An oscillator is a mechanical or electrical construction that produces oscillation based on some variables. This oscillator is used from clock to various metal detectors and even on computers where microprocessors and microcontrollers exist The oscillator in which the RC circuit is used as a feedback path is the RC phase shift oscillator. It generates stable sine waves that are used in low-frequency generations, particularly in the audio frequency range. The amplifier provides a phase shift of 180 degrees in this oscillator. For this amplification either inverting amp or transistor is used. This oscillator consists of an inverting amplifier which is followed by a feedback network with a phase shift of 180 degrees. By combining capacitors and resistors, 180 degrees of phase shift can be gained. Even the Transistor amplifier will give 18o degrees phase shift. This oscillator will produce a sine wave output signal by the use of regenerative feedback from the combination of RC. This article discusses what is RC phase shift oscillator and its working.
RC Phase Shift Oscillator Circuit
The frequency stability is very good in this oscillator. This oscillator can generate pure sine wave signals for various angles of loads. Normally an RC network, the output is expected to lead the input signal by 90 degrees. But coming to reality or in practical condition due to the usage of capacitors which are not ideal, the phase difference will be less.
‘RC’ network phase angle can be written as
Ø = tan-1 XC /R
In this type of RC phase shift network, each offers a definite phase shift and the Burghausen criterion phase shift condition can be satisfied when the RC phase shift is cascaded.
RC Phase Shift Oscillator Circuit Diagram using BJT
The below figure describes the complete view of the RC phase shift circuit by using BJT. A transistor is considered as an active element in the amplifier stage in the RC phase shift oscillator circuit. The resistors R, RC, RE, R1, and Vcc establishes an active region in the above circuit. CE is the bypass capacitor here. Three R sections are identical here. The biasing resistors is having a large value therefore it will not affect the AC operation. The combination of RE-CE offers negligible impedance that why it doesn’t show any effect on AC operation. The collector resistor limits the transistor collector current. Emitter response improves stability. Capacitors Co AND CE are output decoupling capacitor and emitter bypass capacitor.
When the power is switched on, the oscillations will get started. A current with a phase shift of 180 degrees by a small current at transistor amplifier. When the signal will be given as input to the amplifier through feedback, again a phase shift of 180 degrees takes place. The sustained operation will be generated when loop gain equals unity. If the change is needed in the frequency of oscillations then R and C values should be changed because then only frequency oscillations will vary.
Frequency of RC Phase Shift Oscillator
We discussed that these oscillators are capable of generating audio frequencies of range 1Hz to 10MHz. this oscillator consists of a converting amplifier with a feedback network that will generate a phase shift of 180 degrees.
The generalized equation for RC phase shift oscillator frequency is
ƒ = = 1/(2πRC √ 2N)
‘R ‘is resistance
‘C ‘is capacitance and
‘N ‘is the number of RC stages
We can observe that frequency of this oscillator can be changed by changing the capacitor or resistor. In general, capacitors are gang tuned while the resistor is kept constant. In comparison between RC phase shift oscillator and LC oscillators, more number of circuit components are used by the former one than the latter one. This will make us clear that in LC oscillators the output frequency produced cannot deviate much from the calculated value when it is compared with the RC phase shift oscillator. This phase shift oscillators will be used as local oscillators for synchronous receivers, audio frequency generators, and even musical instruments.
There are various advantages in RC phase shift oscillators and they are
- The Circuit of this oscillator is very simple. So there is no complexity while using this oscillator
- The cost is also very less as it comprises resistors and capacitors which are not very costly.
- The frequency stability in the RC phase shift oscillator is very good.
- As the output is sinusoidal, so there is no distortion in the output
- The stabilization arrangements and the negative feedback is not needed in RC phase shift oscillators.
- Another important advantage of the RC phase shift oscillator is it is having a wide range of frequencies i.e. from few Hz to several hundred kHz.
- These are quite apt for low-frequency range, this will make clear that frequencies can be gained by use of R and C large values
There are even some disadvantages in RC phase shift oscillators
- Due to its smaller feedback, the output is very small. This is one of the drawbacks of RC phase shift oscillators.
- When the frequency stability of this oscillator compared with the Wien bridge oscillator, it is very low.
- As the feedback is small, it is somewhat difficult for the circuit to start with oscillations.
- A high voltage battery is needed to develop a large and sufficient feedback voltage. To simplify it high Vcc is needed for large feedback.
The applications of RC Phase Shift Oscillator are as follows
- This oscillator is utilized for the generation of signals over a wide range of frequencies. This oscillator is used in voice synthesis, musical instruments, and GPS units. These oscillators are capable of working at all audio frequencies.
- FET phase shift oscillator is capable of generating a signal for a wide frequency range. The frequency can vary from a few Hz to 200 Hz by using a set of resistors and capacitors together for varying over the range of a 1:10 ratio. Similarly different range of frequencies 200Hz to 2KHz and 2KHz to 20KHz and 20KHZ to 200KHz can be gained by using other or various resistors.
Please refer to this link to know more about Oscillators MCQs
Thus, this is all about an overview of the RC phase shift oscillator, circuit, and its working. Here is a question for you, what are different kinds of oscillators?