V.V. Maslennikov - Dr.Sc. (Eng.), Professor, Faculty of Automatics and Electronics, National Research Nuclear University «MEPhI». E-mail: vmaslennikov@mail.ru
Е.А. Dovgopolaya - Student, Faculty of Automatics and Electronics, National Research Nuclear University «MEPhI». E-mail: e.a.dovg@yandex.ru

It-s important to choose the biquadratic section with frequency properties of amplifier-s low effect on section parameters during the designing of active filter. That-s why the analysis of root-loci and its characteristics under such conditions is more than relevant. At the same time it-s important for the engineer to knack of quickly and easily estimator of like value of the parameter. It is permissible to resort to modern simulation methods and to get the ready-made amplitude-frequency characteristic. However, consideration of necessary searching different sections and using amplifier-s microchips made this task labor-consuming even with using modern PC and corresponding packages of program applications. Therefore in the view of authors on similar occasions it-s more logic in calculation of major parameters of the circuit without amplitude-frequency characteristic building. So the approximate formulas allow neglecting the simulation under analysis of filter-s transfer function pole-s frequencies and Q-factors. Certainly it-s not except of inaccuracy but it-s rather precision to the practice using. Besides, using of approximate formulas is much handy and efficient for the computational resources. The degree of polynomial in the transfer function-s denominator increases per unit with every successive OpAmp which include the in-circuit correction chain when the influence of OpAmp frequency properties is in account. By-turn, the effect is considerable complication of receiving the exact formulas and calculation the necessary parameter according to it. The derivable results are extremely bulky and practically unacceptable to the analysis. (To justify this fact there is an exact formula for the Rauch circuit - classical section which contains only one OpAmp, so it has cubic characteristic equation that could be analytically solved with the Cardano-s method - in this paper.) In that case it could be rationally to resort to the usage of different philosophy. Let-s keep the characteristic equation to the quadratic and take proper account of OpAmp frequency properties in this equation-s coefficients. Then the approximate formulas have moderate error but the calculations are extremely simplified. The prepared work contains the consideration of some routine active selective filters with the realization on OpAmp, receipt of the approximation formulas for it and errors estimation. The last one is especially needed because of lack of such investigation for the method [1]. Also the important aspect viewing this paper is specializing certain of the results [4] with the dispersion of the parameters and emergent compensation in account. There are only frequency properties of OpAmp effect on the pole-s Q-factors in this paper because of known ways of resonance frequency control but the Q-factor control is laborious at times. Moreover, Q-factor fluctuations may costs of section-s instability. Findings let the probable frequency range of active sections be defined.