Design of a Fully Differential Current Mode Operational Amplifier with Improved Input-Output Impedances and Its Filter Applications

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1 Desin of a Fully Diffeential uent Mode Opeational Amplifie with Impoved Input-Output Impedances and Its Filte Applications Mustafa Altun a, *, Hakan Kuntmanb a Istanbul Technical Univesity, Faculty of Electical and Electonics Enineein, Depatment of Electonics and ommunication Enineein, Maslak, Istanbul, Tukey. b Istanbul Technical Univesity, Faculty of Electical and Electonics Enineein, Depatment of Electonics and ommunication Enineein, Maslak, Istanbul, Tukey. * oespondin Autho. altunmus@itu.edu.t; Phone: ; Fax: (Depatment). Abstact In this pape, a MOS implementation of a fully diffeential cuent-mode opeational amplifie (OA) is pesented. To achieve vey low input esistance, a new technique based on a positive feedback is used in the input tansimpedance stae. Additionally, taditional cuent output stae (Abel Goldminz output stae) with impoved output esistance is selected as a tansconductance stae. esults of simulations exhibit 96 db D ain, nealy Ω input, 3 MΩ output esistances and a ain-bandwidth poduct exceedin 9 MHz. The poposed OA is opeated unde ±. V voltae supplies and desined with.3-μm MOS pocess. Futhemoe, OA-based 4 th ode band-pass (BP) filte consistin of Butteth low-pass () and hih-pass () filtes is ealized as an application example. SPIE simulation esults ae included to veify coectness of the desins. Keyds: Fully diffeential OA, OA-based filtes. Intoduction In ecent yeas, cuent-mode appoach has been extensively investiated. ompaed to the voltae-mode cicuits, cuent mode cicuits have some sinificant advantaes such as inheent wide bandwidth, wide dynamic ane and simple cicuity with love voltae supplies [,]. uent-mode opeational amplifie (OA) is one of the useful cuent-mode inteated buildin blocks. The main advantae of usin OA is its ability to eplace with the voltae opeational amplifie (VOA) when applyin the adjoint netk theoem in voltae mode to cuent mode tansfomation [3]. The cicuit symbol of fully diffeential OA is epoted in Fi.. OA ideally exhibits zeo input esistance and infinite output esistance and cuent ain. It is not difficult to incease output esistance and cuent ain to easonable values. On the othe hand, to et vey low input esistance, some complicated neative feedback confiuations have been suested [4,] and it eneally sens fequency esponse of the amplifie. Moeove, these stuctues cause bie poblems in fully diffeential OA s because of its double input. Positive feedback is anothe solution fo ettin bette input esistance [6,7]. This pape pesents a new appoach dependent on positive feedback to decease input esistance. Taditional cuent output stae (Abel Goldminz output stae) is the most commonly used tansconductance stae but it suffes fom its output esistance [8].A novel cuent output stae with Impoved output esistance is poposed in this study. In the liteatue, most of the OA desins ae sinleended. Howeve, fully-diffeential analo sinal pocessin povides seveal advantaes ove sinle-ended analo sinal pocessin. Some MOS implementations of fully diffeential OA have been epoted [9-]. Unfotunately, these OA s do not offe low input esistance and most of them do not have pope fequency esponse. The poposed fully diffeential OA has simple cicuity and offes hih pefomance. Some useful cuent amplifie based filte applications have been pesented in the liteatue [,3]. To demonstate the pefomance of the poposed OA, a new 4 th ode band-pass filte that enables maximal flatness aound the cente fequency is poposed. I i I i - OA I o - I o Fi. icuit symbol of fully diffeential OA. Poposed OA As shown in Fi., the poposed amplifie has vey simple stuctue. W/L of tansistos and D values of the cicuit ae epoted in Table and Table espectively. The amplifie is confiued fom a diffeential input tansimpedace stae followed by a diffeential output tansconductance stae. Shown in dashed lines at the input stae, M, M4 and M, M compose positive feedback loops to educe positive and neative input esistances espectively. Because, only fou moe tansistos ae used fo input esistance impovements, fequency esponse of the amplifie is not notethy affected by these additional tansistos. Equations () and () show input esistances without positive feedback loops and eneally thei values ae not low enouh. in () in m ()

2 VDD M8 M8 M M M3 Vb Vb M M M7 M M9 M4 Vb Vb M4 M Vb Io- Iin- Iin c M7 Io M6 M7 M M M6 M VSS M M3 M3 M4 M M6 Vb4 M9 Vb4 VSS Fi. Schematic of the poposed OA Input esistance equations of the poposed OA ae shown in (3) and (4). The second tems of (3) and (4) mainly affect input esistance value. If we select its value close to zeo, in also oes nea zeo. Moeove, its value must bie than zeo to ovecome the stability poblem. If we choose m3 m4, m m and m m, m, then we will both ovecome stability poblem and obtain vey low input esistances. onsequently we come such a decision that (W/L) M3 (W/L) M4, (W/L) M (W/L) M, (W/L) M (W/L) M, (W/L) M9 (W/L) M. Futhemoe, because positive feedback is inceasin stability poblem, MOS tansisto matchin paametes ae also consideed in this study. To analyze the MOS tansisto mismatches, fistly we need to calculate vaiances of elative cuent ain facto (K) mismatch- σ(δk/k)- and theshold voltae (VT) mismatch- σ(δvt)- fo tansistos M-M4 and M9-M. As it will be epoted in the Simulation esults pat, input esistances stay positive while considein the matchin paametes in st case. m m4 ( ds m3 ds3 ) (3) in m m3 ds4 m ds in m m ( ds9 m ds ) (4) ds m ds Output esistance of taditional cuent output stae (Abel Goldminz output stae) is shown in equation (). M4, M, M6 and M7 in dashed line ae added to the conventional cuent output stae fo ettin vey bi output esistance values. The output esistance equation of the poposed OA is epesented in (6). ompaed to the conventional one, poposed output stae appoximately offes m o times hihe output esistance. mdsds mds3ds7 ( ) 7( 3 ) out out m m m m m m (6) Tansisto M7 opeates like a esistance (c) and impoves fequency esponse of the OA. c is inseted at the hih impedance node fo compensation. D cuent ain and ain-bandwidth poduct ae iven by (7) and (8) espectively. m m ds4 ds3 ds ds6 A i () m4 m (7) m m f GBW π (8) c 3. OA-based filte ealizations The poposed second-ode low-pass and hih-pass filte confiuations ae shown in Fi. 3. Sinle OA is used fo each ealization and matchin conditions ae 3, fo low-pass filte, 4, 4 6 fo hih-pass filte. m ds m ds3 out out m m m3 m ()

3 As shown in Fi. 4, it is possible to ealize bandpass filte by usin and filtes connectin one s output to anothe s input. With usin this BP filte stuctue, we can aane the passband width. I in Filte Filte I out Fi. 3 (a) OA-based low-pass filte topoloy. (b) OA-based hih-pass filte topoloy. The node analysis of the cicuit of Fi. 3 (a) yields cuent tansfe function as follows iout (9) i in s s The natual anula fequency w o and the quality facto of the filte can be expessed as w o () It should be noted that w o and ae othoonally adjustable. It means that w o can be adjusted without distubin the by chanin eithe o and simultaneously while / is constant. Sensitivity analysis of the filte shows that S S / S S / S / which ae equal-less than unity in manitude. The node analysis of the cicuit of Fi. 3 (b) yields cuent tansfe function as follows iout s () i in s s The natual anula fequency w o and the quality facto of the filte can be expessed as w o () It should be aain noted that w o and ae othoonally adjustable. It means that w o can be adjusted without distubin the by chanin eithe o and simultaneously while / is constant. Sensitivity analysis of the filte shows that S S / H P S S / S / which ae equal-less than unity in manitude. Fi. 4 Band-pass filte stuctue fomed by hih-pass and low-pass filtes. 4. Simulation esults A. Simulation esults of the poposed OA SPIE is used fo simulation with the pocess paametes of a.3 μm MOS technoloy. BSIM3 paamete sets ae used fo modellin tansistos of which theshold voltaes ae nealy. V fo NMOS and -.7 V fo PMOS. The tansisto widths ane fom to μm. Table Tansisto dimensions Tansistos W(μm)/L(μm) M, M /.7 M3, M4, M, M6 / M7 /.7 M8 7/.7 M9, M /.7 M, M, M3 /.4 M4 4/.4 M /.7 M6 7.8/.4 M7./ M8 8/ M9 47/ M, M 7/ M, M3 4/.7 M4, M / M6, M7 6/.7 Table D values of the OA Paamete V DD V SS V b, V b V b3, V b4 I D8, I D6 I D8, I D9 Value ±. V.6V, -.3V.3V, -.8V 3uA ua

4 uent Gain (db) Phase (Deee) - -.E.E.E.E3.E4.E.E6.E7.E8.E9.E Fi. Open-loop fequency esponse of the OA Table 3 summaizes the pefomance of the OA. The OA povides 96 db dc ain, 9 MHz UGBW and 6 phase main uaanteein sinle pole behaviou thouhout the UGBW. Input and output esistance values ae also faily ood. Fo tansistos M-M4 and M9-M, σ(δvt) values ae calculated between.mv 3.9mV and σ(δk/k) values ae calculated between.6% -.7%. As a esult, input esistances become Ω (n) and 8 Ω (p) fo the st case (still positive). Because of the class A opeation, speed of the OA is limited by quiescent cuent. Open loop fequency chaacteistic is seen fom Fi.. The tansient esponse to a step input cuent of ±μa is shown in Fi. 6. Output uent (ua) Time (ns) Fi. 6 esponse of the OA in unity-ain feedback to a ± μa input step (fmhz) Table 3 Pefomance paametes of the OA Paamete Powe Dissipation Open-Loop Gain GBW Phase Main (c.p c.4kω) Output Voltae ane Slew ate Input esistance (n) Input esistance (p) Output esistance Input Voltae Offset (n) Input Voltae Offset (p) Value.66 mw 96 db 9 MHz 6 ±.6 V 4uA/ns 4 Ω 9 Ω 3 MΩ.6mV -3.mV Note that if we did not use impedance impovements, input esistance and output esistance values uld be nealy. kω and 34 kω espectively. On the othe hand, output voltae ane uld be bette, nealy ±.9 V. B. Simulation esults of the OA-based filte Shown in Fi. 4, BP filte confiuation is simulated by choosin the passive element values of filte 7.pF, kω which esults in a natual fequency of f o 3.MHz and a quality facto of.77and fo BP filte pf, kω which esults in a natual fequency of f o 3.kHz and a quality facto of.77. So that, cente fequency of the BP filte is MHz and this confiuation enables maximally flat passband. Fom Fi. 7 it can be undestood that the theoetical and simulation esults ae in ood aeement. The dependence of the output hamonic distotion of BP filte on input cuent amplitude is illustated in Fi. 8. Fom Fi. 8, we can see that the hamonic distotion apidly inceases if the input sinal is inceased beyond 8 μa fo the chosen fully diffeential OA implementation. uent Gain (db) Ideal Simulated.E4.E.E6.E7.E8 Fequency (Hz) Fi. 7 Simulated and ideal band-pass filte esponses

5 THD (%) Peak to Peak Input uent (ua) Fi. 8 Total Hamonic Distotion (THD) values of the filte vesus input peak to peak cuent at MHz fequency. onclusion In this k, vey accuate, fully diffeential OA is poposed and simulation esults ae pesented. A novel appoach is used in input esistance impovement and also vey hih output esistance is achieved by modifyin taditional cuent output stae. Due to the simple cicuity of the poposed OA, hihe than 9 MHz GBW is obtained. It also offes vey hih D ain and ±.6V output voltae swin. As an application example, a new OA-based 4 th ode bandpass filte is poposed. Simulation esults of the filte ae the evidences of accuacy of the poposed fully diffeential OA. efeences [] Palmisano G, Palumbo G, Pennisi S. MOS cuent amplifies. Boston MA:Kluwe Academic Publishes, 999. [] Toumazou, Lidey FJ, Haih DG. Analo I desin: The uent-mode Appoach. London- Pete Peeinus Ltd, 99. [3] obets GW, Seda AS. A eneal class of cuent amplifie-based biquadatic. filte cicuits. IEEE Tans. icuits Syst. 99;39:7-63. [4] Suakamponton W, iewuja V, Kumwachaa K, Dejhan K. Accuate MOS-based cuent conveyos. IEEE Tans. Instum. Meas. 99;4: [] Palmisano G, Palumbo G. A simple MOS II. Intenational Jounal of icuit Theoy and Applications 99;3:6: [6] Wan W. Wideband class AB (push-pull). cuent amplifie in MOS technoloy. Electonics. Lettes 99;6:8:43-. [7] Altun M, Kuntman H. A wideband MOS cuent-mode opeational amplifie and its use fo band-pass filte ealization. Poceedins of Applied Electonics, Pilsen, 6. p [8] Abel AF, Goldminz L. Output stae fo cuentmode feedback amplifies, theoy and applications. Analo Inteated icuits and Sinal Pocessin 99; :43. [9] hen KH, Wan H. Desin of cuent mode opeational amplifie with diffeential input and diffeential output. Poceedins of IEEE Intenational Symposium on icuits and Systems, Hon Kon, 997. p [] Jun S, Kim DM. Fully diffeential cuent opeational amplifie. Electonics Lettes 998;34:6-3. [] Kaulbe T. A MOS cuent-mode opeational amplifie. IEEE J. Solid-State icuits 993;8:7:849-. [] Souliotis G, hisanthopoulos A, Haitantis L. uent diffeential amplifies: new cicuits and applications. Int. J. ic. Theo. Appl. ;9:6:3 74. [3] Kilinc S, am U. uent-mode fist-ode allpass filte employin sinle cuent opeational amplifie. Jounal of Analo Inteated icuits and Sinal Pocessin 4;4:47-3.