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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
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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
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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
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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.
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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.
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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.
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