Before knowing about sinusoidal oscillator, we will get some information about oscillator. After that we will learn about Positive Feedback Amplifier, Essential of Transistor Oscillator and Barkhausen Criterion. So let’s start the topic.
Oscillator definition
An oscillator is an electronic circuit that produces a repeating electronic signal in equal intervals of time. They are widely use in myriad electronic devices. Common examples of signals generated by oscillators include signals transmitted by radio and television transmitters, clock signals controlling computers, quartz clocks, etc.
And in other words, The oscillator are electronic circuit or electronic device which convert direct current from the power supply to an alternating current.
Remember : The signal used in oscillator are sine wave an the square wave.
See also this : What is Single Stage Transistor Amplifier?
Sinusoidal oscillator
An electronic device that generates sinusoidal oscillations of a desired frequency is known as a sinusoidal oscillator.
Although we talk about an oscillator as “generating” a frequency, it should be noted that it does not generate energy, but merely acts as an energy converter.
It receives D. C. energy and changes it into A. C. energy of desired frequency. The frequency of oscillations depends on the constants of the device. It may be mentioned here that though an alternator produces sinusoidal oscillations of 50Hz, it cannot be called an oscillator.
First of all, an alternator is a mechanical device with rotating parts whereas an oscillator is a non-rotating electronic device.
And secondly, an alternator converts mechanical energy into A. C. energy while an oscillator converts D. C. energy into A. C. energy.
And in the thirdly, an alternator does not produce high frequency oscillations, whereas an oscillator can produce oscillations ranging from a few Hz to several MHz.
See also this : What is Multistage Transistor Amplifier?
Positive feedback amplifier – Oscillator
With proper positive feedback a transistor amplifier can act as an oscillator that is, it can generate oscillations without any external signal source. In figure-1 you can see a transistor amplifier with positive feedback.
Remember that a positive feedback amplifier is one that produces a feedback voltage ( Vf ) that is in phase with the original input signal. As you can see, this condition is satisfied in the circuit shown in figure-1.
A phase shift of 180° is produced by the amplifier and another phase shift of 180° is introduced by the feedback network. As a result, the signal is shifted by 360° and fed to the input that is, the feedback voltage is in phase with the input signal.
See also this : Feedback in electronics
(1) Note that in figure-1, we can see that a circuit is producing oscillations in the output. However, this circuit has an input signal. This is inconsistent with our definition of an oscillator which means that, an oscillator is a circuit that produces oscillations without any external signal source.
(2) When we open the switch S of fig-1, we get the circuit shown in fig-2. It means the input signal ( Vin ) is remove. However, Vf which is in phase with the original signal is still applied to the input signal.
The amplifier will respond to this signal in the same way as it did to Vin which means, Vf will be amplified and sent to the output. And the feedback network sends a portion of the output back to the input.
Therefore, the amplifier receives one more input cycle and another output cycle is generate. This process will continue as long as the amplifier is on.
Hence, the amplifier will produce a sinusoidal output without any external signal source.
The following points may be carefully noted :
- A transistor amplifier with proper positive feedback will work as an oscillator.
- The circuit only needs a quick trigger signal to start the oscillations. Once the oscillations is start, no external signal source is require.
- In order to get continuous damped output from the circuit, the following condition must be satisfied.
mv Av = 1
Where, mv = Feedback fraction
Av = Voltage gain of amplifier without feedback
This relation is call Barkhausen criterion.
Essentials of transistor oscillator
In the figure below you can see the block diagram of an oscillator. Its essential components are discussed below :
(1) What is tank circuit?
It consists of an inductance coil (L) connected in parallel with a capacitor (C). The frequency of oscillations in the circuit depends on the values of the inductance of the coil and the capacitance of the capacitor.
(2) What is transistor amplifier?
(3) What is feedback circuit?
The feedback circuit provides a part of collector energy to the tank circuit in the correct phase to assist the oscillations, that is it provides positive feedback.
See also this : What is transistor in electronics
What is barkhausen criterion
The barkhausen criterion is states that to produce continuous damped oscillations at the output of an amplifier, the positive feedback must be such that :
mv Av = 1
Once this condition is set in the positive feedback amplifier, continuous undamped oscillations can be obtained at the output as soon as the required power supply is connected.
Mathematical explanation :
The voltage gain of a positive feedback amplifier is given by :
Avf = Av / (1 − mv Av )
If mv Av = 1 then, Avf → ∞
We cannot get unlimited gain in an amplifier. So what does this result represent in physical terms? This means that a vanishingly small input voltage will give rise to finite (that is, a definite amount of) output voltage even when the input signal is zero.
Thus once the circuit receives the input trigger, it will become an oscillator, and generating oscillations without any external signal source.