Op Amp Active BandPass Filter

- basic operational amplifier / op-amp active bandpass filter circuit are an easy way to make effective filters that pass a given band of frequencies.

Bandpass filters are used in many areas of electronics. They are particularly widely used for RF applications where tuned circuits are used. However for lower frequencies, active bandpass filters provide an effective means of making a filter to pass only a given band if frequencies.For these filters the most widely used active element is an operational amplifier, or op amp.

These op amp bandpass filters are easy to design and construct, requiring only a minimum of components. In addition to this, these active bandpass filters provide a very effective level of performance.

What is a bandpass filter

As the name implies a bandpass filter is one where only a given band of frequencies is allowed through.

All frequencies outside the required band are attenuated.

There are two main areas of interest in the response of the filter. These are the pass-band where filter passes signals and the stop-band where signals are attenuated. As it is not possible to have an infinitely steep roll off, there is an area of transition outside the pass-band where the response is falling but has not reached the required out of band attenuation.

Op amp active bandpass filter circuit

The design of band pass filters can become very involved even when using operational amplifiers. However it is possible to simplify the design equations while still being able to retain an acceptable level of performance of the operational amplifier active filter for many applications.

The circuit and design equations represent a good balance between performance and ease of circuit design.

The circuit of an operational amplifier band pass filter showing the two capacitors and three capacitors in the circuit
Circuit of the operational amplifier active band pass filter

From the circuit it can be seen that apart from the operational amplifier itself, the circuit comprises of two capacitors and three resistors.

The formulas or equations for determining the component values in the op amp based active bandpass filter circuit

As only one operational amplifier is used in the filter circuit, the gain should be limited to five or less, and the Q to less than ten. In order to improve the shape factor of the operational amplifier filter one or more stages can be cascaded. A final point to note is that high stability and tolerance components should be used for both the resistors and the capacitors. In this way the performance of the operational amplifier filter will be obtained.

By Ian Poole

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