Operational Amplifier Applications Unit 3 – Low Pass Filter
The characteristics of a practical low pass filter show a more gradual transition from passband to
stopband. You will see in a later discussion that it is possible to alter the circuit to make the
characteristics approach the ideal response.
The cutoff frequency is also called the corner frequency or break frequency and is the point at
which the output voltage equals 70.7% of the passband level. This corresponds to a -3 dB
attenuation and is also known as the 3 dB down point.
The rate at which gain decreases beyond fc is the filter's rolloff.
Low pass filters have a property called order. The order number corresponds to the number of
poles a filter has. The number of poles is also the number of lag networks.
As a filter's order (number of poles) increases, it experiences a sharper rolloff, and its response
curve approaches that of the ideal low pass filter.
Different types of low pass filters have different rolloff characteristics.
For example, the Butterworth filter responses shown exhibit an attenuation rate of n x (-20) dB
per decade, where n is the number of poles.