Electrical Engineering Dictionary
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Active Band-Pass Filter

A band-pass filter is designed to pass a specific range of frequencies. The general representation of a frequency response curve is shown in the lower right. A passive filter would only use LRC components; resistors [R], capacitors [C] and inductors [L], while an active filter uses an amplifier and only resistors and capacitors. This particular circuit is configured with a center frequency of 800Hz, which follows the other Op Amp filter circuit example centered around 1kHz.

This is a second order band-pass filter, but there are a number of common configurations. The TL081 is a JFET operational amplifier, available as a single Op Amp package in an 8 pin DIP package. The supply voltage may be +/- 18 volts, although this amplifier was powered by +12 volts and ground.

Second Order Band-Pass Filter using a TL081 Operational Amplifier
TL081 Band-Pass Filter

Band-Pass Filter Design

The application note that used this circuit indicated that the Q should be set to 10 or less, although it didn't indicate why. The app note also indicated that the Gain [G] should be just larger than the square root of Q. So it appears that the applications of this circuit design might be limited, but it's only offered here as part of the general definition of a band-pass filter.

R1 = Q / (2 * 3.14 * G * C)
R2 = Q / (2 * Q2 - G) * 2 * 3.14 * f * C
R3 = 2 * Q / 2 * 3.14 * f * C
R4 = R3
This circuit uses a frequency of 800Hz, a Q of 5 and a gain of 2.

This circuit was used as an example in an application note, but it does not appear in the latest revision of the data sheet for the TL08x series of parts. However it's hard to tell if the TL0 series of parts is as popular as they once were, or have slowly been replaced by better or cheaper designs.

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