RC Phase Shift Oscillator

    Aim

    To simulate a RC Phase Shift Oscillator circuit.

    Components

    Theory

     An oscillator is a circuit, which generates ac output signal without giving any input ac signal. This circuit is usually applied for audio

     frequencies only. The basic requirement for an oscillator is positive feedback. The operation of the RC Phase Shift Oscillator can be explained

     as follows. The starting voltage is provided by noise, which is produced due to random motion of electrons in resistors used in the circuit.

     The noise voltage contains almost all the sinusoidal frequencies. This low amplitude noise voltage gets amplified and appears at the

     output terminals. The amplified noise drives the feedback network which is the phase shift network. Because of this the feedback voltage

     is maximum at a particular frequency, which in turn represents the frequency of oscillation. Furthermore, the phase shift required for

     positive feedback is correct at this frequency only. The voltage gain of the amplifier with positive feedback is given by

      From the above equation we can see that if . The gain becomes infinity means that there is output without any input.

      i.e. the amplifier becomes an oscillator. This condition is known as the Barkhausen criterion of oscillation. Thus the output contains

      only a single sinusoidal frequency. In the beginning, as the oscillator is switched on, the loop gain Ab is greater than unity. The oscillations

      build up. Once a suitable level is reached the gain of the amplifier decreases, and the value of the loop gain decreases to unity. So the

      constant level oscillations are maintained. Satisfying the above conditions of oscillation the value of R and C for the phase shift network

      is selected such that each RC combination produces a phase shift of 60°. Thus the total phase shift produced by the three RC networks is

      180°. Therefore at the specific frequency f_o the total phase shift from the base of the transistor around the circuit and back to the base is

 

      360° thereby satisfying Barkhausen criterion. We select R₁=R₂=R_3* =R and C₁=C₂=C₃=C

      The frequency of oscillation of RC Phase Shift Oscillator is given by

       At this frequency, the feedback factor of the network is . In order that it is required that the amplifier gain for

      oscillator operation.

     Procedure

      EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment has

      been made. The values are assigned for relevant components.

      EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. The waveform marker is placed at the output of the circuit. GND net is set

      as reference net. The Transient Analysis parameters have been set. The Transient Analysis is executed and output waveform is observed

      in Waveform Viewer.

     EDWin 2000 -> EDSpice Simulator: The circuit is preprocessed. The waveform marker is placed at the output of the circuit. The Transient

      Analysis parameters are also set. The Transient Analysis is executed and output waveform is observed in Waveform Viewer.

 

      Result

      The output waveform may be observed in the waveform viewer.