Post on 21-Feb-2017
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.