Electric Filters

An electric filter is a network that transforms an input signal in some specified way to yield a desired O/P signal.

It is a frequency selective network that favors certain frequencies of input signals at the expense of others. Three very common types of filter are the low pass filter, the band pass filter and the high pass filter.

The signals may be considered in the time domain or in the frequency domain, and correspondingly, the O/P requirements of the filter may be stated in terms of time or frequency.

A Low Pass Filter (LPF) allows signals up to a certain maximum frequency to be passed on, frequencies above this cut-off frequency are rejected to a greater or a lesser degree. Hi-fi treble controls and turn able scratch filters are typical low-pass filters. The low pass filter has the properties that low-frequency excitation signal components, down to and including direct current, are transmitted, while high-frequency components, up to and including infinite ones, are blocked. Thus the magnitude of a low-pass network function has ideally the appearance shown on figure M below:

figure M : Low Pass Filter

The range of low frequencies that are passed is called the pass band or the bandwidth of the filter. As shown in the figure above, it is equal to the value of the highest frequency wc transmitted. This frequency is also called cut-off frequency. 

A High Pass Filter (HPF) blocks frequencies below its cut-off frequency while favoring those above. Hi-Fi bass controls and turn-table rumble filters are typical examples. The high pass filter has the property that low frequencies are blocked (stop band), while high frequencies (pass band), are transmitted.

figure N : High Pass Filter

A Band Pass Filter (BPF) selects a range of median frequencies while alternating or rejecting other frequencies above and below those desired. The tuning dial on an AM radio is an example of  a variable band pass filter. The band pass filter has the property that one band of frequencies (pass band) is transmitted, while two bands of frequencies, namely those below and above the pass band, are blocked (stop bands). The range of frequencies that is passed is called the Bandwidth (BW), and is defined as the difference between the frequencies that define the edges of the pass band. Using w1 and w2 as shown in the figures to define the pass band edges the

  BW = w2 - w1 is obtained.

The center frequency w0 of the pass band is defined as the geometric mean of the band-edge frequencies. Thus :

figure O : Band Pass Filter

Capacitors and Inductors are inherently frequency-dependent devices. Capacitors more easily pass high frequencies and inductors better handle lower frequencies. Thus, most filters traditionally have been designed around combinations of inductors and capacitors. These are called passive filters.

Today, there is a new and often much better way to do filtering. Integrated circuitry, particularly the IC op-amp, can be combined with resistors and capacitors to accurately simulate the performance of traditional inductance-capacitance filters. Since this new approach usually has gain and needs some supply power, filters built this way are called active filters.