Low Input Impedance Current Differencing Unit for Current Mode Active Devices Improved by Positive Feedback and ZC-CDBA Filter Application

Transcription

1 Low nput mpedance Current Differencing Unit for Current Mode Active Devices mproved by ositive Feedback and C-CDBA Filter Application Ersin Alaybeyolu, Arda Güney 2, Mustafa Altun, and Hakan Kuntman stanbul Technical University, Department of Electronics and Communication Engineering, stanbul, Turkey itu.edu.tr, itu.edu.tr 2 Yildiz Technical University, Department of Electronics and Communication Engineering, stanbul, Turkey aguney@yildiz.edu.tr Abstract n current mode analog circuit design, current differencing unit is widely used at the input stage of the current mode analog building blocks. The low input impedance current differencing unit is excellent for current mode circuit realization. n the current study, the positive feedback is used for reducing the input impedance of current differencing unit. The proposed current differencing unit is tested at the input stage of the C-CDBA ( copy current differencing buffered amplifier) recently published as a current mode analog building block. The proposed circuit for C-CDBA is verified with the KH analog filter application. The circuit simulations are verified by using BSM3.8 μm level-7 SCE parameters.. ntroduction Analog signal processing systems can be examined in two main groups as voltage-mode or current-mode circuits, in terms of operating principles. The voltage-mode circuits input signal and the output signal are voltage. The input signal and output signal are current in current mode circuits. Analog building block based on the current-mode, such as current conveyors, current operational amplifiers seem to have a better signal dynamic range and closed-loop bandwidth performance than conventional counterpart voltage mode circuits [-]. C-CDBA ( Copy Current Differencing Buffered Amplifier) and C-CDTA ( Copy Current Differencing Transconductance Amplifier) are recently recommended as current mode analog building blocks by D. Biolek [3]. The C- CDBA and C-DTA increase the universality of the CDBA and CDTA, where a copy of the current through the terminal is available at the -Copy terminal. The classical current mirror is used to copy the terminal output terminal current. The third generation current conveyor is also applicable to duplicate the terminal output current [3]. The input stage of these current mode elements consists of current differencing unit. The and input terminal impedances of the current differencing unit must be ideally zero. n the current study, the input impedance of the current differencing unit block is decreased by positive feedback technique. The -Copy Current Differencing Buffered Amplifier also has two high impedance, -copy terminals and one low impedance terminal. n the last section, a current mode C-CDBA filter application is presented. The proposed filter contains two C- CDBAs, two grounded capacitors and two resistors. The proposed filter has also low passive sensitivities. Low input impedance current differencing unit structures are useful for small value resistors in current mode implementations. Application circuits designed by exploiting smaller value resistors allow the design of integrated circuit structures occupying less area. 2. Reduction method for input impedance The engineer solves problems with the most accurate approach. n electronic circuits, the voltage source internal impedance is ideally zero and the current source internal impedance is ideally infinite. n reality, it is impossible to realize infinite internal impedance for current sources. Designers who use building blocks in order to obtain impedance values close to ideal, benefit from negative or positive feedback. n this work, positive feedback is used to reduce the input impedance of the current differencing unit. Also, comparison of negative and positive feedback is given. Fig., 2 and 3 show a system without feedback, the positive feedback and the negative feedback, respectively. i, p and n indicates the internal impedance value of the system without feedback, positive feedback and negative feedback, respectively. Fig.. A system without feedback. Fig. 2. The positive feedback system. Fig. 3. The negative feedback system. The internal impedance value of a general system is the ratio of the voltage over the system to the current which flows inside of the system. The internal impedance for Fig. is given in Equation. 26

2 V i i () The internal impedance seen from the Fig. 2 with positive feedback system is shown in Equation 2. V p i i i The impedance value is reduced by the ratio of p / i. The ratio of the p / i must be selected lower than unity for positive impedance values. The negative impedance values can also be obtained by using positive feedback. The internal impedance seen from the Fig. 3 with negative feedback system is shown in Equation 3. (2) V n + (3) i i i t is obviously seen from Equation 2 and 3, the positive feedback system decreases the input impedance more than the negative feedback system. 3. C-CDBA CMOS structure The CMOS structure of the C-CDBA is shown in Fig. 4. The equation matrix of the C-CDBA is given in Equation 4. M-M6 transistors belong to the current differencing unit. -8 transistors are the voltage buffer s transistors. M7- transistors are used to copy the current of terminal. The classical current mirror structure consists of M7- transistors. +Vdd M8 M7 Vb Vb2 M M M9 Vb2 M3 M7 M8 2 3 C 4 5 M Vb3 M6 Vb3 M5 M9 6 b M3 M5 Vb4 M6 7 8 Fig. 4. C-CDBA CMOS Realization. The positive feedback transistors to reduce the terminal input resistance are M and M. The positive feedback transistors to reduce the terminal input resistance are and. The input terminal impedances formulas are given in Equations 5 and 6 [6, 9]. i z i zc v w v p vn v z iw i p in iz (4) transistors to the terminal input impedance is given in Fig. 7. The phase margins show that the real part of the impedances are positive. The effect of the M and M transistors to the terminal input impedance is also given in Fig. 8. The desired input impedance level can be obtained by changing these transistors /L ratios. gm g m4 rin ( gds+ gm3 + gds3) (5) gm gm3 gds4 + gm2 + gds2 r g g g m m in+ ds9 m2 ds2 gm9gm 2 gds + gm + gds 9 ( + + ) (6) The input impedances of the and terminal are given in Fig. 5, 6, respectively. The input impedances compared with [7, ] are approximately zero. The effect of the and g g Fig. 5. terminal input impedance. 27

3 The biasing V b, V b2, V b3 and V b4 voltages are selected as 3mV, -4mV, mv and 3mV, respectively. The b current is selected 3μA. The change of terminal current according to the terminal current of the C-CDBA CMOS realization is given in Fig. 9. The terminal current dynamic range was observed between 3μA, -3μA. The terminal output impedance, the change of terminal voltage according to the terminal voltage and the output impedance of terminal of the C-CDBA CMOS realization are given in Fig.,, 2, respectively. The output impedance at terminal was found.5m. The terminal output impedance value is enough to drive the load of our application. The terminal voltage dynamic range was observed between 275mV, -275mV. The terminal output impedance was found 6.4. Fig. 6. terminal input impedance n (ohm) 5 Fig. 9. The change of terminal current according to the terminal current (m) x -6 Fig. 7. The effect of the and transistors to the terminal input impedance. 8 M M 6 p (ohm) 4 2 Fig.. The terminal output impedance (m) x -6 Fig. 8. The effect of the M and M transistors to the terminal input impedance. From Fig. 7, it can be easily observed that the input impedance is reduced by increasing the width of and by decreasing the width of. From Fig. 8, the low input impedance is improved by increasing the width of M and by reducing the width of M. Fig.. The change of terminal voltage according to the terminal voltage. 28

4 The application circuit is given in Fig. 3. The structure of the KH filter has two band pass filter sections, two high pass filter sections and one low pass filter section. One of the high pass filter section has high impedance. The capacitors C and C 2 are.56pf. The resistors R and R 2 are 2k. The filter output of the application circuit with ideal characteristics and the total harmonic distortion for input signal at MHz center frequency. are given in Fig. 4, 5, respectively. Fig. 2. The output mpedance of terminal. in C-CDBA C R C-CDBA 2 C R2 L The performance characteristics of the C-CDBA CMOS structure are seen in Table. The sizes of the transistors are shown in Table 2. Table. C-CDBA simulation results. C C2 ower Supply terminal current dynamic range terminal voltage dynamic range z/n (-3dB) bandwidth z/p (-3dB) bandwidth terminal input impedance terminal phase margin terminal input impedance terminal phase margin terminal output impedance terminal output impedance Vw/Vz (-3dB) bandwidth ower Consumption ±.9V -3μA z 3μA -275mV Vw 275mV 48,674MHz MHz o o M 67.8MHz 33.66μ Fig. 3. C-CDBA filter application [5]. The proposed filter characteristics are in good agreement with the ideal characteristics of the low pass, band pass and high pass filter outputs as seen from Fig. 4. Table 2. C-CDBA transistors sizes. Transistors (/L) M,M9 2μ/.36μ 7.2μ/.36μ M 6.2μ/.36μ 4μ/.54μ M3,M5,M6,M7,M8,M,,M3,,M5, M7,M8,M9 4μ/.36μ M6, μ/.36μ 36μ/.36μ 6 72μ/.36μ 2, 3, 4, 5, 7, 8 3.6μ/.36μ 4. C-CDBA application circuit THD % Fig. 4. C-CDBA filter simulation results. KH filter structure is one of the widely used filter structure in analog signal processing. KH filter is proposed by Kerwin, Huelsman, ewcomb using state-variable synthesis in 967. t is also produced commercially. The most important characteristic of the KH filter transfer function is the adequacy for different type of filter implementation (band pass, high pass, low pass) at the same time. An another important property of KH filter is the conformity for low sensitivity realization in(a) x -5 Fig. 5. The total harmonic distortion for input signal at MHz center frequency. 29

5 The high-pass, band-pass, low-pass filter transfer function, the pole angular frequency and the quality factor Q are given in Equation 7, 8, 9,,, respectively [5, 8]. 2 s G GG s + s + C CC 2 G s s s C CC L C 2 G GG + + GG CC s + s + C CC 2 G GG GG (7) (8) (9) ω () CC Q GC 2 () GC Sensitivity analyses of the proposed filter with respect to active and passive components yield the following Equation 2, 3, respectively [5, 8]. S S S S.5 (2) G G2 C C2 S S S S.5 (3) Q Q Q Q G2 C G C2 5. Conclusion A low input impedance current differencing unit was developed by positive feedback for current mode analog building block input stages. The importance of the low input impedance and the method for reducing input impedance were investigated. The KH filter application based on -Copy Current Differencing Buffered Amplifier was presented. The performance of C-CDBA was verified with the SCE BSM3.8 μm level-7 SCE parameters. 6. References [] C. Toumazou, F. Lidjey, D. Haigh, Analog C Design: The Current-Mode Approach, Exeter, UK, eter eregrinus,99. [2] B. ilson, Trend in current conveyor and current-mode amplifier design, nt. J. Electron., Vol. 23, pp ,992. [3] Biolek D., Senani R., Biolková V., Kolka., Active Elements for Analog Signal rocessing: Classification, Review, and ew roposals, RADOEGEERG, VOL. 7, O. 4,, pp DECEMBER 28. [4] Acar C, Ozoguz S. A versatile building block: currentdifferencing buffered amplifier suitable for analog signal processing filters. Microelectron J 999;3:57 6. [5] E. Alaybeyoglu, A. Güney, H. Kuntman, A ew CMOS C-CDBA Realization ans ts ew Filter Applications, roc. of EUROCO 23, agreb, Croatia. [6] M. Altun and H. Kuntman, Design of a Fully Differential Current Mode Operational Amplifier with mproved nput- Output mpedances and ts Filter Applications, AEU: nternational Journal of Electronics and Communications, Vol.62,. 3, , 28. [7] Kacar, F., Kuntman, H. A ew mproved CMOS Realization of CDTA and ts Filter Application, TJEECS: Turkish Journal of Electrical Engineering & Computer Sciences, Vol.9, o.4,63-642, 2. [8] E. Alaybeyolu, Analog circuit design with new active circuit components, M.Sc. Thesis, stanbul Technical University, stanbul, Graduate School of Science Engineering and Technology, Turkey, 23. [9] M. Altun, Akm modlu ilemsel kuvvetlendirici tasarm ve uygulamalar, M.Sc Thesis, stanbul Technical University, nstitute of Science and Technology, Turkey, 27. [] Uygur A., Kuntman H., Seventh order elliptic video filter with.db pass band ripple employed CMOS CDTAs, AEU: nternational Journal of Electronics and Communications, Vol.6, , 27. [] Dalibor Biolek, Josef Bajer, Viera Biolková, denk Kolka, Michal Kubíek, Copy - Controlled Gain - Current Differencing Buffered Amplifier and its applications nt. Journal of Circuit Theory and Applications, vol. 39, no. 3, pp , 2. 3