Wien's Bridge
Wien Bridge shown in Fig. 2.1 has a series RC combination in one and a parallel combination in the adjoining arm. Wien's bridge in its basic form is designed to measure f r e q u ency. It can also be used for the instrument of an unknown capacitor with great accuracy, The impedance of one arm is
resistance ratio R2/R4 and another express determining the frequency of the applied voltage. If we satisfy
Eq. (1.1) an also excite the bridge with the frequency of Eq. (1.3), the bridge will be balanced.
In most Wien bridge circuits, the components are chosen such that R1= R3 = R and C1= C3= C.
Equation (1.1) therefore reduces to R2IR4 =2 at Eq. (1.3) to f= 1/2ПRC, which is the general equation
for the frequency of fl bridge circuit.
The bridge is used for measuring frequency in the audio range. Resistances R1and R3 can be ganged
together to have identical values. Capacitors C1and C3are normally of fixed values
The audio range is normally divided into 20 - 200- 2 k - 20 kHz range In this case, the resistances can
be used for range changing and capacitors, and C3for fine frequency control within the range. The
bridge can also be use for measuring capacitance's. In that case, the frequency of operation must be
known.
The bridge is also used in a harmonic distortion analyzer, as a Notch filter, an in audio frequency and
radio frequency oscillators as a frequency determine element.
An accuracy of 0.5% - 1% can be readily obtained using this bridge. Because it is frequency sensitive, it
is difficult to balance unless the waveform of the applied voltage is purely sinusoidal.
