8/4/2019 5 FILTER Passive
1/29
FILTER
8/4/2019 5 FILTER Passive
2/29
A filter is a device that passes electricsignals at certain frequencies or
frequency ranges while preventing the
passage of others.Electronic filters are electronic circuits
which perform signal processing
functions, specifically to remove
unwanted frequency components fromthe signal, to enhance wanted ones, or
both.
8/4/2019 5 FILTER Passive
3/29
Passive or active
Analog or digitalHigh pass, low pass, bandpass.
8/4/2019 5 FILTER Passive
4/29
Combinations of resistors (R),
inductors (L) and capacitors (C)
Do not depend upon an externalpower supply
8/4/2019 5 FILTER Passive
5/29
RC and RL filters,
L-section, T-section and -section
8/4/2019 5 FILTER Passive
6/29
An active filter is a type of analog electronic
filter.
Combination of passive and active (amplifying)
components,
Require an outside power source.
Op-amps are frequently used in active filter
designs.
These can have high Q factor and can achieveresonance without the use of inductors.
However, their upper frequency limit is limited
by the bandwidth of the amplifiers used.
8/4/2019 5 FILTER Passive
7/29
Passive filter:
1. Using passive components such as resistors,
capacitors , inductors etc
2. No power consumption.3. low voltage gain.
Active filter:
1. Using active components such as transistor,
op amps etc
2. need power source
3. can have high voltage gain
8/4/2019 5 FILTER Passive
8/29
PASSIVE FILTER
Speaker
Power supply
Filters in power distribution networks etc
ACTIVE FILTER
Instrumentation Communication
Audio application
Biomedical systems
etc
8/4/2019 5 FILTER Passive
9/29
Inductors block high-frequency signals and
conduct low-frequency signals,
Capacitors do the reverse.
Resistors have no frequency-selectiveproperties, but are added to inductors and
capacitors to determine the time-constants
of the circuit, and therefore the frequencies
to which it responds.
8/4/2019 5 FILTER Passive
10/29
Passive filter can be divided into three (3)
types:
i. Low pass filter
ii. High pass filteriii. Band pass filter
8/4/2019 5 FILTER Passive
11/29
Cut-off frequency - a frequency level above
or below which a device fails to respond or
operate efficiently .the cutoff frequency
reduces energy flow through the device andprevents it from working as designed.
Frequency pass-band - The range of
frequencies that can pass through the filter.
8/4/2019 5 FILTER Passive
12/29
Frequency bandwidth - the frequency range
between two specified frequency cut-off
points (c), that are 3dB below the maximum
centre or resonant peak while attenuating orweakening the others outside of these two
points.
8/4/2019 5 FILTER Passive
13/29
Consists of a resistor and capacitor .
The capacitive reactance, (Xc) will be very
large compared to the resistive value of theresistor, R
8/4/2019 5 FILTER Passive
14/29
As a result the voltage across the capacitor, Vc
will also be large while the voltage drop across
the resistor, Vr will be much lower.
At high frequencies the reverse is true with Vc
being small and Vr being large.
A filter in which the signal passes through an
inductor, or in which a capacitor provides a
path to ground, presents less attenuation to
low-frequency signals than high-frequency
signals
8/4/2019 5 FILTER Passive
15/29
= RC
Gain (dB) = 20 log Vout
Vin
8/4/2019 5 FILTER Passive
16/29
8/4/2019 5 FILTER Passive
17/29
an input voltage across the series
combination of a capacitor and a resistor
and using the voltage across the resistor asan output.
http://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Capacitor8/4/2019 5 FILTER Passive
18/29
If the signal passes through a capacitor, or
has a path to ground through an inductor,
then the filter presents less attenuation to
high-frequency signals than low-frequency
signals.
8/4/2019 5 FILTER Passive
19/29
The circuit gain, Av which is given as
Vout/Vin (magnitude) and is calculated as:
Gain (dB) = 20 log VoutVin
8/4/2019 5 FILTER Passive
20/29
8/4/2019 5 FILTER Passive
21/29
By connecting or "cascading" together a single
Low Pass Filter circuit with a High Pass Filter
circuit, produces a Band Pass Filter. Band Pass Filters passes signals within a certain
"band" or "spread" of frequencies without
distorting the input signal or introducing extra
noise.
8/4/2019 5 FILTER Passive
22/29
This band of frequencies can be any width
and is commonly known as the filters
Bandwidth.
Bandwidth is defined as the frequency range
between two specified frequency cut-off
points (c), that are 3dB below the maximum
centre or resonant peak while attenuating or
weakening the others outside of these two
points.
8/4/2019 5 FILTER Passive
23/29
BW = HL
The upper and lower cut-off frequency points
for a band pass filter can be found using the
same formula as that for both the low and highpass filters, For example.
8/4/2019 5 FILTER Passive
24/29
8/4/2019 5 FILTER Passive
25/29
High-pass filters attenuation of frequencies
below their cut-off points.
Low-pass filters attenuation of frequencies
above their cut-off points. Band-pass filters attenuation of frequencies
both above and below
those they allow to pass.
8/4/2019 5 FILTER Passive
26/29
1. Explain the frequency response for thefollowing filter below:-
i. Low pass filter
ii. High pass filteriii. Bandpass filter
8/4/2019 5 FILTER Passive
27/29
The Bode Plot shows the Frequency
Response of the filter to be nearly flat for
low frequencies and all of the input signal is
passed directly to the output, resulting in a
gain of nearly 1, called unity, until it reaches
its Cut-off Frequencypoint ( c ).
8/4/2019 5 FILTER Passive
28/29
This is because the reactance of the
capacitor is high at low frequencies and
blocks any current flow through the
capacitor. After this cut-off frequency point
the response of the circuit decreases giving a
slope of -20dB/ Decade or (-6dB/Octave)
"roll-off" as signals above this frequency
become greatly attenuated, until at very
high frequencies the reactance of the
capacitor becomes so low that it gives the
effect of a short circuit condition on the
output terminals resulting in zero output.
8/4/2019 5 FILTER Passive
29/29
For this type ofLow Pass Filter circuit, all
the frequencies below this cut-off, c point
that are unaltered with little or no
attenuation and are said to be in the filters
Pass band zone. This pass band zone also
represents the Bandwidth of the filter. Any
signal frequencies above this point cut-off
point are generally said to be in the filters
Stop band zone and they will be greatly
attenuated.