RC Oscillator Working and Its Applications

An oscillator is an electronic device which provides good frequency stability as well as waveform by using resistive & capacitive elements. These oscillators are named as phase shift oscillator or RC oscillator. This kind of oscillator includes additional benefits which can be used at extremely low frequencies. In a phase shift oscillator, 1800 of phase can be attained using a phase shift circuit rather than capacitive or inductive coupling. An additional 1800 of phase can be introduced because of the properties of the transistor. Therefore the energy which is supplied back in the direction of the tank circuit can be an exact phase. This article discusses an overview of what is RC phase shift oscillator, working principle, circuit diagram using op-amp and BJT and its applications.

What is the RC Oscillator?

An RC oscillator is a sinusoidal oscillator which is used to generate a sine wave as an output with the help of linear electronic components. The oscillator like tuned LC circuits works at high frequencies, however at low-frequencies, the capacitors and inductors in a tank circuit otherwise time circuit would be extremely large size.

Therefore, this oscillator is more appropriate in low-frequency applications. This oscillator includes a feedback network and an amplifier. The feedback n/w is also named as a phase shift n/w which can be designed with resistors and capacitors. These can be arranged in the form of a ladder. So this is the reason to call this oscillator as a ladder-type oscillator.

Let us talk about the RC oscillator circuit which can be used within the feedback network previous to understanding the working of this oscillator.

RC Oscillator Working Principle

The working principle of the RC oscillator is a circuit which uses the RC network to give the phase-shift which is necessary by the response signal. These oscillators have outstanding frequency strength as well as they can give way a pure sine wave used for a broad range of loads.

RC Phase Shift Oscillator using BJT

The RC phase shift oscillator using BJT is shown below. The transistor used in this circuit is an active element for the amplifier stage. The operating point of DC within the active area of the transistor can be set up by the Vcc supply voltage and R1, R2, RC & RE resistors.


The CE capacitor is a bypass capacitor. Here, the three RC segments are taken as equal & the resistance within the final section can be R’ = R – hie.

The ‘hie’ of the transistor is input resistance which can be added to R’, therefore the network resistance which is known through the circuit is ‘R’.

The R1 & R2 resistors are biasing resistors and these are superior and therefore no consequence on the operation of the AC circuit. Also because of insignificant impedance accessible by the combination of RE – CE, there is also no consequence over AC operation.

As the power is supplied to the circuit, then noise voltage begin the oscillations within the circuit. At the transistor amplifier, a little base current amplifier generates a current which can be 1800 phase shifted.

Whenever this signal in the response to the amplifier’s input, then again it will be phase-shifted with 1800. If the gain of the loop is equivalent to unity after that continued oscillations will be generated.

The circuit can be simplified by using an equivalent AC circuit, and then we can get the frequency of oscillations like the following.

f = 1/ (2πRC √ ((4Rc / R) + 6))

When Rc / R is << 1, then

f= 1/ (2πRC√ 6)

The state of continued oscillations,

hfe = (4Rc/ R) + 23 + (29 R/Rc)

For an RC phase shift oscillator using R = Rc, then ‘hfe’ must be 56 used for continued oscillations.

From the above equations, for altering the oscillation’s frequency, the values of the capacitor and resistor have to be changed.

However, to satisfy the conditions of oscillating, the three-segment values should be altered concurrently. In practical, this cannot be possible; thus the RC oscillator is used like a fixed frequency oscillator used for every practical purpose.

RC Oscillator using Op-amp

Operational amplifier RC oscillators are commonly used oscillators, as compared to the transistorized oscillators. This type of oscillator consists of an op-amp as amplifier stage and three RC cascaded networks as a feedback circuit as shown in the figure below.


This op-amp is operated in inverting mode and hence the output signal of the op-amp is shifted by 180 degrees to the input signal appeared at inverting terminal. And an additional 180 degrees phase shift is provided by the RC feedback network and hence the condition for obtaining the oscillations.

The amplifier’s gain otherwise operational amplifier can be regulated using resistances like Rf & R1. To obtain the necessary oscillations, gain can be adjusted that the product of feedback network gain and op-amp gain is somewhat superior to 1.

This circuit works like an oscillator when the gain of the loop is superior to ‘1’ if the operational amplifier offers the gain superior to 29.

The oscillations frequency can be derived by the following equation

1/ (2πRC√ 6)

The oscillations condition can be given with A ≥ 29.

The gain value of the amplifier can be obtained so that the oscillations take place within the circuit by regulating R1& Rf.

RC Oscillator Applications

The applications of this oscillator include the following.

  • RC oscillators are used in low-frequency applications.
  • The applications of these oscillators mainly include voice synthesis, musical instruments, & GPS units as they perform at all audio frequencies.

Thus, this is all about the RC oscillator and the frequency of this oscillator can be changed with either the capacitors or the resistors. But, generally, the resistors are reserved steadily whereas the capacitors are tuned. After that, by evaluating the oscillators using LC oscillators, we can note that the earlier uses the number of components than the last one. Therefore, the o/p frequency which is generated from these oscillators can move away a lot from the measured value slightly than the LC oscillators. However, they are employed like local oscillators used for musical instruments, synchronous receivers, and audio frequency generators. Here is a question for you, what are the advantages and disadvantages of RC oscillator?

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