Download Lab-3

Filters
• By combining resistor, capacitor, inductor
in special ways we can design circuits that
are capable of passing certain frequency
while rejecting others.
• Some example of filters are low pass filter,
high pass filters and band pass filters.
• Low pass filter circuits pass only the low
frequencies but rejects high frequencies.
Low pass filter
• Combination of R and C.
• It has a frequency sensitive
voltage divider.
• Frequency sensitive part
comes from the C.
• At high frequency capacitors
reactance decreases. Output
voltage decreases too.
• At low frequency capacitor
reactance increases. Output
voltage increases too.
Attenuation Vs Frequency
•
From the circuit we can find that
•
The magnitude or attenuation
•
When ω is 0, attenuation is 1 which means low frequency passes
through the circuit.
When ω is infinity, attenuation is 0, which means high frequency cannot
pass the circuit.
The cut-off frequency is the frequency at which attenuation is
i.e. |Vout|=
|Vin|
At cutoff frequency
•
•
•
Attenuation in dB
•
•
Attenuation is expressed in dB
Decibel / non-decibel conversion
•
•
Notice that when
=0.707 (half power condition), AdB=-3dB
Notice that at the points where dB values are minus are true attenuation; If
dB value is positive can be considered as gain.
Phase shift vs frequency
•
•
•
•
•
For any filter we should consider
phase shift as well as the attenuation.
Phase shift is a measurement to determine
how much output signal has been shifted
in phase related to the input voltage.
Phase shift is
As ω goes 0, phase shift goes 0° (i.e no phase shift). When ω goes to
infinity phase shift goes to -90°.
Phase shift also can be written in terms of cut off frequency:
Where
•
Notice that when ω=ωc , phase shift is -45°
Rectifier
•
•
•
•
A step down transformer reduces the 120VAC to 12VAC
Diodes passes only the positive portion of the signal.
The filter capacitor smoothes the signal.
7805 outputs a regulated +5V
120VAC
7805 IC
5VDC