Wien's Bridge

 Wien's Bridge

It is primarily known as the frequency-determining bridge And is described here not only its use in ac bridges but it also has used in various other useful circuits.

The Wien's Bridge may also be employed in a harmonic distortion analyzer, where it is used as a notch filter, discriminating against one specific frequency.

It has also application in audio and HF oscillators as the frequency-determining device.

from fig at balance we have:

( R1/( 1 jwC1R1 )).R4 = (R2 - j/wC2).R3

Solving and Equating real and imaginary parts we get

and    wC1R2 - 1/wC2R1 = 0 from which w = 1/(R1.R2.C1.C2)^(1/2)

and frequency  

 In most Wien's bridge , the components are so chosen that 

                 R1 = R2 = R          and    C1 = C2 = C

          THe eq reduces to : R4/R3 = 2 

and              f = 1/(2.pi.R.C)

Switch resistors R1 and R2 are mechanically liked so as to fulfill the condition R1 = R2

As long as C1 = C2  are fixed capacitors and R4 = 2R3, the Wien's bridge may be used as a frequency-determining device, balanced by a single control. This control may be directly calibrated in terms of frequency.

This bridge is suitable for the measurement of frequency in the range of 100Hz to 100KHz  with an accuracy of 0.1 to 0.5 percent.

Because of frequency sensitivity, The Wien's bridge is difficult to balance unless the waveform is perfectly sinusoidal in nature.

The bridge is not balanced for any harmonics present in the applied voltage, Thus this harmonics will sometimes produce an output voltage masking the true balance point. This difficulty can be overcome by connecting a filter in series with a null detector.

Wien's bridge may be used for the measurement of capacitance also.