Oscillators Introduction:
An electronic oscillator is an electronic circuit that produces a repetitive, oscillating electronic signal, often a sine wave or a square wave.Oscillators convert direct current (DC) from a power supply to an alternating current signal.They are widely used in many electronic devices.Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.Oscillators are often characterized by the frequency of their output signal:
- An audio oscillator produces frequencies in the audio range, about 16 Hz to 20 kHz.
- An RF oscillator produces signals in the radio frequency (RF) range of about 100 kHz to 100 GHz.
- A low-frequency oscillator (LFO) is an electronic oscillator that generates a frequency below ≈20 Hz.This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.
Any circuit which is used to generate a.c. voltage without a.c. the input signal is called an oscillator.To generate a.c. voltage .the circuit is supplied energy from a d.c. source.If the output voltage is a sine wave function of time, the oscillator is called a "sinusoidal" or "Harmonic" oscillator.Positive feedback and negative resistance oscillators belong to this category.There is another category of oscillators which generate non-sinusoidal waveforms such as square, rectangular, triangular or sawtooth waves.
Oscillators Classification:
Oscillators are classified in the following different ways.
1.According to the waveforms generated:
(a) Sinusoidal oscillator
(b) Relaxation oscillator
Sinusoidal oscillator generates sinusoidal voltage or currents as shown in fig. below.
 |
| sinusoidal wave |
Relaxation oscillator generates voltages or currents which vary abruptly one or more times in a cycle of oscillation as shown in fig. below
 |
| sawtooth wave |
 |
| square wave |
 |
| triangular wave |
2.According to the fundamental mechanisms involved :
(a) Negative resistance oscillators
(b) Feedback oscillators
Negative resistance oscillators use the negative resistance of the amplifying device to neutralize the positive resistance of the oscillator.
Feedback oscillators use positive feedback in the feedback amplifier to satisfy the Barkhausen criterion.
3.According to the frequency generated :
(a)Audio frequency oscillator(AFO):upto 20 kHz
(b)Radio frequency oscillator(RFO):20 kHz to 30 kHz
(c)Very high frequency oscillator(VHF):30 MHz to 300 MHz
(d)Ultra high frequency oscillator(UHF):300 MHz to 3 GHz
(e)Microwave frequency oscillator: above 3 GHz
4.According to the type of circuit used, since wave oscillators may be classified as
(a) LC tuned oscillator
(b) RC phase shift oscillator
Barkhausen criterion(Conditions for oscillation):
The essential conditions for maintaining oscillations are :
1.|AB|=1,i.e. the magnitude of loop gain must be unity
2.The total phase shift around the closed loop is zero or 360 degrees.
Practical considerations:
The condition that |AB|=1 gives a single and precise value of AB which should be set through out the operation of the oscillator circuit.But in practice, as transistor characteristics and performance of other circuit components change with time,|AB| will become greater or less than unity.
Hence, in all practical circuits |AB| should be set greater than unity so that the amplitude of oscillation will continue to increase without limit but such an increase in amplitude is limited by the onset of the nonlinearity of operation in the active devices associated with the amplifier as shown in the figure above. In this circuit, AB is larger than unity for the positive feedback.This is what Barkhausen criterion.This onset of nonlinearity is an essential feature of all practical oscillators.