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Twin T-filter

The circuit above is called a twin T-filter. The current in the resistors is in phase with the voltage. The phase shift in the current in the capacitors will be 180 degrees. At a certain frequency the current amplitues will be same, but in anti-phase. This frequency will therefore be suppressed. The circuit is therefore called a band stop filter, or notch filter. The center frequency equals:

If R = 1k and C = 1n again, this frequency will be 159kHz. This picture below shows the frequency response.

A twin T-filter with feedback can be found on my projects page.