An example of an oscillator circuit that follows the basic development of a feedback circuit is the phase-shift oscillator. This circuit is as shown in Figure. Recall that the requirements for oscillation are that the loop gain, Av, is greater than unity and that the phase shift around the feedback network is 180 (providing positive feedback). Working of RC Phase shift oscillator. The feedback network offers 180 degrees phase shift at the oscillation frequency and the op amp is configured as an Inverting amplifier, it also provide 180 degrees phase shift. Hence to total phase shift around the loop is 360=0degrees, it is essential for sustained oscillations. 3.1 The Phase Shift Oscillator The Phase Shift Network This circuit uses the property of RC filters to cause a phase shift, and by using multiple filters, a feedback circuit with exactly 180° phase shift can be produced. When used with a common emitter amplifier, which also has a phase shift of 180°.

• Rc Phase Shift Oscillator Using Bjt Pdf
• Rc Phase Shift Oscillator Using Op Amp Pdf
• Phase Shift Oscillator Circuit
• Rc Phase Shift Oscillator Pdf
• Bjt Phase Shift Oscillator Pdf
• Rc Phase Shift Oscillator Using Fet Pdf

• Sine-Wave Oscillator 5 4 Phase Shift in the Oscillator The 180 phase shift in the equation Aβ = 1∠– 180° is introduced by active and passive components. Like any well-designed feedback circuit, oscillators are made dependent on passive-component phase shift because it is accurate and almost drift-free. The phase shift.
• The phase-shift oscillator circuit consists of a single transistor amplifier section and a RC phase-shift network. The phase shift network in this circuit, consists of three RC sections. At the resonant frequency f o, the phase shift in each RC section is 60 o so that the total phase shift produced by RC network is.

RC Phase shift oscillator. RC phase shift oscillator or simply RC oscillator is a type of oscillator where a simple RC network (resistor-capacitor) network is used for giving the required phase shift to the feedback signal.

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One of the important features of an oscillator is that the feedback energy applied should be in correct phase to the tank circuit. The oscillator circuits discussed so far has employed inductor (L) and capacitor (C) combination, in the tank circuit or frequency determining circuit.

We have observed that the LC combination in oscillators provide 180^o phase shift and transistor in CE configuration provide 180° phase shift to make a total of 360^o phase shift so that it would make a zero difference in phase.

Drawbacks of LC circuits

Though they have few applications, the LC circuits have few drawbacks such as

• Frequency instability
• Waveform is poor
• Cannot be used for low frequencies
• Inductors are bulky and expensive

Rc Phase Shift Oscillator Using Bjt Pdf

We have another type of oscillator circuits, which are made by replacing the inductors with resistors. By doing so, the frequency stability is improved and a good quality waveform is obtained. These oscillators can also produce lower frequencies. As well, the circuit becomes neither bulky nor expensive.

All the drawbacks of LC oscillator circuits are thus eliminated in RC oscillator circuits. Hence the need for RC oscillator circuits arise. These are also called as Phase–shift Oscillators.

Principle of Phase-shift oscillators

Bridge builder game physics. We know that the output voltage of an RC circuit for a sinewave input leads the input voltage. The phase angle by which it leads is determined by the value of RC components used in the circuit. The following circuit diagram shows a single section of an RC network.

The output voltage V₁’ across the resistor R leads the input voltage applied input V₁ by some phase angle ɸ^o. If R were reduced to zero, V₁’ will lead the V₁ by 90^o i.e., ɸ^o = 90^o.

However, adjusting R to zero would be impracticable, because it would lead to no voltage across R. Therefore, in practice, R is varied to such a value that makes V₁’ to lead V₁ by 60^o. The following circuit diagram shows the three sections of the RC network.

Each section produces a phase shift of 60^o. Consequently, a total phase shift of 180^o is produced, i.e., voltage V₂ leads the voltage V₁ by 180^o.

Phase-shift Oscillator Circuit

The oscillator circuit that produces a sine wave using a phase-shift network is called as a Phase-shift oscillator circuit. The constructional details and operation of a phase-shift oscillator circuit are as given below.

Construction

The phase-shift oscillator circuit consists of a single transistor amplifier section and a RC phase-shift network. The phase shift network in this circuit, consists of three RC sections. At the resonant frequency f_o, the phase shift in each RC section is 60^o so that the total phase shift produced by RC network is 180^o.

The following circuit diagram shows the arrangement of an RC phase-shift oscillator.

The frequency of oscillations is given by

$$f_o = frac{1}{2pi RC sqrt{6}}$$

Where

$$R_1 = R_2 = R_3 = R$$

$$C_1 = C_2 = C_3 = C$$

Operation

The circuit when switched ON oscillates at the resonant frequency f_o. The output E_o of the amplifier is fed back to RC feedback network. This network produces a phase shift of 180^o and a voltage E_i appears at its output. This voltage is applied to the transistor amplifier.

The feedback applied will be

$$m = E_i/E_o$$

Rc Phase Shift Oscillator Using Op Amp Pdf

The feedback is in correct phase, whereas the transistor amplifier, which is in CE configuration, produces a 180^o phase shift. The phase shift produced by network and the transistor add to form a phase shift around the entire loop which is 360^o.

Advantages

Phase Shift Oscillator Circuit

The advantages of RC phase shift oscillator are as follows −

Rc Phase Shift Oscillator Pdf

• It does not require transformers or inductors.
• It can be used to produce very low frequencies.
• The circuit provides good frequency stability.

Disadvantages

Bjt Phase Shift Oscillator Pdf

The disadvantages of RC phase shift oscillator are as follows −

Rc Phase Shift Oscillator Using Fet Pdf

• Starting the oscillations is difficult as the feedback is small.
• The output produced is small.