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Sinusoidal Oscillator
IntroductionOscillators are circuits that produce periodic waveforms.Only input requirement is of DC power supply.Frequency of the generated waveforms may vary from a few
Hz to several KHz.It may be used to generate AC waveforms such as sinusoidal,
rectangular or sawtooth depending upon the type of oscillators used.
Applications Radio Television Communication Systems Computer Industrial controlled applications Function or Signal generator
FeedbackWhen an event is part of a chain of cause-and-effect that
forms a circuit or loop, then the event is said to "feedback" into itself.
Types of feedback : Positive Feedback Negative Feedback
Block Diagram of Oscillators
Principle of OscillatorBasic components required for an oscillator are : 1. Active device 2. Feedback circuit Necessary conditions for an oscillator (aka ‘Barkhausen
conditions’) 1. The product of gain of amplifier 'A' and the gain of
feedback network 'B' has to be unity. 2. The phase shift around the loop is zero or an integer
multiple of 2π.
InitializationHow Oscillations are initiated at First?Thermal noise voltage.Resistors used in feedback circuit acts as small ac voltage
source.This small ac voltage gets amplified and applied to feedback
circuit and output of feedback circuit is fed back to the amplifier as an input.
This process is repeated and at one particular frequency, circuit satisfies the necessary conditions to start oscillation.
Using proper feedback components, it is possible to select the particular frequency.
Sinusoidal Oscillator An oscillator which produces repeatedly a sine wave output is
a sinusoidal oscillator , also known as ‘Harmonic Oscillator’.Types of oscillations
1. Damped 2. Undamped or Sustained
Practical OscillatorsTo obtain the sustained oscillations , oscillator circuit must
satisfy following requirements: 1. Circuit must have positive feedback. 2. The overall circuit gain is given by,
In reality, to get sustained oscillations, at the first time when the circuit is turned on, the loop gain must be slightly greater than one.
Once a suitable level of output voltage is reached, the loop gain must be decreased to unity.
Only then the circuit maintains the sustained oscillation.This can be achieved in the circuit either by decreasing
amplifier gain A or decreasing the feedback gain β.Otherwise, the circuit operates as over damped since Aβ is
greater than 1.
ClassificationDepending upon the frequency determining components,
there are three basic types of oscillators.
Oscillators
RC
RC phase shift
Wien Bridge Twin-T
LC
Hartley Colpitts Clapp Armstrong
Crystal
Principle of Wien Bridge Oscillator0° or 360° phase shift caused by Amplifier circuit.Either we use two stage cascaded transistor in CE
configuration or an Op Amp in non-inverting configuration.0° or 360° phase shift caused by Feedback circuit.This is the basic principle of a Wien bridge oscillator.Basic circuit used in Wien bridge oscillator is a Lead-Lag
Circuit.
Lead-Lag CircuitAt high frequencies, circuit acts as a lag circuit.At low frequencies, circuit acts as a Lead circuit.
Resonant frequencyBasically, lead-lag circuit acts like a resonant circuit.At two extremes, we get zero output voltage, but at one
particular frequency between the two extremes, the output voltage reaches to the maximum value.
Only at this frequency , resistance value becomes equal to capacitive reactance and gives maximum output.
This particular frequency is known as resonant frequency or oscillating frequency which is given by,
Circuit Diagram of Wien Bridge Oscillator
Circuit OperationsCircuit consists of two feedbacks, positive as well as negative.Positive feedback is between output and non-inverting terminal and
negative feedback is between the output and inverting terminal of Op-Amp.
Why negative feedback is required? The loop gain must be slightly greater than one when circuit is
turned on for the first time. For Wien bridge oscillator, the gain of the amplifier must be greater
than three (A>3), which will ensure that sustained oscillations build up in the circuit.
To set this gain negative feedback is essential.
The gain of the non-inverting amplifier is decided by resistors Rf and R1.
For sustained oscillations, resistor Rf must be twice of resistor R1.
For maximum output, resistive value should be equal to reactance value.
What happens if resistor R3 is not equal to twice of resistor R4?If R3 is less than 2R4, the Aβ product will be less than unity and
oscillations cannot be maintained.if R3 is greater than 2R4, the gain significantly increases and
due to excessive gain, distortion may result.
Selection of component’s value Gain components selection and for Wien bridge oscillator A=3. hence, Frequency components selection for maximum output,
The frequency of lead – lag circuit is given by,
Then,
This is the resonant frequency of the sinusoidal or harmonic oscillator.
Presented by
Ajay ShuklaRohin Bisht
S. ChandrikaP. Krishna Chaitanya
Thank You
