Wien-Bridge Oscillator Circuits

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Wien-Bridge Oscillator Circuits
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Why Look At the Wien-Bridge?

• It generates an oscillatory output signal without
  having any input source

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Basics About the Wien-Bridge

• Uses two RC networks connected to the positive
  terminal to form a frequency selective feedback
  network
• Causes Oscillations to Occur

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Basics About the Wien-Bridge

• Amplifies the signal with the two negative
  feedback resistors

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Modification to Circuit
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Analysis

• The loop gain can be found by doing a voltage
  division

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Analysis

• The two RC Networks must have equal resistors and
  capacitors

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Analysis
Need to find the Gain over the whole Circuit
Vo/Vs
Solve G equation for V1 and substitute in for
above equ.
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Analysis
We now have an equation for the overall circuit
gain
Simplifying and substituting jw for s
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Analysis
If G 3, oscillations occur If G lt 3,
oscillations attenuate If G gt 3, oscillation
amplify
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G 3
G 2.9
G 3.05
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Ideal vs. Non-Ideal Op-Amp

• Red is the ideal op-amp.
• Green is the 741 op-amp.

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Making the Oscillations Steady

• Add a diode network to keep circuit around G 3
• If G 3, diodes are off

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Making the Oscillations Steady

• When output voltage is positive, D1 turns on and
  R9 is switched in parallel causing G to drop

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Making the Oscillations Steady

• When output voltage is negative, D2 turns on and
  R9 is switched in parallel causing G to drop

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Results of Diode Network

• With the use of diodes, the non-ideal op-amp can
  produce steady oscillations.

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Frequency Analysis

• By changing the resistor and capacitor values in
  the positive feedback network, the output
  frequency can be changed.

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Frequency Analysis
Fast Fourier Transform of Simulation
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Frequency Analysis

• Due to limitations of the op-amp, frequencies
  above 1MHz are unachievable.

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Conclusions

• No Input Signal yet Produces Output Oscillations
• Can Output a Large Range of Frequencies
• With Proper Configuration, Oscillations can go on
  indefinitely