WEIN BRIDGE OSCILLATOR

  •     This is audio frequency RC oscillator.
  •      The advantage of this oscillator is that the frequency may be varied in the range of 10 Hz to about 1’ MHz whereas in RC oscillators, the frequency cannot be varied.
  •     The oscillator consists of two stages of R-C coupled amplifier and a feedback network.
  •     The block diagram is shown below.
  •      The voltage across the parallel combination of R and C is fed to the input of amplifier 1.
  •     The net phase shift through the two amplifiers is zero.




  •     Now the question is that why we could not feed the out tput of amplifier 2 to amplifier 1 to provide regeneration needed for oscillation operation.
  •     The answer is that amplifier 1 will amplify signals over a wide range of frequencies and hence the direct coupling would result in poor frequency stability.
  •     By adding Wien-bridge feedback network, the oscillator becomes sensitive to a signal of only one particular frequency. Hence good frequency stability is obtained.
  •     It is now essential that the feed back network should not introduce any phase shift between its input and output voltages.
  •      It can be shown that this is achieved at a frequency f= 1/2πRC when β= 1/3.
  •     This is obtained by introducing two resistances R1 and R2 in the feedback network as shown in figure;



  •     We can vary the frequency of this oscillator by varying the two capacitors simultaneously.
  •     We can also change the range of the frequency of the oscillator by using different values of resistors R.




Where f is the frequency of oscillation.

ADVANTAGES:

  •     This gives good frequency stability.
  •     By replacing R2 with a Thermistor amplitude stability  of oscillator output voltage can be increased.
  •     Overall gain is high because of two transistors.
  •     Frequency of oscillations can be changed.
  •     Exceedingly good sine wave output.

DISADVANTAGES:

  •     It requires two transistors and large number of components.
  •     It cannot generate very high frequencies.

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