What is a Twin T Filter?

Problem:
What is a Twin T Filter?

Solution:
A Twin T Filter is a two pole filter topology. It is based on the characteristics of a passive network:



The top and bottom halves of the schematic are the "Twin T's". They are mirror images of each other, except that in the bottom, resistor and capacitor positions have been swapped. When the resistors and capacitors are in the ratios shown, the circuit is capable of producing very low impedances at the resonant frequency, which can lead to bandpass and notch filters have theoretically infinite gain / rejection.

Twin T filters are available in low pass, high pass, bandpass, and notch versions. The Twin T low pass, high pass, and notch filters are unity gain. The bandpass filter has (theoretical) infinite gain. For low pass and high pass filters, it is easy to change the filter type (Butterworth, Chebyshev, and Bessel).

The Twin T topology is suited to operation from a single supply. Current feedback amplifiers can be used, with the restriction that the connection from the op amp output to inverting input must be the recommended feedback resistor value. The Twin T topology cannot be used with fully differential amplifiers.

Schematics for Twin T filters can be found at:

• The Split Supply Expert
• The Single Supply Expert
• The Current Feedback Expert