MultipleFeedback

Multiple-Feedback (MFB) is an active filter topology that uses an operational amplifier with several feedback paths to realize second-order filters. It is widely used for high-quality audio and instrumentation applications due to its compact component count and tunable characteristics. In the typical implementation, the operational amplifier operates in an inverting configuration, with the input signal applied through a resistor to the inverting input, and a combination of feedback components from the output to the same input. The non-inverting input is usually connected to ground. By selecting the values of the resistors and capacitors in these paths, the circuit can realize low-pass, high-pass, or band-pass responses with a controllable quality factor (Q).

The MFB topology is valued for its ability to achieve relatively high Q and sharp cutoff without the use of inductors, making it suitable for integrated and compact designs. Several variants exist, including configurations optimized for low-pass, high-pass, and band-pass filtering. The second-order transfer function of an MFB filter is determined by the relative values of the resistors and capacitors, allowing designers to tailor the center frequency, gain, and Q to meet specific requirements.

Design considerations for MFB filters include the need for an amplifier with sufficient bandwidth to handle the desired center frequency, and sensitivity to component tolerances. Because the pole locations depend on precise R and C values, layout, temperature variations, and resistor/capacitor tolerances can affect performance. Proper component selection and layout are important for achieving the intended response.

Applications of Multiple-Feedback filters include audio signal conditioning, instrumentation front-ends, and any scenario requiring compact, precise second-order filtering without inductors.