Low-Power Class-AB CMOS Voltage Feedback Current Operational Amplifier with Tunable Gain and Bandwidth
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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/TCSII.2014.2327390 Low-Power Class-AB CMOS Voltage Feedback Current Operational Amplifier with Tunable Gain and Bandwidth Fermin Esparza-Alfaro, Salvatore Pennisi, Senior Member, IEEE, Gaetano Palumbo, Fellow, IEEE and Antonio J. Lopez-Martin, Senior Member, IEEE COA [8]-[9] and Voltage Feedback Current Operational Abstract— A CMOS class AB variable gain Voltage Feedback Amplifier, VFCOA [10]-[12], respectively) are therefore Current Operational Amplifier (VFCOA) is presented. The required. Note that being COAs the CM counterpart of VOAs, implementation is based on class AB second generation current they are still bounded to the constant gain bandwidth product. conveyors and exploits an electronically tunable transistorized Instead, the VFCOA combines the constant bandwidth feedback network. The circuit combines high linearity, low power consumption and variable gain range from 0 to 24 dB with nearly property with the possibility of using non-linear resistors in constant bandwidth, tunable from 1 to 3 MHz. A prototype has the feedback loop without penalty in the overall circuit been fabricated in a 0.5-m technology and occupies a total area linearity [13], becoming a very interesting option for of 0.127 mm2. The VFCOA operates using a 3.3-V supply with designing wideband circuits. static power consumption of 280.5µW. Measurements for In this paper a novel low-power class AB VFCOA with maximum gain configuration show a dynamic range (1% transistorized feedback network is presented. Section II THD@300kHz) of 87.5 dB and a THD lower than -40dB for an output current 23 times the bias current. introduces its basic operation. The proposed implementation is shown in Section III, followed by the experimental results in Index Terms— CMOS analog integrated circuits, voltage Section IV. Conclusions are drawn in Section V. feedback operational amplifier, current mode, class AB, current amplifier, quasi-floating gate transistor, current mirror, current II. VOLTAGE FEEDBACK CURRENT OPERATIONAL AMPLIFIER conveyor, operational amplifiers. A. General considerations The VFCOA can be modeled as a single-ended input I. INTRODUCTION differential-output current controlled current source, where the input and positive output terminal are low impedance nodes HE most popular analog building block is the Voltage and the negative output terminal is a high impedance node. Operational Amplifier (VOA). Because of the wide T Fig. 1 shows the VFCOA’s symbol and equivalent circuit with variety of achievable transfer functions, it is used to the feedback configuration used to achieve accurate current implement high complexity circuits such as filters, amplifiers, gain. Main properties of this amplifier will be outlined below. A/D and D/A converters, V-I/I-V converters, etc. [1]-[3]. The VFCOA can be represented with the block diagram in When designing wideband circuits, the main VOA limitation Fig. 1b made up of a trans-impedance amplifier and a current is its constant gain-bandwidth product. On the other hand, the follower. The transimpedance amplifier converts the input Current Feedback Voltage Operational Amplifier (CFVOA) current into a voltage, Z (s)I . The current follower senses the has constant bandwidth independent of the gain [4]-[7]. T in current driven by the transimpedance amplifier and mirrors it Since some sensors and transducers provide an output to the negative output terminal. signal in current form, it is desirable in these cases to process According to the Rosenstark formulation [14], the exact the signal directly in the current mode (CM) domain. High- closed-loop current gain, G , is related to the return ratio, T, performance implementations of the CM equivalents of the F the asymptotic gain, G , and the direct transmission gain, G : VOA and CFVOA, (named Current operational amplifier, o T 1 G G G (1) F 1 T o 1 T Manuscript received; This work has been supported in part by the Spanish Direccion General de Investigacion under grant TEC2010-21563-C02- All these quantities must be calculated with respect to one 01/MIC. controlled source within the feedback amplifier (in this case Fermin Esparza-Alfaro and Antonio J. Lopez-Martin are with the the current controlled voltage source). The asymptotic gain, Department of Electrical and Electronic Engineering, Public University of Navarra, 31006 Pamplona, Spain (e-mail: [email protected]; G, is given by [email protected]). i R Salvatore Pennisi and Gaetano Palumbo are with the Dipartimento di out 2 G 1 (2) Ingegneria Elettrica Elettronica e Informatica (DIEEI), University of Catania, iin R1 Catania I-95125, Italy (e-mail: [email protected]; ZT [email protected]). Copyright (c) 2014 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending an email to [email protected] Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected]. This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/TCSII.2014.2327390 where R1 and R2 are the feedback network resistors. possibility to set a constant R2 for different G while The return ratio T results to be maximizing the return ratio. Moreover, the resulting value of Z (s)R R is not practical because it is in general too small (for T(s) T 1 1,opt R r R r R r instance, if ri= ro+= 50and G =11, then R1,opt is around 16 2 i 1 o 1 0 1 Z (s) (3) T The use of non optimum resistances determines a return r r r r G R i o G 1 ratio reduction with respect to the maximum return ratio, i o 2 R 2 TMAX(s), The forward gain G is lower than 1 and, being divided by o ZT (s) 1+T in (1), is negligible. TMAX(s) (8) r r G 2 r r (G 1) If R , R r ,r , T(s) can be approximated as i o i o 1 2 i 0 given by Z (s) T(s) T (4) R ro ro 2 1 G 2 (G 1) T(s) ri ri and the closed loop bandwidth CL expressed as a function of (9) T (s) r R r the open loop bandwidth, , is MAX 1 o G 2 o (G 1) OL r r R Z (0) i i 2 (1T(0)) T(0) T OL (5) Relationship (9) is plotted in Fig. 2, where the reduction is CL OL OL R 2 expressed in dB versus R2/ri, for four different asymptotic + Comparing (2) and (5), we see that while the asymptotic gains (ri= ro is assumed for simplicity). gain depends on both feedback network elements (R1 and R2), As expected, the return ratio reduction, from its minimum the closed loop bandwidth only depends on R2. Therefore, this value, increases with R2/ri and is greater for lower closed-loop system behaves like a CFVOA with constant closed-loop gains. A tradeoff must hence be met to avoid excessive loop bandwidth provided that different gains are set by changing R1 gain reduction while obtaining a nearly constant closed loop alone. bandwidth. From Fig. 2, it is seen that an acceptable maximum Finally, it is worth mentioning that another advantage of loop gain reduction less than 15 dB is obtained by choosing VFCOAs is the possibility of using non-linear resistors in the R2/ri=20 (for instance, if ri=50, R2=1kcan be set). feedback loop without worsening the linearity of the system dB T/TMAX [13]. This is possible because the virtual ground at the input 0 enforces equal voltage drops across both resistors, allowing cancellation of nonlinear terms. -5 -10 - G =11 Iin Iout I + - -15 ri out ro G =5 - Iout + + ZT(s)Iin r Iout -20 Iin o + IS VFCOA Iout G =2 -25 G =1 I f -30 R 0 10 20 30 40 50 60 70 80 90 100 2 R1 R2/ri (a) (b) Fig. 2. T/T versus R /r for different values of G Fig. 1. The VFCOA: (a) symbol, (b) equivalent circuit with typical feedback MAX 2 i configuration for accurate current gain. III. THE DESIGNED CIRCUIT B. Feedback resistors design A. Block diagram If the feedback resistors R1 and R2 are simultaneously Fig. 3 shows the block diagram of a VFCOA designed changed in such a way that their ratio R2/R1 remains constant, using the most versatile current mode circuit: the second the asymptotic gain (i.e., the ideal gain) in (2) remains generation current conveyor (CCII±) [15]-[17], a three port constant, but return ratio in (3) goes through a maximum. This network whose mathematical relationship between the X,Y, maximum occurs for two optimum values of R1 and R2 so that and Z terminals is represented by the following equation. I 0 0 0V R1,optR2,opt rir0 (6) Y Y thus, evaluating R from (2) and substituting into (6) we get VX 1 0 0 I X (10) 2 I Z 0 1 0VZ riro R1,opt and R2,opt riro (G 1) (7) where IZ I X and IZ I X represent the CCII+ and G 1 This means that for any G there is a different optimum 1 A low R1 value causes both poor matching with R2 and large output offset setting of R1 (and R2).