This document discusses active filters. It defines an active filter as one that uses active components like op-amps along with passive components. This allows active filters to have sharper responses and advantages over passive filters. It describes the different types of active filters - low pass, high pass, bandpass and bandstop - and discusses their frequency response characteristics. Specifically, it outlines the passband and stopband regions and how the gain transitions between these regions. It also covers topics like roll-off rates, filter order and poles.

1. Gandhinagar institute of
technology
 Sub – Analog Electronics(2130902)
 Topic – Active Filter
 Guided by – Prof. Pratik Gohel
 Sem – 3rd
 Branch – Electrical (b3)
 Academic year – 2015&2016

2. Prepared by
 Sodha Manthansinh (140120109057)

3. Active Filter

4. 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

5. 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 filter
 Bandstop filter

6. Merits of active filter
 Low cost
 Flexibility in gain
 No loading problem
 No insertion
 Passband gain
 Small component size
 Use of the inductors can be avoided
 Control impedence

7. Demerits
 Its 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.

8. Frequency response
 Frequency response of a filter is a graph of grain
versus frequency.

10. Angular Velocity
 Sometimes the frequency is expressed as
angular velocity in radians/second.
 The relation between angular frequency and f is
as following

11. Characteristics of terminology
 The frequency response graphs discussed the
previous slide
 A 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

12.  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.

13. 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 dBdecade.

14. 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 .

15. 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=

16. Low pass filter configuration

17.  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.

18. High pass configuration

19.  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.

20. Frequency response of bandpass
filter

21.  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.

22. Bandstop filter
 A bandstpo or bandreject filter is complementary
to the bandpass filter it blocks out the frequency
components within the stopband as shown in
figure.