The oscillator are an electronics circuit that are use to generate a sinusoidal wave or continuous wave at a certain frequency range. The above figure shoes the block diagram of oscillator circuit with positive feedback. This feedback differentiates the amplifier and oscillator. An oscillator convert DC signal to AC output voltage. The output wave form may be wide range of different shapes and frequencies, and can be either complex in shape, or be a simple pure sine wave depending upon the application.
There are various application of oscillator like producing sine wave, square wave, saw tooth wave, triangular wave and pulse. There are various type of oscillator like LC oscillator, Hartley oscillator, Colpitts oscillator etc. the LC oscillator are mostly use in radio frequency because it is easy to implement and good phase noise characteristics.
What is an oscillator?
An oscillator is an electronics circuit which produces a continuous wave without any input. It basically converts the unidirectional current flow of DC source into an AC waveform which is of the desired frequency, as decided by its circuit components. When an amplifier with positive feedback is called oscillator.
An oscillator is a self sustaining circuit, it produce a continuous wave at a single sinusoidal frequency. Thus for an electronics circuit to operate an oscillator it must have follow some characteristics.
- Some form of Amplification
- Positive Feedback (regeneration)
- A Frequency determine feedback network
An oscillator input has summing of feedback amplifier. The gain products of inverting amplifier and feedback amplifier must be unity or garter than unity. It also have phase shift of 360 degree or zero phases shift. The amplifying device must be operational amplifier or Bipolar Transistor is required.
Basic Oscillator Feedback Circuit
The block diagram of basic oscillator is shown below. In this block diagram contains one operational amplifier with positive feedback. The input of operational amplifier is the summation of feedback input and Vin input applied into operational amplifier.
Where: β is a feedback fraction.
Oscillator Gain without Feedback
Oscillator Gain with Feedback
Than oscillator is electronics circuit that produce continuous waveform with the help of feedback network. The feedback network is taken some output fraction of amplifier to feed into the input of amplifier in order to keep the circuit oscillating.
The feedback network is basically an attenuation circuit that has a voltage gain of less than one ( β <1 ). Oscillations start when Aβ >1 and then returns to unity ( Aβ =1 ) once oscillations are sustained.
The basic type of oscillator is:-
- Sinusoidal oscillator – this is also called harmonic oscillator and it is generally a LC tune feedback of RC tuned feedback type oscillator. It is use to generate pure sine wave with constant amplitude and frequency.
- Non-Sinusoidal Oscillators – This type of oscillator is use to generate non sinusoidal wave. In this type of oscillator the output will be quickly change from high to low or low to high. This type of oscillator is also known as Relaxation Oscillators. The example of Relaxation Oscillators is Square-wave, Triangular-wave and Saw toothed-wave.
Barkhausen Criterion or Conditions for Oscillation
The oscillator circuit must have to follow the two criteria for continuous produce wave from, called as Barkhausen’s criteria. These two conditions are
- The close loop gain must be unity.
- The feedback output signal back to input of amplifier must be phase shifted by 360 degrees (which is same as zero degrees). The inverting amplifier is use to phase shift to 180 degree of input and further feedback again phase shift of 180 degree. Thus the total phase shift is 360 degree or same as zero degrees.
In a LC oscillator, a tuned inductor-capacitor circuit alters the phase shift of 180 degree of input signal. Let us see the Barkhausen’s criteria with full explanation;
The input of signal is applied to inverting amplifier, it produce inverting output Vo which is 180 degree phase shift of input signal. The some fraction of output Vo is taken out by feedback as a input signal. The output of feedback is in out of phase 180 degree of Vo, this feedback output is summing with original input signal. This process will continue.
Let us consider the input voltage Vi is applied at the input of an inverting amplifier, then inverted output voltage will be:-
V0 = – A Vi
The output voltage of feedback circuit with feedback gain β is
Vf = – β V0
The negative sign indicted phase shift of 180 degree of V0.
Put V0 in Vf = – β V0 , we get,
Vf = A β Vi
In oscillator the feedback output must drive the amplifier; hence the Vf must act as Vi. For achieving Vf = Vi , the A β in the above expression should be 1, i.e.,Vf = Vi when A β = 1.
That condition the product of amplifier gain and feedback gain is unity is called as Barkhausen criterion for oscillation.
Basic Oscillator working principle
The working principle of oscillator can be understood by analyzing the faction of LC tune circuit which is shown below. In LC circuit contains inductor and per charge capacitor. In figure the capacitor discharge via inductor, which result in the conversion of electrical charge to electromagnetic field. This electromagnetic field is store in inductor. Once the capacitor is fully discharge there will be no current flow from capacitor to inductor.
However by them, the stored electromagnetic field in inductor generate back electro motive force (EMF) which result the current start flow form inductor to capacitor. This flow of current will continue until the electromagnetic field collapses which result in the back conversion of electromagnetic to electrical charge, causing the cycle to repeat. However the capacitor is charge with opposite polarity, due to which get an oscillating wave as the input.
The electrical energy is lose in real resistance of the inductor, in the dielectric of the capacitor due to this loses the oscillation steadily decreases until they die away completely and the process stops.
Once the power supply is turn “ON” oscillation start. The gain of oscillator is
Where the A is voltage gain of amplifier and gain of feedback network is β
Condition for Oscillation: Aβ < 1: the output signal will die out (Damped oscillations)
Aβ > 1: the output signal will build up.
Aβ = 1: the output signal will be steady, undamped oscillations (stable oscillations)
In other word the gain product of amplifier and feedback is greater than unit the output will be distorted; if gain product is less than unity the output will be Damped oscillations. If gain product is equal to unity the output will be constant amplitude are called undamped oscillations or stable oscillations.
Type of Oscillator
There is much type of oscillation, but it broadly classified into two main categories
- Harmonic Oscillators or Linear Oscillators – In this type the current flow the active to passive component. And the feedback is decided the frequency. This type of oscillators produces sine wave.
- Relaxation Oscillators – In this type of oscillator the exchange of energy in between active and passive component and the frequency will be decided by the time constant of charging and discharging time constant involved in the process. This oscillators generate non-sinusoidal (saw-tooth, triangular or square) wave-forms.
The main types of Oscillators include:
- RC Phase Shift Oscillator
- Hartley Oscillator
- Voltage Controlled Oscillator
- Crystal Oscillators
- Tuned Collector Oscillator
- Wien Bridge Oscillator
- Colpitts Oscillator
- Pearson-Anson Oscillators
- Armstrong Oscillator
- Gunn Oscillator
- Cross-Coupled Oscillators
- Ring Oscillators
- Clapp Oscillators
- Tri-tet Oscillators
- Dynatron Oscillators
- Opto-Electronic Oscillators
- Pierce Oscillators
- Electron Coupled Oscillators
- Meissner Oscillators
- Robinson Oscillators
- Delay-Line Oscillators
- Royer Oscillators
- Multi-Wave Oscillators
The oscillation can be also classified into various types depending upon the parameter like shape of wave, feedback etc. These types of classification are given below:-
- Based on the Shape of the Output Waveform -Sine Wave Oscillators, Square or Rectangular Wave oscillators, Sweep Oscillators (which produce saw-tooth output waveform), etc.
- Based on the Feedback Mechanism -Positive Feedback Oscillators and Negative Feedback Oscillators.
- Based on the Frequency of the Output Signal -Low-Frequency Oscillators, Audio Oscillators (whose output frequency is of audio range), Radio Frequency Oscillators, High-Frequency Oscillators, Very High-Frequency Oscillators, Ultra High-Frequency Oscillators, etc.
- Based on the Nature of the Frequency of Output Waveform -Fixed Frequency Oscillators and Variable or Tunable Frequency Oscillators.
- Based on the type of the Frequency Control Used -RC Oscillators, LC Oscillators, Crystal Oscillators (which use a quartz crystal to result in a frequency stabilized output waveform), etc.
The design of oscillators is simple and cheap. It is the easiest way to specific Frequency of a signal. For example, LC oscillators is used to generate a High Frequency signal, RC oscillators is used to generate a Low Frequency signal and Op-Amp based oscillators is used to generate a stable frequency.
There are some common example of oscillators is listed below:-
- Quartz watches (which uses a crystal oscillators)
- Audio systems and video systems
- It is use in radio, TV and other communication system
- It is used in computers, stun guns, metal detectors, ultrasonic, inverters, and radio frequency applications.
- To generate clock pulses for microprocessors and micro-controllers
- In alarms and buzzes system.
- operate decorative lights like dancing lights
LC Oscillators Summary
The basic requirements are as follow:-
- It must contain reactive component ie “inductor” and “capacitor”, which connected to DC supply.
- LC circuit, oscillations become damped over time due to component and circuit losses.
- It uses positive feedback and Voltage amplification to overcome these circuit losses.
- The phase difference is 360 degree or 0 degree.
- The gain product of inverting amplifier and feedback circuit must be unity.