Active Filters

Active Filters is a circuit that is designed to pass a specified band frequencies while attenuating all the signals outside that band. It is a frequency selective circuit.

The filters are basically classified as active filters and passive filters. The passive filter networks use only passive elements such as resistors, inductors and capacitors. On the other hand, active filter circuits use the active elements such as op-amps, transistors alongwith the resistors, inductors and capacitors. Modern active filters do not use inductors as the inductors are bulky, heavy and nonlinear. The inductors generate the stray magnetic fields. The inductors dissipate considerable amount of power.

Advantages of Active Filters

Overall the active filter using resistors, capacitors and op-amps has following advantages over the passive filters

i) Flexibility in Gain and Frequency Adjustment : The op-amp gain can be easily controlled in ‘le closed loop fashion, hence active filter input signal is not attenuated. The passive filters need the attenuation. The active filters can be edgily tuned.

ii) No Loading Effect : The op-amp has high input impedance and low output impedance. Hence;ft active filter using op-amp does not cause loading of the source or load.

iii) Cost : Due to availability of modern ICs, a variety of cheaper op-amps are available. The inductors are absent which makes the modern active filters more economical than passive filters.

The most commonly used filters are :

1) Low Pass (LP) filter

2) High pass (HP) filter

3) Band Pass (BP) filter

4) Band Reject (BR) filter. This is also called as Band Stop Filter (BS) or Band Elimination (BE) filter.

Each of these filters use op-amp as an active element and resistors and capacitors. Let us see the frequency response characteristics of these types of filters.