources) have been realizedas cascode current-mirrors. Each transistor in the amplifier needs to match one or two othertransistors and in order to improve the matchning a finger-structure common centroid layouttechnique has been used for all transistors.248Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 06,2010 at 12:24:55 UTC from IEEE Xplore.  Restrictions apply. MEHI' IK,A FFF F perOêFrequency, HzFigure 5: Transfer-functions of a. COA b. CTAIII Experimental ResultsThe COA has been implemented in a 2.4/¿m industry standard analog CMOS procès. A chipphoto is shown in Fig. 4. The COA was found to have a open loop current gain as shown in Fig.5a. It is seen that the DC-gain is approximately 72 dB and the GB is 2 MHz . The offset-currentwas measured to about 0.8 /¿A compared to a signal range of ±700//A.In Fig. 5b measured results from the feedback configuration of Fig. 2b are shown. Thefeedback resistor R\ is lOkfi and different values of R2 ranging from 1 kíí to infinite havebeen used. The measurements confirm the expected constant bandwidth characteristics of theamplifier.IV ConclusionA CMOS current mode opamp has been analyzed and experimental results have been presented.With a structure based on three current-conveyors a versatile current-mode building-block hasbeen achieved. The current-mode operational amplifier is found to have the current process¬ing proporties as the traditional balanced voltage-mode operational amplifier as well as thetransimpedance amplifier.References[1] A. S. Sedra and K. C. Smith : "A second-generation current conveyor and its applications,"IEEE Transactions on Circuit Theory , 1970, Vol. CT-17, pp. 132-134.[2] E. Bruun : "A differential-input differential-output current-mode operational amplifier,"International Journal of Electronics, 1991, Vol. 71, pp. 1047-1056.[3] E. Bruun : "A constant-bandwidth current-mode operational amplifier," Electronics Letters,1991, Vol. 27, pp. 1673-1674.[4] G. W. Roberts : "All current-mode frequency selective circuits," Electronics Letters, 1989,Vol. 25, pp. 759-761.[5] D. Nelson and S. Evans : "A New Approach to Op Amp Design," Comlinear CorporationApplication Note 300-301, March 1985.249Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on July 06,2010 at 12:24:55 UTC from IEEE Xplore.  Restrictions apply. 

CMOS Current-mode Operational Amplifier

Thomas Kaulberg

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