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Proc. of Int. Conf. on Recent Trends in Information, Telecommunication and Computing, ITC Design and Simulation of High Stability 2-Stage Differential Op-Amp Integrator in 180nm CMOS Technology Anu1, Amit Kumar2 and Neeraj Kr. Shukla3 VLSI Design Group, Department of Electrical, Electronics & Communication Engineering ITM University, Gurgaon (Haryana), India Abstract— Integrator is an Op-Amp circuit application which does the mathematical operation of integration, i.e. the output voltage is proportional to the integral of input voltage. The stability of Op-Amp circuit is analyzed by Gain and Phase Margin curves. This paper also discusses the power consumption of Op-Amp as well as Integrator analog circuit. The circuits are simulated and analyzed at 180nm standard CMOS process. The GainBandwidth product of Operational Amplifier is analyzed at different bias voltages. The power of Integrator is 7.844mW which is evaluated by using the Op-Amp as the lower block of the Integrator. The Unity Gain Bandwidth of Operational Amplifier is 15 MHz at 0.7V biasing voltage and 21 MHz at 0.4V biasing voltage with power consumption of 7.158mW and 6.998mW, respectively. Index Terms— Gain Margin, Integrator, Low Power, Operational Amplifier, Phase Margin. I. INTRODUCTION Over the last few years, the electronics industry is evolving in VLSI domain where its major segment of total worldwide sales is dominated by CMOS Chips. In CMOS chips, transistors are scaled down to small geometries which results in higher chip density and improved functionalities. The device scaling helps to accommodate more number of transistors in a chip in addition to lower voltage requirement and improved speed of operation. In low-distortion applications, signals can be scaled down at the expense of lower signal power [1]. However, with decreasing supply voltage, the conflicting requirements of large signal swing and low distortion makes the design task more challenging design task [1]. The aim of the design methodology is to propose simple yet accurate results for the design of Op-Amp and integrator that could be used for analog to digital converter in various applications like sensor signal conditioning, voice-band and audio signal processing. In the literature, the reported techniques for low-voltage and high stability circuit design [2]–[8] enabled integrators to operate with low-voltage power supplies with certain issues with the stability. However, these techniques have rarely resulted in low power consumption with improved stability which is required for portable applications [9]. Double-Sampling Scheme (DSS) is one possible solution for reducing power consumption of an integrator. However, it requires complex bootstrapping schemes in low-voltage systems because previously proposed, non-bootstrapping design techniques such as switch Op-Amp and Op-Amp reset-switching techniques [5]–[7] are applicable only to Single-Sampling Scheme (SSS). Dynamic-SourceFollower Integrators [10] used low-power Switched-Capacitor Integrator which could be applied to a 2ndDOI: 02.ITC.2014.5.525 © Association of Computer Electronics and Electrical Engineers, 2014 order ∑∆ modulator. At 0.18 μm CMOS technology, the Integrator offers a reduction in power dissipation (compared with conventional Op-Amp-based Integrators) upto 9.8 μW with an input capacitance of 0.1 pF, input frequency of 20 kHz and sampling frequency at 500 kHz. In a CMOS based Op-Amp, circuit techniques that are fully compatible with low-voltage submicron CMOS processes were addressed to solve the DC Gain issues with unity gain frequency, like Switched-Op-Amps (SO) [11]–[15] and the Op-Amp-reset Switching Technique (ORST) [16][17]. However, SO circuits face a tradeoff between speed and accuracy due to slow transients, while ORST stages have higher power consumption and settling issues due to unity gain feedback during the reset phase [17][18]. Low Power Op-Amp Design with Current Compensation Technique [19] is used to implement a single output two stage CMOS Op-Amp with at 1.8 V supply voltage and it is designed in 0.18μm CMOS technology having very low power consumption with a high driving capabilities. The Op-Amp had open loop gain of 73.57db, the Gain Bandwidth Product (GBW) as 1.094 MHz and 4.35μW power consumption at a Phase Margin of 65.86º. Another approach for designing of a low power Op-Amp could be done by using Class AB CMOS Fully Differential technique [20] in which an Op-Amp has been designed in a low-cost 0.18 μm CMOS technology with 0.8 V single voltage supply using Cadence Spectre with a BSIM3v3.2. The Op-Amp operates in the subthreshold operation which results in ultra low-power consumption and enhanced slew-rate. The Phase Margin and DC open-loop gain of the Op-Amp with a load capacitance of 10 pF are 65º and 51 dB, while the simulated unity gain frequency is 40 kHz, with a Phase Margin of 65º degrees. The power consumption of the Operational Amplifier is 1 μW with a slew-rate of 0.12 V/μs [20]. This work comprises of five sections where section I presents introduction. Section II consists of the design of two stage Operational Amplifier with an analysis on power and stability. The section II also sheds light on the Integrator design using two stage differential amplifiers. Section III discusses the simulation of Op-Amp and Integrator based on the various parameters like operating frequency, Gain Margin, Phase Margin, power and bandwidth. Finally, the conclusions are presented in section IV. II. TWO STAGE DIFFERENTIAL OP-AMP INTEGRATOR Due to the continuous scaling of supply voltage and channel length, the Op-Amp design has started offering the design challenges in terms of speed, power, gain, etc. and their tradeoffs. Gain Margin and Phase Margin are the measures of stability in closed-loop, dynamic-control systems which indicates the absolute stability and relative stability of the system [16][17]. The Phase Margin and Gain Margin of a minimum phase system should be positive for an Op-Amp and Integrator to be stable. The schematic of two stage differential OpAmp having coupling capacitor and load capacitor of values 2.2pF and 10 pF respectively, is shown in Fig 1. The Op-Amp Integrator performs the mathematical operation of integration with respect to time. It offers output voltage proportional to the input voltage over time. The schematic of Integrator comprising of two stage differential Op-Amp as the lower block, Fig 2. The values of resistor and capacitor used in the Integrator design are 50kΩ and 10pF respectively. III. SIMULATION AND RESULT The design simulation of Op-Amp that comprises of input and output waveform with basic amplification function is presented here, Fig 1. The transient analysis, DC and AC analysis of operation amplifier are presented in Fig 3 and Fig 4. It also discusses the stability of Operational Amplifier. This section also contains the simulations of Integrator with sine wave provided as input to it obtaining a cosine wave as output verifying the mathematical operation of integration. The transient analysis of two stage differential Op-Amp is shown in Fig 3 in which a signal of small magnitude (mV) is amplified by the amplifier. The input voltage of Op-Amp ranges from 50mV to 100mV which is amplified approximately to 1.85 V. When a sinusoidal wave is applied to the input terminals of the amplifier, the amplified output waveform of a continuous sinusoidal wave is showing the AC simulations of Op-Amp as shown in Fig 4. In the presence of negative feedback to the amplifier, a zero or negative Phase Margin (PM), where the loop gain exceeds unity guarantees instability. Thus, positive PM is a safety margin that ensures proper operation of the amplifier circuit for minimum phase systems. The magnitude verses frequency curve of Op-Amp is shown in Fig 5. The Gain Margin curve depicting the unity gain bandwidth of Op-Amp with a biasing voltage of 0.7V is shown in Fig6 and the Phase Margin curve of Op-Amp is also shown in Fig7. The stability 193 Fig1 Schematic Diagram of 2-Stage Differential Op-Amp Fig2 Differential Two Stage Op-Amp Integrator of the system could be concluded from Phase and Gain Margin curves by evaluating phase crossover frequency and gain crossover frequency. The unity gain bandwidth of Operational Amplifier is measured as 15 MHz at 0.7 V biasing voltage and 21 MHz at 0.4 V biasing voltage respectively. A system is said to be stable when the Gain and Phase Margin are positive. The Gain Margin and Phase Margin of Op-Amp are 13.096 dB and 59.582 ◦ with supply voltage as ±2.5V and 0.7V biasing voltage. Also, the Gain Margin and Phase Margin of Op-Amp are 8.388 dB and 26.674 ◦ with supply voltage as ±2.5V and 0.4V biasing voltage. This indicates that the Op-Amp circuit is stable. The power consumption of Op-Amp is 7.158mW and 194 Fig3 Transient Analysis of Differential 2 Stage Op-Amp Fig4 AC Analysis of Differential 2 Stage Op-Amp 6.998mW at a biasing voltage of 0.7V and 0.4V, respectively. Simulation results of Operational Amplifier with biasing voltage of 0.7 V and 0.4 V at a supply voltage are given in Table 1 and Table 2 respectively. Fig5 Magnitude V/S Frequency Curve Fig6 Gain Margin 195 Fig7 Phase Margin TABLE I: SIMULATIONS OF OPERATIONAL AMPLIFIER WITH A VOLTAGE SUPPLY OF ±2.5V AND VBAIS AS 0.7V S. No. 1 2 3 4 5 6 Parameters Biasing voltage UGB Gain (3dB) Gain Margin Phase Margin Power Dissipation Values 0.7V 15 MHz 34.6 dB 13.096 dB 59.582 ◦ 7.158 mW TABLE II: SIMULATIONS OF OPERATIONAL AMPLIFIER WITH A VOLTAGE SUPPLY OF ±2.5V AND VBAIS AS 0.4V S. No. 1 2 3 4 5 6 Parameters Biasing voltage UGB Gain (3dB) Gain Margin Phase Margin Power Dissipation Values 0.4 V 21.10 MHz 60.0379 dB 8.388 dB 26.674 ◦ 6.998mW The sinusoidal signal applied across the terminals of an Integrator. Its output is a cosine function, i.e., an integrated output of an applied signal, Fig8 and Fig9. The simulations of Integrator circuit at supply voltage of 2.5V with biasing voltage at 0.4 V has a power dissipation of 7.844mW at operating frequency of 10kHz, Table 3. Fig8 Input Waveform of Integrator IV. CONCLUSIONS The full custom design simulation of a two stage CMOS Op-Amp and Integrator have been simulated and analyzed. Tables and graphs of different operational parameters for Op-Amp and Integrator are presented. The Phase and Gain Margin, both have positive values and the phase cross over frequency is seen to be more than the gain cross over frequency of the two stage differential Op-Amp. This shows that the system is stable. 196 Fig9 Output Waveform of Integrator TABLE III: SIMULATIONS OF INTEGRATOR S.No 1 2 3 4 5 6 Parameters Supply voltage Bandwidth Vin+ VinVbais Time Period Values ± 2.5 V 15.03MHz 50mV to -50mV 30mV to -30mV 0.4V 100μs 7 8 Frequency Power 10kHz 7.844mW The power of Op-Amp and Integrator is also evaluated. The unity gain bandwidth of Operational Amplifier is 15 MHz at 0.7 V biasing voltage and 21 MHz at 0.4 V biasing voltage with power consumption of 7.158mW and 6.998mW, respectively. The power of Op-Amp is 7.158mW at 0.7V biasing voltage and 6.998mW at 0.4V and the power of Integrator circuit is 7.884mW. REFERENCES [1] G. Ahn,N. Ozaki, H. Youra, K. Yamamura, K. Hamashita, K. Takasuka, “A 0.6-V 82-dB Delta-Sigma Audio ADC Using Switched-RC Integrators,” IEEE Journal Of Solid-State Circuits, vol. 40, no. 12, Dec. 2005. [2] T. Adachi, A. Ishinawa, and K. Takasuka, “A 1.4 V switched capacitor filter,” Proc. 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Sansen, “A 900-mV low-power A/D converter with 77-dB dynamic range,” IEEE J. Solid-State Circuits, vol. 33, no. 12, pp. 1887–1897, Dec. 1998. [13] A. Baschirotto and R. Castello, “A 1-V 1.8-MHz CMOS switched Op-Amp SC filter with rail-to-rail output swing,” IEEE J. Solid-State Circuits, vol. 32, pp. 1979–1986, Dec. 1997. 197 [14] M. Waltari and K. Halonen, “1-V 9-bit pipelined switched-Op-Amp,” IEEE J. Solid-State Circuits, vol. 36, no. 1, pp. 129–134, Jan. 2001. [15] B. Vaz, J. Goes, and N. Paulino, “A 1.5-V 10-b 50 MS/s time-interleaved switched-Op-Amp pipeline CMOS ADC with high energy efficiency,” Dig. Symp. VLSI Circuits, pp. 432–435, Jun. 2004. [16] M. Keskin, U. Moon, and G. Temes, “A 1-V 10-MHz clock-rate 13-bit CMOS delta-sigma modulator using unitygain-reset Op-Amps,” IEEE J. Solid- State Circuits, vol. 37, no. 7, pp. 817–823, Jul. 2002. [17] D. Chang, G. Ahn, and U. Moon, “A 0.9 V 9 mW 1 MSPS digitally calibrated ADC with 75 dB SFDR,” Dig. Symp. VLSI Circuits, pp. 67–70, Jun. 2003. [18] D. Chang and U. Moon, “A 1.4-V 10-bit 25 MSPS pipelined ADC using Op-Amp-reset switching technique,” IEEE J. Solid-State Circuits, vol. 38, no. 8, pp. 1401–1404, Aug. 2003. [19] P. Sheetal1, N. Tripathi, “Low Power Op-Amp Design with Current Compensation Technique,” International Journal of Advanced Research in Computer and Communication Engineering, vol. 2, issue 9, Sept. 2013. [20] M. R. V. Bernal, S. Celma, N. Medrano, and B. Calvo, “An Ultralow-Power Low-Voltage Class-AB Fully Differential Op-Amp for Long-Life Autonomous Portable Equipment,” IEEE Transactions On Circuits And Systems, vol. 59, no. 10, Oct. 2012. ABOUT THE AUTHORS 1 Anu, completed her B.Tech in Electronics and Communication Engineering from Ganga Institute of Technology and Management, Kablana in 2012. She is now pursuing her Master of Technology (M.Tech) in VLSI Design at ITM University, Gurgaon. Her interest includes Digital Design, ASIC Design, VLSI Testing and Verification. 2 Amit Kumar, received the B.E. (Hons.) degree in Electronics and Communication Engineering from C.I.T.M. Faridabad in 2010 and the M.Tech. degree in VLSI Design from N.I.T. Kurukshetra in 2012. He joined ITM University, Gurgaon, (Haryana) India as Asst. Professor in the Department of Electrical, Electronics & Communication Engineering during July-December 2013. His areas of interest include VLSI domain and in particular Analog IC Design, Operational Amplifers and Mixed signal systems. 3 Neeraj Kr. Shukla, (IETE, IE, IACSIT, IAENG, CSI, ISTE, VSI-India), an Associate Professor in the Department of Electrical, Electronics & Communication Engineering, and Project Manager – VLSI Design at ITM University, Gurgaon, (Haryana) India. He received his PhD from UK Technical University, Dehradun in Low-Power SRAM Design and M.Tech. (Electronics Engineering) and B.Tech. (Electronics & Telecommunication Engineering) Degrees from the J.K. Institute of Applied Physics & Technology, University of Allahabad, Allahabad (Uttar Pradesh) India in the year of 1998 and 2000, respectively. He has more than 50 Publications in the Journals and Conferences of National and International repute. His main research interests are in Low-Power Digital VLSI Design and its Multimedia Applications, Digital Hardware Design, Open Source EDA, Scripting and their role in VLSI Design, and RTL Design. 198