Transcript Chapter 1 (Part 4) - Basic Filter

Chapter 1
Part 4  Introduction to
Basic Filter
1
What is filter?
Filter is a circuit that passes a specific
range of frequencies while rejecting
other frequencies.
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Introduction to Filters
 What


is passive filter /active filter ?
A passive filter consists of passive circuit
elements such as capacitors, inductors
and resistors .
An active filter uses active devices such as
op-amps combined with passive elements.
3
Introduction to Filters

The passive elements
provide frequency
selectivity.
 The active devices
provide voltage gain,
high input impedance
and low output
impedance.
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Types of filters
Four basic types of filters:

1. LOW PASS FILTER
3. BAND PASS
FILTER

2. HIGH PASS FILTER
FILTER
4. BAND STOP
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Types of filters
Low Pass Filter (LPF)
High Pass Filter (LPF)
Pass all frequencies
below cutoff frequency
and reject all
frequencies above cutoff
frequency
Pass all frequencies
above the cutoff
frequency and reject all
frequencies below the
cutoff frequency
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Types of filters
Band
Pass Filter (BPF)
Passes all frequencies
within a band of
frequencies and reject all
other frequencies
outside the band.
Band Stop Filter ( Notch)
Reject all frequencies
within a band of
frequencies and passes
all other frequencies
outside the band.
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Ideal filters;LPF,HPF,BPF and BPF
8
Simple RC LPF Circuit
9
Simple RC LPF response curve or
Bode Plot.
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Characteristics of Filters
 The
most common way to describe the
frequency response characteristics of a
filter is to plot the filter voltage gain
(Vout/Vin) in dB as a function of frequency
(f).
 What is cutoff Frequency?
The frequency at which the output power
gain drops to 50% of the maximun value is
called cutoff frequency.
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Characteristics of Filters (cont..)
When the output power gain drops to
50%, the voltage gain drops 3 dB (0.707 of
the maximun value.
 The filter network voltage gain in dB is
calculated from the actual voltage gain (A)
using the equation
A(dB) = 20 log A
Where A=Vout/Vin

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Characteristics of Filters (cont..)
What is Roll-Off Rates (ROR)?
ROR is given in dB per decade
(dB/dec) or dB per octave (dB/oct).
# dB/dec = (20/6) x # dB/oct.
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Characteristics of Filters (cont..)

What is Q or Shape Factor (SF)?
14
BPF frequency response
15
Typical LPF bode plot
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Examples
3.1 What determines the Q of a filter?
3.2 What is the bandwidth of a filter with a
Q of 75 and a center frequency of 375
MHz?
3.3 If the -6B bandwidth of a filter is 3.1
KHz, determine the -60dB bandwidth
if the filter shape factor is 1.45?
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END OF CHAPTER 1
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