Gandhinagar institute of technology

Sub – Analog Electronics(2130902)Topic – Active FilterGuided by – Prof. Pratik GohelSem – 3rdBranch – Electrical (b3)Academic year – 2015&2016

Prepared bySodha Manthansinh (140120109057)

Active Filter

What is a Filter?A filter is basically a “ frequency selective “

circuit. It is designed to pass a specific band of frequencies and block input signals of frequencies outside this band.

Classification of Filters

What is Active Filter?The active filters use active devices such as an op-amp

or transistors alongwith the passive components.Active filters have more sharp frequency response

characteristics and have many other advantages over passive filters.

Types of active filter:Low pass filter High pass filter Bandpass filterBandstop filter

Merits of active filterLow costFlexibility in gain No loading problemNo insertionPassband gainSmall component sizeUse of the inductors can be avoidedControl impedence

DemeritsIts has need dc supply.Active filter are limited in their frequency

range .op-amp had a finite gain bandwidth product.

Active filters can not handle of large amount of power.

Frequency responseFrequency response of a filter is a graph of

grain versus frequency.

Angular VelocitySometimes the frequency is expressed as

angular velocity in radians/second.The relation between angular frequency

and f is as following

Characteristics of terminologyThe frequency response graphs discussed

the previous slideA low pass filter passes all the frequencies

below its cut-off frequency and stop all the frequencies which are above the critical frequency.

The critical frequency forms the boundary between the two bands called as pass band and stop band of the filter.

Frequency response of an ideal low pass filter

Pass band it is the band or range of frequency which

are allowed to pass out through the output by filters without any attenuation.

Stop band it is the band or range of frequencies

which are not allowed to pass through to the output by the filter.

Roll off rates The gain falls off rapidly in the stop band as

shown in fig the rate at which if falls off is called as the roll off rate .

If the increase the filter rate order by 1 then the roll off rate is increased by 20 dB\decade.

Poles and Order For the general purpose high pass and low

pass filters the term “poles” and “order” will have the same meaning.

That means the number of poles is equally to the order of filters this also tell the roll of filters .

Frequency response characteristics of filters

Frequency response characteristics of a filter is the graph of gain filer on Y-axis versus frequency on X-axis

Filter gain=

Low pass filter configuration

it shows that a low pass filter has a constant gain from 0 to a high cut off frequency fc.

The frequencies between 0 to fc are known as “ passband frequencies” whereas the frequencies beyond fc are known as the “stopband frequencies”.

At f= fc the filter gain makes a sudden transition to zero. Therefore all the frequencies beyond fc are completely attenuated.

Previous slide figure shows the frequency response of a practical low-pass filter. gain does not change suddenly at f= fc. Instead as f increase, the gain reduces gradually.

High pass configuration

Its “stopband” extends from f=0 to f=fc where fc is the cut off frequency. the “passband” will be for all frequencies above Fc.

The gain of an ideal high-pass filter is 0 over its stopband constant over its passband.

The gain make a sudden transition from 0 to 1 at f= fc show in figure.

Frequency response of bandpass filter

Its “passband” extends between the two cut-off frequencies fL anfd fH with fH>fL. The frequencies outside this passband lie in the “stopband “.

Thae gain of an ideal bandpass filter is 0 over the stopband and constant over its passband.

The gain will make sudden trasitions from 0 to 1 at f=fL and from 1 to 0 f=fJ as shown in figure.

Bandstop filterA bandstpo or bandreject filter is

complementary to the bandpass filter it blocks out the frequency components within the stopband as shown in figure.