Basically, there are two types of oscillator circuits. They are Sinusoidal and the non Sinusoidal. The first essentially produce sine waves and the other generates square wave, triangular wave, pulses or sawtooth wave etc. The circuit, which generates square waves, is generally referred to as Multivibrator. According to the style of producing the waves, oscillator circuits are classified into three sub groups: Tuned circuit [LC] oscillator circuits, RC oscillator circuits & Crystal oscillator circuits.
How a tuned coil and capacitor circuit [LC] or oscillator works?
This is the most basic type of oscillator circuit, which essentially consists of a tank circuit. It is a combination of a single capacitor and a single inductor [coil] connected in PARALLEL. The simple setup of this type of tank circuit is given below –
The tank circuit and the damped oscillations produced by it (Waveform not shown)
Here Fig: 1a shows the circuit of tank circuit and the Fig: 1b is the output waveform of the tank circuit. It is clear that the amplitude of the output wave produced by the tank circuit is NOT constant. It decreases with respect to time. Such oscillations are called as damped oscillations. These oscillations die out after some time. However, theoretically these oscillations continue for infinite time and do not stop. Such wave is not at all useful to us in the practical electronic circuit. So it is necessary to convert it into undamped or sustained oscillations. The specific circuit, which produces such sustained oscillations, is called as basic oscillator circuit.
In such circuit, the tank circuit is the most important part, which is used as the basic element to produce the damped wave, and then it is converted to undamped wave with the help of some other circuit. The frequency of oscillations of circuit in which the tank circuit [LC type] is used is given by –
What is the Barkhausan’s Criterion for oscillations?
The Barkhausan’s criterion has three important rules for a circuit which is to be used as the oscillator circuit:
a) The circuit must have a tank circuit, which will produce damped oscillations.
b) The circuit must contain an amplifier whose gain must be greater than unity.
c) The circuit must have a feedback circuit and it must provide positive feedback.
The basic circuit using above rules; to satisfy the Barkhausan’s Criterion is given below –
Basic circuit for satisfying the Barkhausan’s criterion
Here the tank circuit produces the damped oscillations, which are amplified by the amplifier (note that the gain of this amplifier is greater than unity). The fraction of this amplified output is then fed back to the input of amplifier via the feedback circuit. Here note that the amplifier must have its output and input IN PHASE so that the feedback via the feedback circuit will also be positive feedback. Thus, the Barkhausan’s Criterion will be satisfied and the oscillations will start up. These oscillations will now be sustained oscillations, because of the presence of the amplifier. Thus, the amplifier and the feedback circuit are actually responsible to convert the damped into undamped oscillations. The losses produced in the damped waves are filled up by this circuit, at every step and thus we get the constant amplitude of the output waves.
What are the LC type oscillator circuits? How they work?
The LC oscillator circuit is the one which contains one or number of coils together with combination of capacitors. Any electronic or electric circuit that generates alternating voltage is called as an oscillator circuit. However, to generate the AC voltage from the oscillator circuit we must provide the DC voltage to the circuit. Thus, out of DC voltage the AC voltage is produced by the oscillator circuit.
An oscillator circuit is mainly used in communication electronics. For example, in radio and television broadcasting, the transmitter generates the high frequency and then it is radiated in air through the antenna. Similarly, the transmitted frequency is then received by the receiver and converts it back to the required form. For this conversion, also we need high frequency oscillator circuits.
How the Hartley oscillator circuit works?
Basic circuit of Hartley oscillator
The above circuit uses an NPN transistor along with the tank circuit of capacitor [C] and the primary of the transformer [L]. Therefore, the damped oscillations are produced in it, when the power supply is connected to the circuit. These damped oscillations are then fed to the base of the transistor via the feedback path consisting of base capacitor [Cb]. This input is then amplified by the transistor and the high output fill up the losses took place in the tank circuit. So at the output of the transistor through the collector capacitor [Cc] pure alternating current having sinusoidal oscillations are obtained. The frequency of output of the circuit is given by –
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How a Colpitt’s oscillator circuit works?
– this oscillator circuit is similar to the Hartley oscillator circuit; only this uses two capacitors and a single coil. The circuit is given on the next page. Here the transistor is used as an amplifier, which is, connected in common collector configuration. The fraction of the output is fed back to the input of the transistor at base, via the collector capacitor [Cc]. The capacitors [C1 & C2] along with the coil [L] form the tank circuit to produce the damped oscillations. They are then fed to the base and amplified and then available at the collector of the transistor.
Basic circuit of Colpitt’s oscillator
Since the transistor is connected in CC configuration, its output is in phase with its input at base. So here, the Barkhausan’s criterion is satisfied and the oscillations are setup. The frequency of the oscillations produced is given by –
How an RC type oscillator circuit works?
You may know that the tank circuit is the only circuit, which can produce the oscillations. However, similar to this, the combination of resistor and capacitor can also effectively produce good quality oscillations; when this combination is properly used in some practical circuits like Phase Shift Oscillator and Wien Bridge Oscillator. The rules of Barkhausan’s Criterion are equally applicable to the RC oscillator circuits. So here, also, the feedback should be positive, and the circuit must contain an amplifier whose gain must be greater than unity. So using resistor and capacitor combination, we shall see some oscillator circuits.
How a Phase Shift Oscillator circuit works?
Basic circuit of phase shift oscillator
Here the transistor is used as an amplifier. The most important part of this circuit is the RC network wired around the R1-R3 and C1-C3. This network provides a phase shift of [180°] to the waveform being produced by the circuit. Now since the transistor is connected in common emitter configuration it also provides a phase shift of another [180°]. The RC network is connected in series with the base of the transistor; so the two independent phase shifts of both section of the circuit are added and the total phase shift becomes [360°].
Now when the phase shift of a circuit is 360° then its output must be in phase with its input. So in this way the Barkhausan’s Criterion is satisfied and the oscillations start-up. The gain of the transistor in common emitter configuration is always greater than unity. Hence, it amplifies the base signals at its collector and oscillations are produced. The frequency of these oscillations is given by –
How a Wein-Bridge Oscillator circuit works?
It is special type of oscillator circuit, which uses the typical Wein-bridge circuit. Its typical oscillator circuit is given below –
Circuit of Wein bridge oscillator
The circuit is very simple to understand and its working is entirely based upon the operation of the Wein-bridge. The Wein-Bridge is wired around resistors R1-R4 & C1, C2. Here note that the C1-C2 combination is known as the Gang Capacitor [i.e. when we change the value of one capacitor (C1) the value of other capacitor (C2) simultaneously changes. Thus, the frequency of the circuit can be changed by adjusting the value of the gang capacitor [C1 & C2] together.
Transistors T1 & T2 are used as amplifiers and the feedback path is provided via the uppermost line in the circuit. Each transistor provides a phase shift of 180°, so the total phase shift becomes 360° and hence the Barkhausan’s criterion is satisfied and the circuit produces oscillations.
The frequency of oscillations of the circuit is given by –
Generally the feedback circuit is a single resistor or group of resistors arranged to reduce the amount of signals that should flow from output to input of the amplifier.
The Hartley and Colpitt’s oscillator circuit are very useful in the generation of high frequency and very high frequency waveforms for the transmitter and receiver circuit – in a communication system.
Have a look at the video related to this article.
Test your knowledge…
1) The Hartley’s oscillator circuit has _____ capacitor and _____ resistors.
2) The phase difference produced by each combination of RC network in Phase shift oscillator is _____.
3) The tank circuit produces _____ oscillations.
4) The frequency of crystal oscillator circuit is given by _____.
5) To satisfy the Barkhausan’s criterion, the gain of the amplifier should be equal to ____.
6) To produce the tank circuit, the capacitor and a coil are connected in ____ to each other.
7) The frequency of phase shift oscillator is given by _____.
8) In Wein-bridge oscillator circuit, the combination of capacitors is called as _____ capacitor.
9) In Colpitt’s oscillator circuit, the transistor is connected in _____ configuration.
10) To satisfy the Barkhausan’s criterion, the circuit must have _____ feedback.