# Wien Bridge

The Vienna bridge is a type of bridge circuit that was developed by Max Wien in 1891 [1] The bridge consists of four resistors and two capacitors . At the time of the Wien bridge’s invention, bridge circuits were a common method of measuring component values ​​by comparing known values. Often an unknown component will be put into one arm of a bridge, and then the bridge will be reduced to zero by adjusting the other arms or by changing the frequency of the voltage source. For example, see Wheatstone Bridge .

The Wien bridge is one of several common bridges. [2] Wien’s bridge is used for precise measurement of capacitance in terms of resistance and frequency. [3] It was also used to measure audio frequencies.

The Wien bridge does not need to be the same value as R or C. At some frequency, the reactance of the series 2 – 2 arm will be an exact multiple of the shunt x – x arm. If the two arms R3 and R4 are adjusted to the same ratio, then the bridge is balanced.

When the bridge is balanced:

\omega ^{2}={1 \over R_{x}R_{2}C_{x}C_{2}}And{C_{x} \over C_{2}}={R_{4} \over R_{3}}-{R_{2} \over R_{x}}\,।


If one chooses 2 = x and 2 = x then the equations become simpler; The result is r4 = r3 .

In practice, the values ​​of R and C will never be exactly equal, but for the values ​​fixed in the above equations that the 2 and X arms , the bridge will equilibrate on some and some ratio of R4 / R3 .