Electronics Principles V11

Clive W. Humphris

ACTIVE FILTERS: RC Low Pass Buffer Active Filter.  

Active filters differ from passive types as they comprise both active and passive components. The active part, which can be a transistor or in this case an Op-Amp provides gain or attenuation only at the frequencies in the pass band.

This example uses an amplifier configured as a buffer circuit with a gain of 1. The filter response is -20dB per frequency decade, the same as for the passive type. The difference being that this design would not be affected by the next stage loading (input impedance), owing to the buffer amplifier.

R1 and C1 form the low pass filter circuit, the output of which is connected to the non-inverting input to the Op-Amp. From the Op-Amp theory you will recall that there is virtually zero voltage across the Op-Amp inputs, also negligible input current, therefore, only a very small current flows through Rf, which means the voltage across C1 is almost identical to Vout.

By reversing the positions of R1 and C1 and the circuit can be changed into a high pass filter.

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