Wien’s bridge and applications

AC BRIDGES• Ac bridges are the bridges which are formed by

network of four impedances with a ac source supply to the bridge

• These impedances may be various reactance

1. Capacitive2. Inductive3. Resistive

• Under balanced condition (when the current across the detector is zero)

Wien’s bridge

• Under the balanced condition this bridge involves frequency as a function of capacitances and resistors let us see?

Operation of Wien’s bridge

applications• The wien’s bridge is primarily known as a frequency determining bridge but it can be applicable

in audio oscillators as frequency determining devices

• Wein’s bridge operation range is (100Hz-100KHz)

• It is used in total harmonic distortion analyser(THD) as a notch filter

Which discriminates unwanted frequencies of a given response of a system

Wien’s bridge oscillator• The Wien-bridge oscillator is a unique circuit

because it generates an oscillatory output signal without having a sinusoidal input source

• Instead, it uses capacitors with initial voltages to create the output.

• Here Wien bridge is a lead-lag network. The phase’-shift across the network lags with increasing frequency and leads with decreasing frequency. By adding Wien-bridge feedback network, the oscillator becomes sensitive to a signal of only one particular frequency. This particular frequency is that at which Wien bridge is balanced and for which the phase shift is 0°

• This oscillator is similar to the previous one

• if we examine the gain of the oscillator is dependent on resistors R5 and R6

• The gain is

A=R3 + R4 / R4

Under practical conditions R3 = 2 R4 in wien bridge

The gain must be A=R3 /R4 +1 so the gain must A=3 is ideal condition to work as an oscillator

operation of oscillator

Time

0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0msV(R5:2)

-4.0V

0V

4.0V

Time

0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0msV(R5:2)

-4.0V

0V

4.0V

Time

0s 100us 200us 300us 400us 500us 600usV(R5:2)

-20V

0V

20V

ADVANTAGES OF WIEN’S BRIDGE DISADVANTAGES OF WIEN’S BRIDGE

• As the bridge is frequency sensitive it is difficult to balance the bridge for non sinusoidal waves

• Bridge is not balanced for any harmonics present in applied ac voltages

• This bridge is not cost effective since a perfect variable capacitor is expensive

• This bridge is suitable for measuring of frequencies

• It has accuracy ranging from 0.1 to 0.5 per cent

Thank you

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