Wien Bridge Notch Filter

Figure 1.  Circuit Diagram for an Active Wien Bridge Notch Filter

A Wien bridge is a bridge circuit consisting of 4 arms: one arm has a capacitor in series with a resistor, another arm consists of a capacitor in parallel to a resistor, and two arms simply contain a resistor each. A Wien bridge is commonly used to measure unknown capacitance values, but it may also be used as a filter.

The components of a Wien bridge may be chosen and adjusted to make it balanced at a given frequency. When a Wien bridge is balanced, the reactive effects of the capacitors in the circuit cancel each other out, leaving nodes A and B at the same potential. Note that this balancing of the bridge occurs only at a single frequency.

The circuit in Figure 1 is just a Wien Bridge fed into a difference amplifier. If the frequency of the input signal Vin is equal to the frequency to which the Wien bridge is tuned when it is balanced, then the voltage levels at A and B become equal.  This causes both inputs of the op-amp to be at the same voltage level, resulting in zero output voltage.  Thus, in effect, this circuit acts as a notch filter, since it does not let an input signal whose frequency is equal to the notch frequency to pass to the output.