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ISSN: 2278 – 7798 International Journal of Science, Engineering and Technology Research (IJSETR) Volume 2, Issue 1, January 2013 A description of OTA the topologies of OTA and discussion of telescopic type of OTA Sourabh Singh Chouhan, Devendra Singh Ajnar, Pramod Kumar Jain Abstract— This paper gives a description about of OTA (Operational Transconductance Amplifier).It gives design concept of telescopic type of OTA and a brief description about different topologies of OTA. The .18µm CMOS process is used for design and simulation of this OTA. This OTA uses a biasing current of 15.6µA with vdd and vss of 1.25 V and -1.25V successively. The designand simulation of this OTA is done using CADENCE spectre enviroment with UMC 0.18 µm technology file. The simulation results of this OTA shows that the open loop gain of about 78.16dB. this OTA is having CMRR 230dB. Index Terms —OTA,Topologies of different OTA, Cadence, CMRR , CMOS IC Design. I. INTRODUCTION Due to recent development in VLSI technology, the size of transistors decreases and power supply also decreases. The OTA is a basic building block in most of analog circuits with linear input output characterstics. The OTA is widely used in analog circuits such as neural networks, Instrumentation amplifire, ADC and filter circuits. The Operational transonductance amplifire has similarity with Operational amplifire (OPAMP) in which both have differantial inputs. The basic difference between OTA and OPAMP is, OPAMP is voltage controlled voltage source (VCVS) and OTA is voltage controlled current source (VCCS) i.e. the OPAMP output is in form of voltage since OTA is transconductance amplifire so it converts voltage into current. Figure 1. The bottom transistor Qb is used as a current source; unnder normal circumstance, its drain voltage V is large enough that the drain current Ib is devided between Q1 and Q2 is a sensitive function of the difference between V1 and V2. B. Simple transconductance amplifire The schematic for the transconductance amplifire is shown in figure 2. The circuit of a differential pair and single current mirror, like the one shown in figure 2. Which is used to subtract the drain current is thus equal to I1-I2 II. BASIC OF OTA A. Differential pair Input signals represented as a difference between two voltages. These circuits all use some variant of the differential pair shown in Figure as an input stage. Because the differential pair is so universal useful. Sourabh Singh Chouhan, Dept of Electronics & Instrumentaton, S.G.S.I.T.S,., (e-mail: [email protected]). Indore, India, 09407407167, Devendra Singh Chouhan, Dept of Electronics & Instrumentation, S.G.S.I.T.S,Indore, India, Pramod Jain , Dept of Electronics & Instrumentation, S.G.S.I.T.S,Indore, India, Figure 2 C. Wide range amplifires A simple transconductance amplifire will not generate output voltage below Vmin, which , in turn , is dependent on 1 All Rights Reserved © 2013 IJSETR ISSN: 2278 – 7798 International Journal of Science, Engineering and Technology Research (IJSETR) Volume 2, Issue 1, January 2013 the input voltages. This limitation often is a source of problems at the system level, because it is not always possible to restrict the range of input coltages. We can remove this restriction however , by addition to the circuit, as shown in figure 3 voltage is VDD=1.25 and VSS=-1.25V. The input AC a signal is 7.49mV, 10GHz. the OTA is characterized by various parameters like Gain at dc (AV),Unity gain bandwidth,Input common mode range (Vin(min) and Vin(max)), Load capacitor (CL). The design parameter of this OTA is shown in below in below table II. There are several different OTA’s are used in which this OTA is a simple OTA is a simple OTA with and high gain. The OPAMP is characterized by various parameters like open loop gain, Bandwidth, Noise etc. the performance Measrures are fixed Due to Design parameters such as Transistor size, Bias current and etc. In this paper we describe design of OTA amplifier and design is done in 0.19µm technology. Figure 3 D. Various topologies of ota There are four topologies of OTA. (1) Single stage OTA. (2) Two stage OTA. (3) Gain boosted OTA. (4) Folded cascode OTA. E. Comparision between different topologies The table presents a comparioson of basic OPAMP paramerters for different configuration described above described topologies in table 1 Figure 4 Operation Transconductance Amplifier TABLE II TRANSISTOR SIZE Topology Two stage Telescopic Cascode Gain boosted Folded cascode TABLE I Gain Outputswing High Highest Medium Medium Speed Power Low Highest Medium Low High Medium Medium Highest Medium Medium High Medium III. OPERATIONAL TRANSCONDUCTANCE APMLIFIRE (OTA) DESIGN Figure 4 shows the schematic of Operational Transconductance Amplifier (OTA). In this OTA the supply DEVICE PMOS Transistor NMOS Transistor W/L(µm) 10/01 05/01 IV. SIMULATION RESULTS The design of this Operational transconductance Amplifier (OTA) is done using Cadence Tool. The simulation results are done using Cadence Spectre Enviroment using of the OTA shows that the open loop gain of approximately 78.16dB. The OTA has GBW of about 30.40MHz. 2 All Rights Reserved © 2013 IJSETR ISSN: 2278 – 7798 International Journal of Science, Engineering and Technology Research (IJSETR) Volume 2, Issue 1, January 2013 The Table III shows the simulated result of the OTA. The AC response which shows gain and phase change with frequency is shown in figure 5. Figure 6 shows schematic for CMRR anakysis. TABLE III Specification Values CMOS technology 0.18µm Open Loop Gain 78.16dB Supply voltage +1.25v Load capacitance 1.0pF CMRR 230dB Gain Margin 35.06Db Phase Margin 90.25o Unity Gain BW 30.46MHz Figure 6 Schematic for CMRR analysis V. SIMULATED CHARACTERSTICS OF OTA Figure 7 Common mode and differential mode gain Figure 5 AC response showing gain and phase change with frequency 3 All Rights Reserved © 2013 IJSETR ISSN: 2278 – 7798 International Journal of Science, Engineering and Technology Research (IJSETR) Volume 2, Issue 1, January 2013 [5] Y. Tsividis, Operation and Modeling of the MOS Transistor, 2nd ed. Boston, MA: McGraw-Hill, 1998. [6] D. A. Johns and K. Martin, Analog Integrated Circuit Design. New York: Wiley, 1997. [7] Phillip E. Allen and Douglas R. Holberg “CMOS analog circuit design” ,second edition, Oxford university press, 2007,pp. 269-274. Sourabh Singh Chouhan He has currently persuing M.Tech with specialization in Microelectronics and VLSI Design at S.G.S.I.T.S, Indore, India. He received his Bachelor degree in Electronics and Instrumentation Engineering from Institute of Engineering and Technology, Indore. D. S. Ajnar Figure 8 Calculation of Unity Gain BW VI. CONCLUSION In this paper we present a simple Telescopic Cascoded Operational Transconductance Amplifier (OTA) topology for low voltage and low power applications. This OTA can be used in low power, low voltage and high time constant applications such process controller, physical transducers and small battery operated devices. This work can be in biquad filter design, ADC design and instrumentation amplifiers because of its high gain, high CMRR and low power consuption. REFERENCES [1] J. H. Botma, R.F. Wassenaar, R. J. Wiegerink, “A low voltage CMOS Op Amp with a rail-to-rail constant-gm input stage and a class AB rail-to-rail output stage”, IEEE 1993 ISCAS, Chicago, pp.1314-1317.. [2] Paul R. Gray, Paul L.Hurst, Stephan H.Lewis and Robort G.Mayer “Analysis and design of analog integrated circuits”,Forth Edition, John Wiley & sons, inc.2001, pp.425-439. [3] Adel S. Sedra, Kenneth C.Smith “Microelectronic Circuits”, Oxford university press, Fourth edition ,2002,pp.89-91. [4] Jin Tao Li, Sio Hang Pun, Peng Un Mak and Mang I Vai “Analysis of Op-Amp Power-Supply Current Sensing Current-Mode Instrumentation Amplifier for Biosignal Acquisition System”,IEEE conference,August-2008,pp.2295-2298. He has received the B.E. degree in Electronics and Communication Engineering from D.A.V.V University, India in 1993 and M.E. Degree in Digital Techniques & Instrumentation Engineering from Rajiv Gandhi Technical University Bhopal, India in 2000. He has been teaching and in research profession since 1995. He is now working as Associate Professor in Department of, Electronics & Instrumentation Engineering S.G.S.I.T.S, Indore, India. His interest of research is in Designing of analog filter and Current-Conveyor. P.K. Jain He has received the B.E. degree in Electronics and communication Engineering from D.A.V.V. University, India in 1987 and M.E. Degree in Digital Techniques & Instrumentation Engineering from Rajiv Gandhi Technical University Bhopal, India in 1993. He has been teaching and in research profession since 1988. He is now working as Associate Professor in Department of Electronics & Instrumentation Engineering, S.G.S.I.T.S. Indore, India. His interested field of research is analog circuit design. 4 All Rights Reserved © 2013 IJSETR