20120140504017

125 views
91 views

Published on

Published in: Technology, Business
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
125
On SlideShare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
Downloads
1
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

20120140504017

  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 160 A STUDY OF MULTIPLE OTA-C ACTIVE LOW PASS FILTER Dr. Rajeshwari .S. Mathad Department of Electronics, Basaveshwar Science College, BAGALKOT, India ABSTRACT A review paper on comparative study on single structure OTA-C low pass filter and multiple structure OTA-C low pass filter. OTA single structure LC filter has a limitation in obtaining -3dB cut off frequency in the range from KHz to MHz. The variation in the frequency with the bias current is inconsistent, because fine tuning is not possible in obtaining -3dB frequency. In order to eliminate inconsistency, and to obtain fine tuning the LC filter is developed using multiple OTAs representing the transconductances gm 1=gm= 1=gm 2 > gm. Under the stated relation a sharp variation in -3dB cut off frequency is observed by tuning the respective transconductances of the system of OTAs in respect of the inequality. The -3dB frequency obtained are linearly related with bias current which is due to the suited value of floating inductance of the filter. Such types of filters are useful in high frequency applications and in instrumentation such as channel analyser. Keywords: OTA-C Operational Transconductance Amplifier-Capacitor. I. INTRODUCTION In recent years Operational Transconductance Amplifier based high frequency integrated circuits, filters and systems were widely investigated. Currently, high frequency, high linearity, and low power are the three main concerns of OTAs. With many efforts, researchers have made significant progress in these three aspects of OTAs. The usefulness of OTAs in comparison with conventional Op-Amps in the design of both first order and second order active filters are well documented. Electronically tuneable circuits attracted considerable attention in the design of analog integrated circuits because different values of resistance, inductance or capacitance can be obtained by the same device. Electronic tuning is one the most attractive feature of the Operational Transconductance Amplifier (OTA), since this makes it possible to tune analog devices in applications such as, filters, oscillators etc. OTA is a differential voltage controlled current source INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2014): 7.8273 (Calculated by GISI) www.jifactor.com IJARET © I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 161 (VCCS) where the output current is controlled by an applied input voltage. Tune ability of a device can be obtained by varying the transconductance (gm) of the OTA which is controlled by the bias current or voltage. Analog designs are viewed as a voltage dominated form of signal processing. But current- mode signal processing circuits are preferred over the voltage-mode signal processing circuits, due to their advantages such as higher signal bandwidth, larger dynamic range, greater linearity, low power consumption, and simple circuitry. Designs for active realizations of passive elements using high performance active devices are a rich area of research. OTA applications were extended in a variety of analog circuits, from simple components as variable resistors and active inductors, to more complicated circuits such as filters and oscillators. The OTA can be connected to create active element, simulating the property of an inductance in order to be an alternative choice of an on-chip inductor instead of a passive spiral inductor which usually has low Q-factor. Low Q factors degrade circuit performance, such as phase noise and gain. On-chip inductors also consume much larger IC areas compared to the active devices. Compared to the on-chip spiral inductors, an active inductor has a higher Q factor and occupies a smaller IC area due to its composition of only active devices and capacitors, and thus is attractive to researchers. This proposed scheme possesses many advantages. Firstly, the structure is very simple and easy to design. No external resistor is required, which can save the area in case of fabricating on a silicon chip. Moreover, the cut off frequency can be easily tuned electronically by adjusting the bias current of OTA, changing the system’s configuration is also very easy and comfortable. This will be useful in abiding when the values of passive devices are deviated. In this paper there is a comparison between single structure OTA-C low pass filter and multiple OTA-C low pass filter. It has been proved that the fine variation of cut off frequency can be achieved in multiple OTA-C low pass filter by simulating the floating inductance. OTAs are manufactured by various manufacturers which are given by different numbers. But with the use of LM13600 and CA3080 OTA’s, it is possible to achieve extremely linear transconductance characteristics with respect to amplifier bias current. In this paper for experimentation, LM13600 OTA is used, which consists of two current controlled transconductance amplifiers each with differential inputs & push pull outputs. The two amplifiers shares common supplies but operate independently. Linearalizing diodes are provided at the inputs to reduce distortion and allow higher levels of signals. Controlled impedance buffers which are especially designed to complement the dynamic range of the amplifier are provided. In the ideal OTA, the output current is a linear function of the differential input voltage, which can be expressed as follows, Iout= (Vin+ − Vin− ) gm . The transconductances gm is given by, gm = Ibias/ 2VT Where VT: thermal voltage = 26 mV at room temperature Ibias : bias current of OTA. [1-7]
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 162 Figure 1: Circuit symbol of OTA 1.1 Derivation of first order OTA-C Filter Consider a first order OTA-C low pass filter as shown in Figure 2. From Figure 2 we get I1 = gm (Vin - V0) From above equation, the transfer function for first order OTA-C low pass filter is given by, gm/ C T(s) = -------------- S + gm/ C Figure 2: First order active low pass filter using single OTA 1.2 Realizations of first order low pass filter using multiple OTAs. First order active low pass filter using multiple OTAs is shown in figure 3 in which the series arm is a simulated floating inductance using three OTAs. From figure 3 it can be inferred that, V3 = I2/SC I2 = (Vin - V0 ) (Vin - Vo) gm2 Solving above equations we have V3 = ------------------------- SC I4 = -gm Vo I3 = -V3gm1 , But we see that I3 + I4 = 0 Therefore transfer function of the designed filter is [8] V0 = T(s) = gm1gm2 / Cgm Vin S + (gm1gm2/Cgm)
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 163 Figure 3: First order active low pass filter using multiple OTA 1.3 Floating Inductor Lot of active elements have been used to simulate the inductance. To achieve any required cut-off frequency in passive filter, we require different values of resistors & inductors as calculated from mathematical expression; which may or may not be available. To solve this problem, we can use OTAs to simulate the inductance to achieve the required inductance values by varying its gm. An active inductor has an impedance inverter that “inverts” a real capacitor into a virtual inductor. Its input emulates a real inductor’s voltage and current. Fig. 3 illustrate diagram of a floating active inductor using three OTAs. Floating inductance simulation using Op amp is very difficult, but using OTA it is easy, just by varying the bias current inductance value can be changed. Figure 4: Three- OTA based floating active inductor We infer that the inductance can be electronically tuned by varying the external bias current IB of the three OTAs [9].
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 164 II. EXPERIMENTAL SETUP The stated circuits of “Fig 2” and “Fig 3” are simulated using Proteus professional 7 shown in “Fig 5” and “Fig 6”. Same circuits are arranged on bread board using usual method, the output of the filter is measured. In first order low pass filter using single OTA bias current of the device is varied to vary the transconductance gm , of the device to obtain different cut off frequencies. For a first order low pass filter using four OTAs with transconductance gm1 gm1, gm2 and gm, by adjusting proper values of bias current of gm1 gm1, gm2 and gm low pass filter action can be obtained and fine variation of cut off frequencies can also be obtained. Figure 5: Circuit diagram of OTA- C Low Pass filter Figure 6: Circuit diagram multiple OTA- C Low Pass filter
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 165 III. RESULTS AND DISCUSSION In OTA-C low pass filter shown in “Fig-5” bias current can be varied from 1 µA to 2mA and the cut off frequencies obtained are from 147KHz to 267MHz respectively. Observations are presented in the table 1. Table 1 Bias Current IB Maximum gain in dB Frequency at -3dB 1µA 0dB 147 KHz 10µA 0dB 1.48MHz 100µA 0dB 14.5 MHz 500µA 0dB 72.8 MHz 1mA 0dB 142 MHz 2mA 0dB 267 MHz Now for single OTA-C filter structure, by varying the bias current with the difference of 10µA, that is from 50µA to 100µA the cut off frequencies obtained are from 7.47MHz to 14.5MHz. For every change in 10µA of bias current there is change in cut off frequency approximately of order 1.5MHz, corresponding observations are given in table 2. Table 2 Bias Current IB Maximum gain in dB Frequency at -3dB 50µA 0dB 7.47 MHz 60µA 0dB 9.00 MHz 70µA 0dB 10.5MHz 80µA 0dB 11.6 MHz 90µA 0dB 13 MHz 100µA 0dB 14.5 MHz Figure 7: Frequency response of Proteus professional 7 simulated OTA-C low pass filter circuit, with pass band from 7.47MHz at -3dB gain. Maximum gain is 0dB. The bias current is 50µA
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 166 In case of multiple OTA-C structure the transconductances of all the three OTAs i,e gm 1 gm 1 and gm 2 are kept constant. The bias current of three OTAs is kept at 2mA each. But by varying the bias current of gm from 1µA to 100µA the cut off frequencies obtained are from 412 KHz to 11.2MHz. For every change in 10µA of bias current from 50µA to 100µA the difference between cut off frequencies are of order of 0.2MHz or 200 KHz. Where as in case of single OTA- C structure this difference is 1.5MHz. Therefore multiple OTA-C structure is more advantageous compared to single OTA-C structure. The required cut off frequency can be obtained just by varying the bias current. The corresponding observations are given in table 3. Table 3 Figure 8: Frequency response of Proteus professional 7.5simulated multiple OTA-C low pass filter circuit, with gm 1=gm 1 = gm 2= 2mA gm = 50µ A with maximum gain 0 dB and frequency of 10 MHz at - 3dB Bias current of gm 1 =gm 1 Bias current of gm 2 Bias current of gm Maximum gain in dB Frequency at -3dB 2mA 2mA 1µA 0dB 412KHz 2mA 2mA 10µA 0dB 3.8MHz 2mA 2mA 50µA 0dB 10 MHz 2mA 2mA 60µA 0dB 10.4MHz 2mA 2mA 70µA 0dB 10.6MHz 2mA 2mA 80µA 0dB 10.8MHz 2mA 2mA 90µA 0dB 11MHz 2mA 2mA 100µA 0dB 11.2MHz
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 167 IV CONCLUSIONS The discussion of observations concludes that we can achieve a tuneable range of cut off frequencies of a filter with change in bias current in respect of the relation stated. The consistency in gain and increase in -3dB frequency is observed with increase in bias current. The advantage of using single OTA and multiple OTAs can be summarized as follows: 1. In case of first method, by using single OTA, we can change cut-off frequency by changing gm but the degree of freedom for changing gm is only one ie bias current of single OTA. Fine variation in cut off frequency is not observed. 2. In case of second method, by using multiple OTAs cut-off frequency can be changed by tuning gm 1gm 1 or gm gm 2 separately or in combination. So we have larger tuneable parameters than the first method. By this method fine variation in cut off frequencies can be obtained. Gain roll off rate is at -20dB per decade. The Low Pass filter structure using OTA has a several advantages in high frequency applications. The structure is very simple and easy to design. No external resistor is required, which can save the area in case of fabricating on a silicon chip. Moreover, the cut off frequency can be tuned electronically by adjusting the bias current of OTAs in combination for specific application. These filters are used in high frequency mixtures and in medical applications where conventional op- amp filters can’t be used [10]. V. ACKNOWLEDGEMENTS I am very much thankful to Prasant K. Mahapatra and et al for the design idea which has been taken from the paper “Realization of Active Filters Using Operational Transconductance Amplifier (OTA)” J. Instrum. Soc. India 35(1) 1-9 and encouraged for further work. VI. REFERENCES [1] Sergio Franco “Design with operational amplifiers and analog integrated circuits”, 3rd ed, pp 133-141. [2] Randall L. Geiger and Edgar Sanchez- Sinencio, “Active Filter Design Using Operational Transconductance Amplifiers: A Tutorial”, IEEE Circuits and Devices Magazine, Vol.1, [1985], pp 20-32 [3] Deliyannis, Theodore L. et al "Single Operational Transconductance Amplifier (OTA) Filters" Continuous-Time Active Filter Design Boca Raton: CRC Press LLC, 1999. [4] Achim Gratz “Operational Transconductance Amplifiers” http://Synth.Stromeko.net/diy/OTA.pdf [5] Datasheet- National semiconductor Corporation, “LM13600/LM13700 Dual Operational Transconductance Amplifiers with linearing Diodes and Buffers.” 2004. [6] D.Prasad, D.R.Bhaskar, A.K.Singh “New Grounded and Floating simulted inductance Circuits using Current Differencing Transconductance Amplifiers” April 2010, Radio engineering, VOL.19, NO.1. [7] Neha Gupta, Meenakshi Suthar, Sapna Singh, Priyanka Soni, “Active Filter Design Using Two OTA based Floating Inductance Simulator”, International Journal of VLSI & Signal Processing Applications, Vol.2,Issue 1, Feb 2012, (47-50), ISSN 2231-3133.
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 4, April (2014), pp. 160-168 © IAEME 168 [8] Prashant K. Mahapatra, Manjeet Singh and Neelesh Kumar, “Realisation of active filters using operational Transconductance Amplifier (OTA),” Journal of Instrumentation Soc. of India, 1999, 35(1), pp 1-9. [9] You Zheng “Operational transconductance amplifier for Giga Hertz Applications”, A thesis submitted to the, Department of Electrical and Computer Engineering, Queen’s University Kingston, Ontario, Canada (September, 2008). [10] Rajeshwari S. Mathad, M. M. Mutsaddi, S. V. Halse, “Design of OTA-C Active Low pass Filter using multiple OTAs ” IOSR Journal of Applied Physics (IOSRJAP) ISSN – 2278- 4861 Volume 1, Issue 4 (July-Aug. 2012), PP 08-12. [11] Dr. K. Ravi Chandrudu, “Voltage Profile Improvement using Series Hybrid Active Filters”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 5, Issue 1, 2014, pp. 59 - 72, ISSN Print: 0976-6480, ISSN Online: 0976-6499. [12] Mohammed Arifuddin Sohel, Dr. K. Chennakeshava Reddy and Dr. Syed Abdul Sattar, “Linearity Enhancement of Operational Transconductance Amplifier using Source Degeneration”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 3, 2013, pp. 257 - 263, ISSN Print: 0976-6480, ISSN Online: 0976-6499.

×