GATE Electrical Engineering - Complete Master Library

 

Operational Amplifiers – Complete Theory

Page 15 – Active Low Pass Filter

An Active Low Pass Filter allows low-frequency signals to pass while attenuating high-frequency signals.

                                              

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Basic Components

• Operational amplifier
• Resistor (R)
• Capacitor (C)

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Working Principle

• At low frequencies → capacitor behaves like open circuit
• Signal passes through amplifier
• At high frequencies → capacitor offers low impedance
• Signal gets attenuated

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Cutoff Frequency

fc = 1 / (2πRC)

Where:

• R → Resistance
• C → Capacitance

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Frequency Response

• Passband → constant gain
• Cutoff frequency → gain reduces to 0.707
• Roll-off → −20 dB/decade

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Advantages of Active Filters

• No inductors required
• High input impedance
• Easy gain control

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Applications

• Audio signal processing
• Noise filtering
• Communication systems
• Signal conditioning circuits

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GATE Important Points

• Cutoff frequency formula: fc = 1 / (2πRC)
• Roll-off slope = −20 dB/decade
• Uses op-amp + RC network
• Important for frequency response questions