Definition of oscillator
Oscillators are those electronic devices that produce a sinusoidal wave form at their output upon application of dc voltage at their input terminals. Although other types of wave forms can also be obtained from them but the basis wave form that they produce is sinusoidal. There can be different types of oscillators like feedback oscillator and negative feedback oscillator. Here we will be see how we can classify feedback oscillator along with their working principle.
How many types of feedback oscillators are there?
There can mainly three types of oscillators which can be classified as under on the basis on component used in them like resistor, capacitor inductor etc.
1. RC oscillator
2. LC oscillator
3. Crystal oscillator
What are RC oscillators?
These are the oscillators that involve resistor and capacitor in their circuitry for the purpose of required feedback to the amplifier. These categories can further be classified in to two types that are given as:
1. Wien's bridge oscillator
2. Phase shift oscillator
What is Wien's bridge oscillator?
It is so important type of feedback circuit that it provides a standard as the variable frequency testing circuit in audio frequency range. The operation of this circuit is very simple. Also it is less complex than LC oscillators. It is actually an audio oscillator that is used in the frequency range of audible range generally generated through speakers. And it is seldom used in testing of those electronic circuits that operate upon audio frequencies. Usually a sinusoidal wave form is generated by this type of oscillator that occurs to be variable both in frequency and amplitude so that it may be adjusted according to the concerned application. Other than sinusoidal waveform some other wave forms can also be generated with help of them like square wave but it utilizes the same principle for that purpose too. The amplitude generated by them is generally in the range of about 25V and frequency occurs in range of about 20Hz to 20 KHz. The output impedance of the audio oscillator is in the range of 75 to 600 ohm.
The most common oscillators as Wien's bridge oscillator and phase shift oscillator and both of them apply RC network in their operation. There are some very important features regarding RC oscillators are:
• Their simple and straight forward circuitry and design.
• Production of pure sine wave form
• The stability of the frequency it its output port
It is actually a type of feedback oscillator where the function of the Wien's bridge is to provide inversion of phase. There inversion of phase does not mean to shift the phase by 180 degree only but the shift in phase can be by any angle. It is basically an AC bridge such the balance is achieves at the particular frequency of the whole frequency under given range. Here is a circuit diagram that shows the circuitry set of a Wien's bridge network. There are capacitors, resistors and an operational amplifier represented by a triangular symbol getting input from the net work formed by passive components.
The bridge is balanced when it follows the following condition:
Z1 Z4=Z3 Z2
Here the values of resistors R1 and R2 are kept equal by the means of mechanical knobs that can be adjustable the user during the operation.
There are some conditions for oscillations to occur one of them is that he phase shift between E1 and E2 should be equal to zero which is possible in one situation when the bridge is made to be balanced. The frequency of oscillations is given by:
This equation represents the condition of resonance of the bridge in which it does not depend upon the value of that of the second arm of bridge.
Second condition for making oscillations to occur is the product of gain constant of amplifier and constant H should come to be greater than unity. The requirement of the bridge is to provide feedback to the operational amplifier so the value of E2 should remain always greater than one and phase shift between the two i.e. E1 and E2 remain to be zero.
Wien's bridge oscillator circuit
The phase angle of both the impedances i.e. Z1 and Z2 are same when it is in resonance situation because both R3 and R4 are pure resistances. If we assume
Then for amplifier gain, G, to be equal to the constant Alfa a condition should be satisfied which is given as:
From the discussion so far it has become clear that frequency of oscillation 's' is equal to that of resonance. And Ea does not come to be equal to E1 at any frequency other than resonance frequency. This results in the consequence that the difference of Ea and Eb does come to be in phase with E1.
Some points about Wien's bridge circuitry
The capacitor that is used in above circuit is of air by variable capacitor which is mounted on the same shaft. The change is frequency of the bridge is done by means of this capacitor. But this leads to only small variations in frequency while for large variations there exit a switch associated with resistor and by changing the positions of this switch and desired value of resistance can be introduced in this circuitry. This action of changing the resistance shifts the value of resistor directly to another range thus the frequency can be shifted to another range. Thus different frequency ranges can be selected with this resistance shifting switch. We have read about negative feedback which actually means to apply a portion of output signal back to input terminal but in inverted form that actually leads to attenuation of the signal i.e. the reverse of amplification that you may have question in your mind that
Why do we need a negative feedback in amplifiers?
The answer to this question is that the negative feedback results in stable amplitude of the output wave by automatic gain control system present in circuitry.
In the present circuitry negative feedback is given with the help of resistors R1 and R2. There it is to be emphasized that resistor R2 is often chosen to be of temperature sensitive type whose temperature coefficient is positive. R2 is an incandescent lamp operated at a lower temperature to produce the luminescence less than its usual level. Since the resistance offered by the lamp is varied as the amplitude of driving signal changes thus it changes the voltage division of the bridge arm in the same accord. This result is the case that amplitude of oscillations also increases this leads to the similar increase in value of R2. The negative feedback is reduced causing a decrease in gain of amplifier and thus amplitude of oscillations also gets back to its normal level.
Practical Wien's bridge oscillator circuit
This result can also be understood by the following method:
Amplitude of oscillations is function of value of product GH i.e. how much the value of this product varies greater that unity affect the amplitude of oscillations. Taking the case when feedback constant is constant then the only variable reaming is G that decides the amplitude level. In this way when an increment is made in G the similar increase comes to be visible in amplitude also till the increase is limited by the non linear characteristic transistors. When the output amplifier comes to increase the increased current through resistor R2 increases its resistance. In this way it reduces the value of H. Now since, initial value of G was allowed to increase and here at the later stage value of H is found to be decreasing, thus net product of both remains constant i.e. HG is content.
This far we have studied about operation of a Wien's bridge oscillator now we will try to gather some information about Phase shift oscillator by answering the following question:
What are advantages of Wien's bridge oscillator?
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