Application of Op-amp Fixators in Analog Circuits
R. Rohith Krishnan, S. Krishnakumar · International Journal of Engineering and Technology · 2016
Nullor elements have applications not only in analog behaviour modeling but also in analog circuit design and analysis.Fixator-norator pair, the emerging tool in analog design is a combination of a nullor and sources.A method for the realization of fixator-norator pair is discussed in this paper.Application of fixator-norator pair into a circuit makes it possible to perform the AC and DC designs in a linear like way.Fixator fixes a critical biasing spec at the design, whereas the pairing norator finds the value of power conducting components or DC sources that meets the design.A scaling amplifier design, an active load design and a CMOS differential amplifier design are provided as examples to demonstrate the procedure and the methodology.Keyword-Fixator-norator pair, Nullor, Op-amp, Analog circuits.I. INTRODUCTION Source allocation and biasing design are the major steps in the design of analog circuits.The process becomes harder as the length of the circuit increases.Nonlinearity is one of the problems that make the design process difficult and time consuming.A linear network is much easier to design as it obeys superposition principle.Thus, a method that performs linear like analysis on nonlinear circuit is effective.That is why the fixator-norator pair (FNP) getting attraction.A FNP can fix a critical biasing spec of an active device at a desired point on the characteristic curve.Thus the additivity property of superposition is applicable in such circuits where each critical operating point is fixed using appropriate FNPs.A fixator is a nullator plus source(s).FNPs are realized to use in analog design using ideal controlled sources with very high gain, but in this paper, we use operational amplifiers.The former one can be used only in circuit simulators but the later one can be used in both simulators and practical labs.Thus such a study gains pedagogical importance.Verhoeven et al. [1] proposed a method in which transistors are DC isolated with the help of controlled sources.These transistors are biased only at the end using local biasing sources.A similar methodology is presented by Reza Hashemian [2] called 'local biasing'.Here no controlled sources are used for biasing purpose but the total number of local biasing source are reduced to half by replacing other half using storage elements like inductors and capacitors.The major difficulty with local biasing is the large number of scattered supplies.Source transformation methods can be used to reduce their number, but such processes are tedious and time consuming.Later, Reza [3]-[5] introduced 'fixator-norator pair', which can avoid many of the problems associated with the earlier methods.Fixators and norators are always used in pair and their use is temporary in the circuit.They can be replaced with actual circuit components when the design process is completed.Circuit simulators cannot directly model the pair; hence in his works, ideal controlled sources with very high gain are used to represent fixator-norator pairs.The remaining part of this paper is arranged as follows.Section II introduces op-amp fixator and its working.A comparison is also made between op-amp fixator and its theoretical counterpart.Section III gives the case study with three examples.Finally section IV concludes the methodology.