Low-Voltage CMOS Current Feedback Operational Amplifier and Its Application

Transcription

1 Low-oltage MS urrent Feedback perational Amplifier and Its Application Soliman A. Mahmoud, Ahmed H. Madian, and Ahmed M. Soliman A novel low-voltage MS current feedback operational amplifier (FA) is presented. This realization nearly allows rail-to-rail input/output operations. Also, it provides high driving current capabilities. The FA operates at supply voltages of ±.75 with a total standby current of 3 µa. The circuit exhibits a bandwidth better than MHz and a current drive capability of ± ma. An application of the FA to realize a new all-pass filter is given. PSpice simulation results using.5 µm MS technology parameters for the proposed FA and its application are given. Keywords: urrent feedback op-amp, low-voltage, variable gain amplifier, all-pass filter. I. Introduction In recent years, great interest has been devoted to the analysis and design of current feedback op-amp and current-conveyor integrated circuits []-[], mainly because these circuits exhibit better performance, particularly higher speed and better bandwidth, than classic voltage-mode operational amplifiers (A). The current feedback operational amplifier (FA) close-loop bandwidth is independent of its close-loop gain (provided that the feedback resistance is kept constant and much higher than the FA inverting input resistance) [7] unlike A-based circuits, which are limited by a constant gain-bandwidth product. The FA, shown in symbolic form in Fig. (a), is a four-port network which has a describing matrix of the following form: I I = I I. () Manuscript received July, 6; revised Nov. 9, 6. Soliman A. Mahmoud (phone: , amahmoud@frcu.eun.eg) is with Electrical Engineering Department, German University, airo, Egypt. Ahmed H. Madian ( ahmed_madian@hotmail.com) and Ahmed M. Soliman ( asoliman@ieee.org) are with Electronics and ommunications Department, airo University, airo, Egypt. riginally, FAs were implemented using only bipolar process technology. This technology is intrinsically well suited to process signals in the form of currents giving the high bipolar junction transistor (BJT) transconductance. More recently, several MS realizations for the FA have been reported in the literature []-[7], [9]-[]. The FA has always been seen as an extension of the second generation current conveyor (II); therefore, the design approach was to cascade a II+ with a voltage follower to realize the complete circuit []. The obtained bandwidth was always a degraded version of the II+ bandwidth. Several MS FA implementations have been presented to provide offset Soliman A. Mahmoud et al. ETI Journal, olume 9, Number, April 7

2 in- in+ r in- I ( ) I z out transistors (M, M ) and (M, M 3 ) produce a positive voltage shift for the input voltage applied on transistors M and M 3. All transistors are operating in the saturation region; the control voltage applied to transistor gates M and M controls the shifting value as r in+ r in+ (a) i ( ) = +, () DD in+ in- r in- II+ Fig.. (a) urrent feedback op-amp symbol and (b) FA block diagram []. (b) compensation [], high current drive capability [6], [7], and suitability for high frequency applications compensation [5]. The low-power/low-voltage issue, which is increasingly important in very large scale integrated (LSI) circuits, was partially addressed in []. In this paper, a novel MS current-feedback operational amplifier is presented. The FA is capable of operating under a minimum supply voltage ( Tp + Tn + DS,sat ) and with reduced power dissipation. The new circuit includes a class AB output stage exhibiting high current drive capability and good power conversion efficiency. A rail-to-rail input and output voltage operation is also nearly achieved. This paper is organized as follows. In section II, the circuit description and MS realization of the proposed FA are illustrated. In section III, PSpice simulations of the proposed FA using MS.5 µm technology are also given. In section I, an application of the FA to realize a new all-pass filter structure is given. In section, conclusions are drawn. II. Proposed MS urrent Feedback p-amp As stated in the introduction, the FA could be realized by the II which is cascaded with a voltage follower [], as shown in Fig. (b). The MS realization of the proposed FA, which offers both low-voltage and high drive capability will be described. The MS realization of the proposed FA shown in Fig. consists of two matched parallel connected n-differential pairs, (M, M ) and (M 3, M ); two matched biasing current source transistors, (M 5, M 6 ); a cascoded current mirror formed of matched transistors, (M 7, M, M 9 ); and two pairs of matched source follower transistors, (M, M ) and (M, M 3 ). Transistors M 5 and M 6 carry equal bias currents ( ), while c + r out out i ( ) = +, (3) where i and i are the output voltages from the source followers, is the high input impedance voltage, and is the low input impedance terminal. The circuit regions of operation can be explained as follows: and voltages are closed to the negative supply voltage SS (SS, < Tn+ SS), so the current source transistor M 5 and, hence, the differential pair M 3 and M are cut-off. Therefore, the small and large signal behaviour of the whole circuit results only from the contribution of the differential pair M and M, biased with current source transistor M 6. In the middle range ( Tn + SS, < + Tn DD ), both input pairs (M, M ) and (M 3, M ) are active and the small and large signal behaviour of the whole circuit results from the contribution of both differential pairs. Finally, when, are very close to the positive supply voltage DD (+ Tn DD, DD), the current sources of the shifters M and M are cut-off. Therefore, the small and large signal behaviour of the whole circuit contribution results only from the differential pair M 3 and M, biased with current source transistor M 5. This ensures a rail-to-rail operation. It is apparent that this structure does not provide a constant transconductance over the variations of the input voltages and. A feed forward section could be added to guarantee a constant transconductance over the variations of the input voltages and ; however, this is not a real drawback so long as the loop gain is sufficiently high. Indeed, variations of the open-loop parameter were greatly reduced by feedback action. The structure of the FA input stage (voltage follower) requires the terminal to have low input impedance, so a suitable buffer circuit should be used to fulfill this condition and to provide a rail-to-rail swing capability. Transistors (M - M ) fulfill the required buffering action with a rail-to-rail swing capability, as shown in Fig.. Transistors M and M 5 form the push-pull output stage at the terminal. Transistors M 6 and M 7 are level-shifting transistors, providing proper biasing for transistor M 5. This push-pull action of transistors M and M 5 reduces the power dissipation. To prevent crossover distortion, both transistors M and M 5 must be N when no current is withdrawn from DD ETI Journal, olume 9, Number, April 7 Soliman A. Mahmoud et al. 3

3 DD M 7 M M M 9 M 3 B3 M 9 M M M 9 B3 M 3 M 3 M 3 sh sh M 6 M M sh M 36 M 3 sh i M M i i M 3 M i M M 3 M M 3 M 33 M 5 M 6 M 35 I SB M 5 M M 39 B M M 7 M 37 B M 5 M 6 M M 7 SS Fig.. MS realization of the proposed FA. the terminal (standby mode), this current should be small and controllable. This is achieved by using a suitable gate voltage of M, which sets the voltage level shift between the gates of M and M 5. The standby power consumption of the overall circuit for dual power supply is given by = (I + I + I I ). () PSB DD SB B Bsh + B The last term in the above equation is the current passing through the level shift transistors (M 6, M 7 ). This current can be kept small by choosing a small aspect ratio for transistors (M 6, M 7 ). The class AB output stage enables the circuit to derive the heavy resistive and capacitive load with low standby power dissipation and no slewing. It is worth mentioning that smaller miller compensation capacitors can be connected between the gate and drain of transistors M and M to ensure good transient response under all loads. Transistors M 7 and M force the current in transistors M and M 3 to be equal to the current in transistors M and M ; therefore, I M + I M3 = I M + I M. (5) From the above equation, the matched differential pair transistors carry equal currents; therefore, =. (6) The current follower stage, as shown in Fig., is made up of transistors (M, M ). It conveys the terminal current into the terminal current; therefore, I = I. (7) Finally, a suitable buffer must be available between the and terminals. It is similar to the buffer between the and terminals and consists of transistors M 3 to M 39 ; therefore, =. () It is worth mentioning that, the proposed FA input stage is a dual circuit. This means that when the input stage which is formed of transistors M to M 6 changes to PMS, the current source formed from transistors M 7 to M 9 and the biasing circuits M to M will be NMS and vice versa. For small-signal analysis, when both differential stages are properly working, the open-loop gain T(s) is given by g T(s) = g + (r //r ) ds ds g (rds //rds ) ( r //r //r ) g ( r //r ). ds 7 m m ds ds3 m ds m(or m ) ds5 + g m3(or m ) In the above equations, g mi and r dsi are the transconductance and the drain to source resistance of the i-th transistor where i is the transistor number. As a result for the feedback, as shown in Fig., the voltage gain between the terminals and becomes (9) (s) Av(s) = =. () (s) + T(s) Soliman A. Mahmoud et al. ETI Journal, olume 9, Number, April 7

4 For high values of T(s), Av(s) tends towards. The FA input resistance at the terminal and the output resistance at the terminal is approximately given by (rds//rds5) rin = rout. () T() The FA output resistance at terminal is simply obtained as r = r //r. () ds ds If higher output resistances are needed, cascoded topologies can be used to increase this value and to improve the linearity performance. The FA dc open-loop gain can be given as T() = r in r + r out. (3) Derivative of the output voltage 6 k in k to k out D(v(7)) D(v(5)) D(v()) D(v(3)) Fig.. Derivatives of the output voltage of the proposed FA for different gains. Gain= in (m) III. Simulation esults The performance of the proposed FA circuit was verified by performing PSpice simulations with supply voltages ±.75 using.5 µm TSM MS technology parameters and transistor aspect ratios given in Table. Figure 3 shows the output voltage swing of the proposed FA when used to utput voltage swing (m) Table. Transistor aspect ratios. Transistor W (µm) L (µm) M -M, M 3, M, M 6, M 7.5 M 5, M 6, M 5, M M -M 3, M 3-M M 7 - M 9, M 9 - M M, M, M, M M 6, M 7, M 36, M 37 M 9, M 5, M, M, M k in k to k out Gain = v(7) v(5) v() v(3) in (m) Fig. 3. FA-based variable gain amplifier output voltage. Derivative of the terminal offset voltage 6 terminal offset voltage v xoff (m) off d(v(6)) v(6) l (µa) Fig. 5. terminal offset voltage and its derivative versus terminal input current I. realize an amplifier with different gains. The input voltage was applied at the non-inverting input terminal voltage, the output voltage obtained at the terminal. The inverting input is terminated with kω, while the terminal is terminated with resistance values of kω, kω, kω, and kω. The total standby power dissipation is.56 mw. Figure gives the derivative of the output voltage of the proposed FA versus the input voltage for different gains. Figure 5 shows the variation of the offset voltage across the terminal versus the variation in the input current applied across the terminal (I ) when is equal to zero. The terminal input resistance is less than 36 Ω and the offset voltage is less than m. Figure 6 shows the terminal output current swing versus terminal input current I. Figure 7 shows the magnitude response of the FA when it is used to realize a variable gain amplifier, where in is the A-varying signal with magnitude and the inverting terminal is terminated with a kω and the terminal is terminated with a variable resistance with values of kω, kω, kω, and kω. The FA shows a constant bandwidth ETI Journal, olume 9, Number, April 7 Soliman A. Mahmoud et al. 5

5 Table. Performance comparison between FA introduced in [5], [6], and the proposed FA. Parameters FA [5] FA [6] Proposed FA MS technology (TSM).5 µm. µm.5 µm Power supply ( DD, SS) (.5, -.5 ) (.5, -.5 ) (.75, -.75 ) Total power dissipation.5 mw N.A..56 mw Input voltage dynamic range N.A. -. to to.65 terminal offset voltage while and are grounded N.A. < m < m urrent driving capability N.A. - µa, µa - ma, + ma Ω < Ω < 36 Ω FA bandwidth MHz 6 MHz MHz * N.A.= not available..5 terminal output current (ma) I E out Input/output refereed noise (µ) I(r) I x (ma) Fig. 6. terminal output current swing versus the terminal input current I. Magnitude response (db) Gain= k in k to k out -3.k k k.m M M M vdb(7) vdb(5) vdb() vdb(3) Frequency (Hz) Fig. 7. Magnitude response of the FA-based variable gain amplifier..k k k.m M M M (inoise) (onoise) Frequency (Hz) Fig.. Input and output referred noise spectral densities. for different gains. The FA has a 3 db bandwidth of MHz and a phase margin of 6. The input and output referred noise spectral densities shown in Fig. are less than.5 µ/ Hz. The power supply rejection-ratio (PS) from the positive supply to the output is db, and from the negative supply to the output is 9 db. Table gives a performance comparison between the FA introduced in [5], [6] and the FA proposed in this paper. I. Application: New FA-Based All-Pass Filter The proposed FA is used to realize a new second-order all-pass filter structure which is shown in Fig. 9. The filter consists of three cascading blocks: a weighted differential voltage integrator, a weighted differential voltage adder integrator, and a weighted differential voltage adder amplifier. By cascading the differential voltage adder integrator (N-) times, an N-order all-pass filter can be realized. By direct analysis, the following transfer function is obtained: 6 Soliman A. Mahmoud et al. ETI Journal, olume 9, Number, April 7

6 in 3 out open-loop bandwidth and reduce the voltage-transfer error. The FA block is suitable for low-voltage, low-power applications and is characterized by low voltage-transfer errors and highoutput driving current capability. An application example realizing the proposed second-order all-pass filter was given. Table gives a summary of the simulation results and a favorable comparison between the proposed FA and the FA which was introduced in [5] and [6]. Fig. 9. Proposed FA-based grounded- second-order all-pass filter. out in = ( ) 3 s s s + s ( ) + ( ) + ( ) ( ) () From (), the ω o, Q, and D gain H of the filter are given by ω ( ) o =, Q =, H ( ). 3 =. (5) Figure shows the ideal and simulated magnitude and phase responses of the second order all-pass filter given in Fig. 7, where = kω, = kω, 3 = kω, and = =.5 nf.. onclusion A new MS FA was presented, analyzed, and simulated. This FA has been demonstrated to improve the input stage Phase response (degree) Magnitude response () Ideal phase response Simulated magnitude response Ideal magnitude respnose -. vp() vp() vp() v() -.5.k 3.k k 3k k 3k.M 3.M M 3M Frequency (Hz) Fig.. Ideal and simulated magnitude and phase responses of the second-order all-pass filter based on the proposed- FA. Simulated phase response eferences []. Toumazu and J. Mahattanakul, A Theoretical Study of the Stability of High-Frequency urrent Feedback p-amp Integrators, IEEE Trans. ircuit Syst. I, vol. 3, 996, pp. -. [] S.A. Mahmoud and A.M. Soliman, Novel MS- Balanced- Input Balanced-utput Filter Using the urrent Feedback perational Amplifier, Int. J. Electron., vol., 99, pp [3] A.M. Soliman, Applications of the urrent Feedback perational Amplifier, Analog Integrated ircuits Signal Processing, vol., 996, pp [] A. Assi, M. Sawan, and J. hu, An ffset ompensated and High-Gain MS urrent Feedback p-amp, IEEE Trans. ircuits and Syst. I, vol. 5, no., Jan. 99, pp [5] A.M. Ismail and A.M. Soliman, Novel MS urrent Feedback p-amp ealization Suitable for High Frequency Applications, IEEE Trans. ircuit Syst. I, vol. 7,, pp [6] S.A. Mahmoud, H.. Elwan, and A.M. Soliman, Low oltage ail to ail MS urrent Feedback perational Amplifier and Its Applications for Analog LSI, Anal. Int. ircuits Signal Processing, vol. 5,, pp [7]. Mita, G. Palumbo, and S. Pennisi, Low-oltage High-Drive MS urrent Feedback p-amp, IEEE Trans. ircuit Syst.-II, vol. 5, 5, pp [] S.A. Mahmoud and A.M. Soliman, New MS- Biquad Filter Using the urrent Feedback perational Amplifier, IEEE Trans. ircuit Syst.-I, vol. 6, Dec. 999, pp [9] J. Bayard and M. Ayachi, TA- and FA-Based L Sinusoidal scillators Analysis of the Magnitude Stabilization Phenomenon, IEEE Trans. ircuit Syst.-I, vol. 9, Aug., pp [] B.J. Maundy, I.G. Finvers, and P. Aronhime, Alternative ealizations of MS urrent-feedback Amplifiers for Low- oltage Applications, Anal. Int. ircuits Signal Processing, vol. 3, Dec., pp [] K. Manetakis and. Toumazou, urrent-feedback p Amp Suitable for MS LSI Technology, Elect. Let., vol. 3, no., June 996, pp [] J. Bales, A Low-Power, High-Speed, urrent-feedback p Amp with a Novel lass AB High urrent utput Stage, IEEE J. ETI Journal, olume 9, Number, April 7 Soliman A. Mahmoud et al. 7

7 Solid State ircuit, vol. 3, no. 9, Sep. 997, pp [3] E. Bruun, A Dual urrent Feedback p Amp in MS urrent onveyor, Electron. Lett., vol. 3, 99, pp [] A.M. Soliman, Generation of urrent onveyor-based All-Pass Filters from p-amp-based ircuits, IEEE Trans. ircuit Syst.- II, vol., Apr. 997, pp Soliman A. Mahmoud was born in airo, Egypt, in 97. He received the BSc degree with honors in 99, the MSc degree in 996, and the PhD degree in 999, all from the Electronics and ommunications Department, airo University, Egypt. He is currently an Associate Professor at the Electrical Engineering Department, Fayoum University, Egypt. He is currently also a isiting Associate Professor at the Electrical and Electronics Engineering Department, German University, airo, Egypt. In 5, He was awarded the Science Prize in Advanced Engineering Technology from the Academy of Scientific esearch and Technology. His research and teaching interests are in circuit theory, fully-integrated analog filters, high-frequency transconductance amplifiers, low-voltage analog MS circuit design, current-mode analog signal processing, and mixed analog/digital programmable analog blocks. Ahmed H. Madian was born in Jeddah, Saudi Arabia, in 975. He received the BSc degree with honors and the MSc degree from the Electronics and ommunications Department, airo University, Egypt, in 997 and, respectively. He is currently a esearch Assistant in the Electronics Engineering Department, Micro-Electronics Design enter, Egyptian Atomic Energy Authority, airo, Egypt. His research interests are in circuit theory, low-voltage analog MS circuit design, current-mode analog signal processing, and mixed/digital applications on filed programmable gate arrays. Ahmed M. Soliman was born in airo, Egypt, on November, 93. He received the BSc degree with honors from airo University, airo, Egypt, in 96, the MS and PhD degrees from the University of Pittsburgh, Pittsburgh, PA, USA, in 967 and 97, respectively, all in electrical engineering. He is currently a Professor at the Electronics and ommunications Engineering Department, airo University, Egypt. From September 997- September 3, Dr. Soliman served as a Professor and the hairman of the Electronics and ommunications Engineering Department, airo University, Egypt. From 95 to 97, Dr. Soliman served as a Professor and the hairman of the Electrical Engineering Department, United Arab Emirates University, and from 97 to 99, he was the Associate Dean of Engineering at the same University. He has held visiting academic appointments at San Francisco State University, Florida Atlantic University, and the American University in airo. He was a isiting Scholar at Bochum University, Germany (Summer 95), and with the Technical University of Wien, Austria (Summer 97). In November 5, Dr. Soliman gave a Lecture at Nanyang Technological University, Singapore. Dr. Soliman was also invited to visit Taiwan and gave Lectures at hung uan hristian University and National entral University of Taiwan. In 977, Dr. Soliman was decorated with the First lass Science Medal, from the President of Egypt, for his services to the fields of engineering and engineering education. Soliman A. Mahmoud et al. ETI Journal, olume 9, Number, April 7