Limit cycle behavior in a class-AB second-order square root domain filter

Abstract

Publication informationAnalog Integrated Circuits and Signal Processing, 68 (2) Abstract-This paper shows how an unwanted limit cycle can be exhibited by a second-order CMOS companding filter. The filter employs the quasi-quadratic law of the MOS transistor in strong inversion and saturation to achieve compression together with a Class-AB topology to extend the dynamic range. In the zero-input case, the filter operates in the manner expected of an externally-linear circuit. However, when a standard linear IC design technique is applied to it, unwanted zero-input sustained oscillations may be observed. Simulations from PSpice and measurement results from a semi-custom realization in a 0.8µm CMOS process are used to explore this behavior. This work highlights an aspect of the behavior of such filters that must be taken into account by analog designers. 2Limit Cycle Behavior in a Class-AB Second- Order Square Root Domain FilterCarlos A. De La Cruz-Blas, and Orla FeelyAbstract-This paper shows how an unwanted limit cycle can be exhibited by a second-order CMOS companding filter. The filter employs the quasi-quadratic law of the MOS transistor in strong inversion and saturation to achieve compression together with a Class-AB topology to extend the dynamic range. In the zero-input case, the filter operates in the manner expected of an externally-linear circuit. However, when a standard linear IC design technique is applied to it, unwanted zero-input sustained oscillations may be observed. Simulations from PSpice and measurement results from a semi-custom realization in a 0.8µm CMOS process are used to explore this behavior. This work highlights an aspect of the behavior of such filters that must be taken into account by analog designers.

A systematic class AB state space synthesis method based on MOSFET square law and translinear square‐root cells

“…This principle has proven to be successful in the extension of the dynamic range and low power consumption in log domain filters [20,21]. However, there is a very limited number of different SRD filter circuits designed by using Class AB circuit structure [31][32][33][34][35].…”

“…Then in 1999, generalized Class AB filter synthesis theory was proposed in the log domain . Researchers interested in these advantages and many papers have been presented on this issue . Class AB circuit structure is a very useful method, but until now, this method has been rarely used in the design of SRD circuits .…”

“…This method uses BJT technology and cannot be applicable directly for SRD circuits. Therefore, in this paper, differential Class AB structure is reorganized for SRD circuits, which are based on CMOS technology rather than BJT.Some applications of Class AB are available for SRD filters [31][32][33][34][35]. For example in [31], researchers proposed a different Class AB linear transconductance model that was applied to first order SRD low-pass filter applications.…”

“…Some applications of Class AB are available for SRD filters [31][32][33][34][35]. For example in [31], researchers proposed a different Class AB linear transconductance model that was applied to first order SRD low-pass filter applications.…”