• Phase Shift Oscillator Derivation
• Bjt Phase Shift Oscillator

And is met when the overall phase shift of the feedback is 360. 6.2.1 Transistor Oscillators Phase Shift Oscillator Figure 6.1 shows the circuit for a phase shift oscillator, in which the feedback circuit employs three cascaded RC sections to shift the phase by 180. 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 is a sinusoidal oscillator used to produce sustained well shaped sine wave oscillations. It is used for different applications such as local oscillator for synchronous receivers, musical instruments, study purposes etc. The main part of an RC phase shift oscillator is an op amp inverting amplifier with its output fed back into its input using a regenerative feedback RC filter network, hence the name RC phase shift oscillator.

By varying the capacitor,the frequency of oscillations can be varied. The feedback RC network has a phase shift of 60 degrees each, hence total phase shift provided by the three RC network is 180 degrees. The op amp is connected as inverting amplifier hence the total phase shift around the loop will be 360 degrees. This condition is essential for sustained oscillations. We have already discussed about RC phase shift oscillator using transistor.

Circuit Diagram

Components Required

1. Resistors (1.2KΩ, 1KΩx2)
2. Potentiometer (50KΩ)
3. Capacitors (0.01µFx3)
4. 741 Op Amp

Output Waveform beginning

Sustained Output Waveform

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.

• At the oscillation frequency each of the resistor capacitor filter produces a phase shift of 60° so the whole filter circuit produces a phase shift of 180°.

• The energy storage capacity of capacitor in this circuit produces a noise voltage which is similar to a small sine wave, it is then amplified using op amp inverting amplifier.

• By taking feedback, the output sine wave also attenuates 1/29 times while passing through the RC network, so the gain of inverting amplifier should be 29 in order to keep loop gain as unity.

• The unity loop gain and 360 degree phase shift are essential for the sustained oscillation.

• RC Oscillators are stable and provide a well shaped sine wave output with the frequency being proportional to 1/RC and therefore, a wider frequency range is possible when using a variable capacitor.

• However, RC Oscillators are restricted to frequency applications because at high frequency the reactance offered by the capacitor is very low so it acts as a short circuit.

Why it uses 3 RC stages?

• Number of RC stages help improve the frequency stability. The total phase shift introduced by the feedback network is 180 degrees, if we are using N RC stages each RC section provide 180/N degree phase shift.

• When 2 RC sections are cascaded, the frequency stability is low. For 3 sections cascaded the phase change rate is high so there is improved frequency stability. However for 4 RC sections there is an good phase change rate resulting in the most stable oscillator configuration. But 4 RC sections increases cost and makes circuit complexity.

• Hence phase shift oscillators make use of 3 RC sections in which each section provides a phase shift of 60 degree. The latter is generally used in high precision applications where cost is not much regarded and only accuracy plays a major role.

Design

Frequency of oscillation (F):
Gain of the Op Amp inverting amplifier (G):
Attenuation offered by the feedback RC network is 1/29, so the gain of inverting amplifier should be 29
Use R_i=1.2 KΩ
So, R_f=35KΩ
Use 50KΩ potentiometer and adjust its value to obtain output on CRO

Components Pin out

i-St@r Lab

Simulated output using Multisim

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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

Free antivirus app for laptop. 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

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

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.

Virtual dj 7 crack download 64 bit. 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.

Phase Shift Oscillator Derivation

The frequency of oscillations is given by

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

Where

$$R_1 = R_2 = R_3 = R$$

$$C_1 = C_2 = C_3 = C$$

Bjt Phase Shift Oscillator

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

$$m = E_i/E_o$$

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

The advantages of RC phase shift oscillator are as follows −

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

Disadvantages

The disadvantages of RC phase shift oscillator are as follows −

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