Filters

1. 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.

2. 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.

3. 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

4. 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.

5. Phase shift vs frequency • For any filter we should considerphase shift as well as the attenuation. • Phase shift is a measurement to determinehow much output signal has been shiftedin 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°

6. 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 Rectifier 5VDC 120VAC