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A STUDY OF STABILITY OPERATIONAL AMPLIFIER FOR VOLTAGE CONVERTORS Prof. Sanjay Shrivastava
Ass. Prof., Guru Ramdas Khalsa Institute of Science & Technology Jabalpur, MP, India Email Id:-harshilshri@gmail.com
ABSTRACT
The Unity Gain Bandwidth (UGB) of the op amp designed for the Capacitance to Voltage Convertor circuit should comprise a value greater than 10MHz. After designing the operational amplifier (op amp) the UGB value comes out to be 90 MHz (Mega Hertz) with the numerical of Phase Margin (PM) equivalent to 710 and the excellent DC gain value equals 80.12 decibels (dB). The operational amplifier worked with a reference or bias current of 12 uA. This reference current is same as that of the DC current applied to the operational amplifier. The operational amplifier is designed using the 180 nm technology. Further the operational amplifier is implemented using the Miller Compensation technique having two stages. High gain enables the operational amplifier to work efficiently and the high bandwidth enables it to be able to work at higher speed application. The whole design works on the +5V supply with the load Capacitance value 10pF and compensation Capacitance value 2.4pF with excellent input linearity.
Key words: Operational amplifier, Tower 180nm technology, Frequency compensation, Slew rate, CVC (Capacitance to Voltage Convertors) MEMS Accelerometer and CADENCE
1. INTRODUCTION
Microelectronics has been the enabling technology for the design, development and manufacturing of the hardware and the software systems recently in past 10 years.
The continuous increase and rapid boost in the level of the integration (incorporation) of the electronic devices on a single substrate or on a single die structure has led to the rapid development of increasingly complex system. But the semiconductor technology used in the integrated circuit design has also progressed tremendously. In the present era of constant technical progression the VLSI circuits have developed reaching a point where integration of millions of active components (diodes, BJT’s, FET’s) onto a single chip of a particular dimension has been achieved.
The present prevalent IC technology are able to incorporate complete system on chip with the combination of both analog and digital domain aspects which earlier used to provide the partition boundary between analog and digital domain.
In the early 1980s many experts predicted that the analog circuits would become obsolete in the market and the digital signal will over power the analog domain which would to demise of analog circuits. Digital signal processing algorithms were becoming
increasingly more powerful and popular, and moreover due to the advancements in the integrated circuit technology which made the circuits compact, the implementation of the algorithms became easy. Movement or shifting of signal processing in the dawn of the 21st century from the analog domain to the digital domain has not been fully accomplished, there are still many cases where analog signal processing circuits is prevalent and these cases mainly include high performance complex systems. The various applications where the analog signal processing is still useful and extensively prevalent are:
Natural Signal Processing
Digital Communications
Disk Drive Electronics
Sensors
Wireless Receivers
Optical Receivers
Memories & Microprocessors
In the analog signal processing analog signal which is continuous in nature is processed by means of filters, op amps,
2 OTA and analog signal conditioning circuit.
Being irreplaceable analog circuit design is being carried out extensively. All of the operational amplifier designed will be used in the designing of the Capacitance to Voltage convertor circuits useful for the design of open loop MEMS accelerometer at 180 nm technology. MEMS (Micro electrical and mechanical systems) symbolize a technology used in the gyroscopes and accelerometer. It is a technology used especially for the moving parts and the devices are at microscopic level. MEMS devices are known by different names in different regions such as in Japan they are known as micro machines and in Europe they are referred to as micro system technology. They are the combination of both mechanical and electrical at micrometer scale. In MEMS designing the components are having the size range from 1 micrometer to 100 micrometers and the MEMS devices made from these components have the size range from 20 micrometers to 1 millimeter. But the components area occupation can be more than 1000 mm2. The major components of the MEMS devices are:
Mechanical Elements
Sensing Mechanism
Application specific Integrated circuits (Microcontroller)
One of the chief and main criterion’s in the designing procedure are some components which are illustrating and subsequently implementing some sort of mechanical functionality even if the above said equipment’s or components are not mobile or moving. These are the perfect blend of semiconductor & micro fabrication. Micro- Electro-Mechanical Systems (MEMS) devices can very much fluctuate from the comparatively simpler structures having no presence of element which is under motion to the extremely complex electromechanical system having more than one rotating element or more than one element in motion, working in the control vicinity of the integrated (non-segregated) electronics.
These are the basic building block of the MEMS devices out which the Micro Sensors and Micro Actuators club together to form the transducer column. Micro sensors convert the mechanical energy into the electrical energy. The actual (major) potential of the MEMS devices starts when these micro sensors are integrated on the silicon chip.
Accelerometers are the MEMS devices which are based on the linear motion.
Accelerometers sensors are commonly used to measure the displacement of a mass.
One of the major elements of the accelerometer is the CVC circuit whose chief component is an operational amplifier.
Operational Amplifiers are the basic building block of any electronic circuits.
These are most widely used in the analog as well as in mixed signal circuit domain i.e. in regulators, signal processing units, Low Pass Filter’s (LPF’s) and data convertors (ADC’s and DAC’s) [14]. The main functions of the op amps in these circuits are buffering, filtering and most important of all the amplification of the incoming signals up to a certain level. As it is widely known that the single stage op amps are faster and having superior frequency response than that of the multi-stage op amps. But as op- amps open loop/DC gain doesn’t satiate the standards of the small voltage circuit values thereby they might not be able to stifle the non-linear effects in open loop scenario in order to get desired certainty [14]. So for getting better responses, op amps with higher amount of open loop/DC gain and better stability are used. For the better stability frequency compensation technique is applied.
A High Gain and High Stability Operational Amplifier for the Capacitance to Voltage Convertors at 180 NM Technology. The basic current mirror (CM) circuits configurationally constitute one of the major part in most of the modern day electronic analog and mixed signals circuits. The curtailed value of voltage Cascode Current Mirror has a higher voltage fluctuation with minimal proposed
3 voltage specifications besides some of the basic conventional cascode current mirror circuits [10].
Operational amplifiers are the modern day most recurring elements which are universally used in the analog and mixed signal (VLSI) analysis.
Operational amplifier has significant number of performance parameters on the basis of which its nature can be characterized.
Open Loop Gain/DC gain: Operational amplifier’s (op-amp’s) DC gain predispose the meticulousness or accurateness for the non-unity feedback system with which it is used in the operational amplifiers (op amps). The magnitude value of gain varies by the order of four times depending upon the magnitude of the application where it is being analyzed [1, 2, 3].
Small Signal Bandwidth (SSB)/Unity Gain Bandwidth (UGB): The high frequency behavior of the operational amplifier (op- amp) plays a very vital or pivotal part in numerous electronics applications. As the operating frequency for a specific application shows an increasing trend, the DC gain (open loop DC gain) shows a falling trend inducing greater intensity of errors in the non-unity feedback control system. The Small Signal Bandwidth (SSB) parameter is widely known by the “unity gain frequency” or “unity gain bandwidth”
represented as fu. This parameter can also be used for the -3dB frequency [1, 2, 3].
Large signal Bandwidth: In many such practical systems the op amp must operate in the large transient signals. In this phenomenon the nonlinear behavior makes it impossible to characterize speed by small signal properties. Due to the nonlinearity process the op amp goes into the nonlinear region and shows nonlinearity [1, 2, 3].
Linearity: Open Loop operational amplifier suffers from substantial nonlinearity. They exhibit a nonlinear relationship between its differential drain current and the input voltages provided either at the inverting (negative) or at non-inverting (positive) pin
of the designed operational amplifier (op- amp’s) [1, 2, 3].
CMRR: Common Mode Rejection Ratio (CMRR) may be expressed accordingly with the ratio of difference mode gain value Ad when differential signal is enforced to that of common mode gain value Acm when common mode signal is exercised. On logarithmic scale it is given as the 20 log of above ratio.
PSRR: Two stage unbuffered op amps have been used in many application such as in telecommunication systems. But this two stage op amps suffers from the problem of poor Power-Supply-Rejection-Ratio’s (PSRR). Defined as per capability shown by amplifier to maintain its respective output voltage making its DC power-supply voltage i.e. VDD diversified. The ripple content in the power supply contributes too much noise at the output (o/p) terminal of the operational amplifier (op amp). PSRR (Supply Rejection Ratio) mainly expressed by the ratio of differential gain (AV) maintaining supply voltage i.e VDD equivalent to 0V to that of the gained value obtained from the ripple nature of the supply from the output when the differential input is entrenched to zero value (Add).
Frequency Compensation: Whenever the transfer function of the open loop or closed loop system is established then the poles and zeros formed either lies on the right half or on the left half of the jw plane.
Frequency compensation is used so as to reduce the effect of poles which are near to the jw from affecting the characteristically frequency dependent behavior of the system.
2. MATERIALS & METHOD
For the designing process and implementation of an operational amplifier (op amp) having a higher value of open loop-DC gain along with high stability and
4 excellent CMRR and PSRR the various steps involved are as follows:
The phase Margin can also be calculated as the minimum phase that can be added to a stable system so that the respective system reaches the edge of unstability that is the stable system becomes the marginally stable system. Phase Margin can be calculated from the gain cross over frequency (wgc). The gain cross over frequency also known as the 0 dB frequency connonated as wgc which can be calculated by substituting the magnitude of the system equal to 1 or 0dB in terms of decibels. After the calculation of the gain cross over frequency (wgc) we can put the respective value of wgc into the equation for the calculation of phase response.
3. RESULTS AND DISCUSSION
The implementation of the 2 stage operational amplifier is depicted in the figure below. The first stage of implementation comprises of a differential grade with a gain value equals 44.805 decibels. Subsequently 2nd leg comprises of a common source (CS) amplifier. In the schematic design we have used PMOS as the current element and NMOS as the input signal terminals by providing them with the input signal. The second stage of the design is connected (attached) to the Miller compensation circuit comprising of a compensation capacitor CC. The output is taken across a capacitive load CL=10 pF.
The frequency compensation techniques are implemented so as to obtain the gain phase response smooth and stable. The AC response is usually a gain phase plot so as to obtain open loop/dc gain along with the Phase Margin (PM) for corresponding illustration.
From the schematic drawn it can inferred that all the transistors whether it is NMOS or PMOS are in saturation region. The enforcement of input voltages to the Operational Amplifier’s (op-amps) inverting and non-inverting terminals is being
executed. Magnitude value of input voltage is sweeped from 0V (Zero Voltage) to +5V with a step size of the sweep equals 0.1.
Ideally the dc magnitude is kept at 2.5 V so as to obtain proper AC response during AC analysis. These analysis are done under the parametric analysis. The AC magnitude of the input signal is 1uV or 2.5 uV depending upon the requirement of the circuit. The nature of the source is dc source with AC magnitude in order to get proper AC, DC and transient response.
4. CONCLUSIONS
From the analysis of the various analog circuits discussed in the preceding sections we can deduce that we have been able to achieve op-amp’s open loop/DC gain equivalent to 80.12 dB with the Phase Margin (PM) value equals 710 and Unity Gain Bandwidth (UGB) numerical value attained of 90 MHz (Mega Hertz). Now this design can be exclusively used in the charge to voltage convertor circuit for the design of open loop MEMS accelerometer using 180nm technology.
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