What is an Oscillator?
An Oscillator is a positive feedback electronic circuit in which the input signal and the feedback signal are In Phase with each other. It can be used to generate oscillating signals like a square wave, triangular wave, sine wave, etc (without any Input)
Feedback in electronic circuits
” The process of injecting some portion of output signal of a circuit back as input to the same circuit is known as feedback. “
Consider the following system which shows amplification of input voltage Vi by factor ‘A’.
Output Voltage = Amplification factor * Net Input Voltage ┉┉ ➀
Vo = A*Vi
Note that amplification factor of the system is given by (Vo/Vi ).
connecting feedback to the above system gives,
Here, ‘β’ fraction of the output current Vo is feedback as input to the system, i.e Vf = βVo ┉┉ ➁
Also, in the case of oscillators, the feedback element provides 0o or 360o of total phase shift to the feedback signal.
So, the total input to the system becomes ( Vi + Vf )
From equation ➀ we get,
Vo = A * ( Vi + Vf )
Vo = AVi + AVf
Vo = AVi + AβVo (from eq ➁)
Vo ( 1 – Aβ) = AVi
Vo/Vi = A/(1 – Aβ)
⇒ Gain of a system with positive feedback is given by A/(1 – Aβ).
It can be clearly noted that whether the input signal in the system will be amplified or attenuated or will remain sustained depends on the value of ‘Aβ’.
Therefore 3 cases arise:
Note that ‘A’ represents the amplification of the signal without feedback and ‘β‘ represents the fraction of output voltage that will be fed back to the system.
Case 1) Aβ < 1 :
Here decaying oscillations are generated by the oscillator. Below is an example of decaying oscillations.
Case 2) Aβ > 1 :
Here, growing oscillations are generated. Below is an example of growing oscillations.
Case 3) Aβ = 1 :
Here, sustained oscillations are generated by the oscillator. Below is an example of sustained oscillations.
So, Sustained Oscillations are generated only when Aβ = 1.
Barkhausen Criterion:
These are the two conditions that a system must satisfy to behave as an Oscillator:
1) The total phase shift of the signals in the close loop system must be equal to 0 or 360 degrees.
2)The magnitude of Aβ should be equal to 1.
NOTE: Oscillators can generate oscillations even without giving any external input. This is because of the presence of thermal noise in the system. The noise itself will act as input to the oscillator.
Characteristics of a positive feedback circuit
- It is also called regenerative feedback.
- The gain of a positive feedback circuit is always greater than the gain without any feedback.
- The feedback signal is in phase with the input signal.
- It is an essential part of any oscillator circuit.
Types of Oscillators
Oscillators are broadly classified into two types: (on the basis of the nature of the output signal)
- Sinusoidal
- Non – Sinusoidal
The oscillators which generate a sinusoidal (Either sine or cosine) wave as output are called sinusoidal oscillators. Examples:
- RC Oscillators (Made with combinations of Resistors and Capacitors)
- LC Oscillator (Made with combinations of Inductors and Capacitors)
- Crystal Oscillators
The oscillators which generate a non-sinusoidal wave (like a square wave, or triangular wave) as output are called non-sinusoidal oscillators. Examples include:
- Relaxation Oscillators
- MultiVibrators
RC Phase Shift Oscillator
The RC phase-shift oscillator consists of a single transistor in the ‘Common Emitter’ configuration and a phase-shift section consisting of three sections of RC combination. Each resistor and capacitor combination provides a phase shift of 60 degrees to the signal. As there are three sections of RC, the total phase shift will be 180 degrees. The transistor also provides a phase shift of 180o to the input signal. Therefore, the total phase shift is 360o in the circuit.
The frequency of oscillations produced by an RC phase shift oscillator is given by:
Fo = 1/(2πRC𢆦) where
R = R1 = R2 = R3
C = C1 = C2 = C3
The RC phase-shift oscillator generates sinusoidal waves with frequency Fo = 1/(2πRC𢆦).
Examples of other commonly used oscillators include:
- Wein Bridge Oscillator
- Hartley Oscillator
- Colpitts Oscillator
- Crystal Oscillator
Applications:
Some uses of the oscillators include:
- Electronic devices like computers have microprocessors that work on clock pulses. The oscillators provide these clock pulses.
- Signal generators generate different signals which are used in laboratories.
- Day-to-day devices like TV, computers, Inverters, Audio, Video systems, etc.
- Metal detectors, Quartz watches.
Frequently asked questions:
1. What is the total phase shift of a signal in an oscillator circuit?
Ans) The total phase shift of an input signal across the loop (i.e. from input to output, from output to feedback element, from feedback element to input signal) must be equal to 0 degrees or 360 degrees.
2. What is the significance of feedback factor ‘β’?
Ans) Feedback factor represents the portion of output signal which is fed back as input to the system. The value of β changes from system to system.
3. What is the role of thermal noise in oscillators?
Ans) Any unwanted disturbance in an electronic system is termed as ‘Noise’. Thermal noise is the unwanted signal generated in a system because of the excitations and de-excitation of charge carriers in the system. This thermal noise itself is sufficient for an oscillator to generate oscillating signals.