Low‐noise and low‐sensitivity filters using current‐mode coupled biquads

• Published in: International journal of circuit theory and applications 2019-11, Vol.47 (11), p.1713-1735
• Main Authors: Jurisic, Drazen, Moschytz, George S.
• Format: Article
• Language: English
• Subjects: active filters; analog circuits; coupled biquads; Coupled modes; Coupling; current‐feedback operational amplifiers; filtering theory; Noise; Noise sensitivity; output thermal noise; second‐generation current conveyors; Thermal noise; Tolerances
• ISSN: 0098-9886; 1097-007X
• DOI: 10.1002/cta.2695

Summary In this paper, it is shown that by adding coupling to biquad pairs to obtain a fourth‐order band‐pass filter, the output thermal noise is reduced. It is shown analytically that minimum output thermal noise is obtained when the biquads are identical and the negative‐feedback factor used for coupling is the same as for the minimum sensitivity to component tolerances and ambient changes. Furthermore, by optimizing the coupled biquads for maximum dynamic range, the output noise is further decreased. The effect of coupling on the output thermal noise of two coupled biquads (CO) is compared with that of a non‐coupled cascade (CA) realizing the same filter transfer function. Minimum noise is analytically shown to be obtained when the biquads inside the coupled structure are the same as those optimal for minimum sensitivity, ie, identical. The output thermal noise of an optimized coupled structure of two biquads is shown to be approximately 20% lower than in the equivalent cascade.