Design Of A Simple Bandpass Filter Of A Third Octave Equalizer

As they are spread across almost all applications based on electrical circuits, frequency filters form one of the most complex issues on electronic devices. There exist a great number of different design methods based on various approaches. A large group of these filters is based on active components, namely amplifiers, that are equipped with the appropriate feedback to achieve the desired transfer function. In terms of applications operating at audible frequencies it became to be a rule that one or two operational amplifiers are applied, taking into consideration that their input impedance and amplification factor are infinitely high, whilst their output impedance is infinitely low. The simulation results presented in this paper show that these considerations may not be so strict. That means the input impedance may be in tens of kiloohms and the amplification factor of 40 dB without the feedback loop can do enough. Moreover, the operational amplifiers may be replaced by a simple circuit consisting of two transistors, as it is shown in this paper. This should be considered when calculating manufacturing costs of several designs. In this paper, a detailed description of a band pass filter for a third octave equalizer is presented. However, it is obvious, that the same approach can be applied in other designs.