RC Phase Shift Oscillator Circuit Working and Applications

In this topic we would like to introduce you a transistor oscillator circuit, is called as an RC Phase shift oscillator. First of all we need to know what is an oscillator. The oscillator is an electronic circuit which performs as a sine wave generator. It is working only for a DC power supply. The oscillator is widely used in frequency variable signal generators, so it is a common basic electronic circuit. There are different types of oscillators are available which is based on their performance and components used, like Hartly Colpitts oscillator, Op amp RC phase shift oscillator, Wein bridge oscillator, etc. Here we are going to study how we can obtain the RC Phase shift Oscillator using a silicon transistor. In an oscillator the frequency generating circuit is depending upon the usage of circuit elements. A basic oscillator circuit contains the following parts

RC Phase Shift Oscillator
RC Phase Shift Oscillator

RC Phase Shift Oscillator

It is a linear electronic oscillator circuit that produces a sine wave output. It consists of an inverting amplifier element, such as Op-Amp or transistor with its output fed back to its input through a phase-shift network consisting of capacitors and resistors in a ladder network. The feedback network ‘shifts’ the phase of the amplifier output by 1800 at the oscillation frequency to give positive feedback. Phase-shift oscillators are often used at audio frequency by audio oscillators.

The filter produces a phase shift that increase with the frequency. It must have a maximum phase shift of more than 1800 at high frequencies so the phase shift at the desired oscillation frequency can be 1800. The most common phase-shift network cascades three identical resistors, capacitor stages that produce a phase shift is zero at low frequencies and 270° at high frequencies.

RC Phase Shift Oscillator Using Op-Amp

An RC phase shift oscillator is a sinusoidal oscillator used to produce the well shaped sine wave oscillations. This oscillator is used in numerous applications like as a local oscillator for synchronous receivers, study purposes, musical instruments. The essential component used in an RC phase shift oscillator is an operational amplifier inverting amplifier with its output fed back into its input using a regenerative feedback RC filter n/w, hence the name is called an RC phase shift oscillator. The frequency of oscillations can be changed by changing the capacitor.

RC Phase Shift Oscillator using Op-Amp
RC Phase Shift Oscillator using Op-Amp

The feedback RC network has a phase shift of 60 degrees each, hence the total phase shift provided by the three RC network is 180 degrees. The op amp is connected as an inverting amplifier, so hence the total phase shift around the loop will be 360 degrees. This condition is essential for continued oscillations.

Working of RC Phase Shift Oscillator

  • The feedback network offers 1800 phase shift at the oscillation frequency and the operational amplifier is arranged as an Inverting amplifier, and it also provide 180 degrees phase shift. Hence, the total phase shift around the loop is 360 degrees, it is essential for sustained oscillations.
  • At the oscillation frequency each of the capacitor, resistor, filter produces a phase shift of 60° so the whole filter circuit generates 180° of a phase shift of.
  • The energy storage capacity of a capacitor in this circuit produces a noise voltage which is like to a small sine wave, it is then amplified using an op amp inverting amplifier.
  • By taking feedback, the o/p sine wave also reduces 1/29 times while passing through the RC network, so the gain of the inverting amplifier should be 29 in order to keep the loop gain as unity.
  • The unity loop gain and 360 degree phase shift are essential for the continued oscillation.
  • RC Oscillators are constant and provide a well shaped sine wave output with the frequency being proportional to 1/RC and therefore, when we are using a variable capacitor a wide frequency range is possible.
  • Though, RC Oscillators are controlled to frequency applications because at high frequency the reactance offered by the capacitor is very low so it acts as a short circuit.

Frequency of oscillation (F)

F= 1/2πRC√2N

1/2πRC√2×3

1/2πRC√6

For 6.5kHz= 1/2πx1000x0.01×10-6X√6

Gain of the Op Amp inverting amplifier (G)

G= -Rf/Ri

=29

The feedback RC network offers an attenuation that is 1/29, so the gain of the inverting amplifier should be 29

Use Ri=1.2 KΩ

So, Rf=35KΩ

Use 50KΩ potentiometer and adjust its value to obtain output on a CRO

RC Phase Shift Oscillator Using BJT

The circuit implementation using NPN type bipolar transistor is based on examples from the popular electronics hobby magazine. The circuit generates a sinusoidal wave at its output. The RC phase-shift network is used to produce a positive feedback loop. Rb and RC resistors polarize the transistor for only DC current, Rs is a separating resistor. The circuit diagram as shown above.

RC Phase Shift Oscillator Circuit using BJT
RC Phase Shift Oscillator Circuit using BJT

RC Phase Shift Oscillator Using FET

The circuit implements the RC phase shift oscillator with FET. Note that the topology is used for positive feedback and voltage series feedback.

RC Phase Shift Oscillator Circuit using FET
RC Phase Shift Oscillator Circuit using FET

Advantages of RC Phase Shift Oscillator

  • This circuit is very simple and cheap as it comprises resistors and capacitors (not bulky and expensive high-value inductors).
  • It provides good frequency stability.
  • The output of this circuit is sinusoidal that is quite distortion free.
  • The phase shift oscillator circuit is simpler than the Wein bridge oscillator circuit because it does not need negative feedback and the stabilization arrangements.
  • They have a wide range of frequency (from a few Hz to several hundred of kHz).
  • They are particularly apt for low frequencies, say of the order of 1 Hz, so these frequencies can be easily gained by using R and C of large values.

Disadvantages of RC Phase Shift Oscillator

  • The output is small and It is due to smaller feedback.
  • The frequency stability is not as good as that of the Wien bridge oscillator.
  • It is difficult for the circuit to start oscillations as the feedback is usually small.
  • It needs high voltage (12 V) battery so as to develop a sufficiently large feedback voltage

Applications of RC Phase Shift Oscillator

FET phase-shift oscillator is used for generating signals over a wide frequency range. The frequency may be varied from a few Hz to 200 Hz by employing one set of resistor with three capacitors ganged together to vary over a capacitance range in the 1 : 10 ratio. Similarly the frequency ranges of 200 Hz to 2 kHz, 2 kHz to 20 kHz and 20 kHz to 200 kHz can be obtained by using other sets of resistors.

Thus, this is all about different types of RC Phase Shift Oscillators and its applications. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or to implement electrical and electronics projects,  Please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what is the function of RC Phase Shift Oscillator?

2 Comments

  1. what happens whaen we interchange the r and in this feedback ckt

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