Oscillator

An oscillator is a circuit that produces a repetitive waveform on its output with only the dc supply voltage as an input.A repetitive input signal is not required.The output voltage can be either sinusoidal or nonsinusoidal depending on the type of oscillator.

The basic concept of an oscillator is illustrated in figure(a).Essentially an oscillator converts electrical energy in the form of dc to electrical energy in the form of ac.A basics sinusoidal oscillator consists of an amplifier for gain( either discrete transistor or op-amp ) and a positive feedback circuit that produces phase shift and provides attenuation,as shown in figure(b).

The basic oscillator concept showing three common types of output waveforms sine wave,square wave and sawtooth.

Oscillator Principles:

Positive Feedback:

Positive feedback is characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift,resulting in a reinforcement of the output signal.This basic idea is illustrated in figure.As you can see the in phase feedback voltage Vf is amplified to produce the output voltage which in turn produces the feedback voltage.That is,a loop is created in which the signal sustains itself and a continous sinusoidal output is produced.This phenomenon is called oscillation.

Conditions for Oscillation:

Two conditions are required for a sustained state of oscillation.

1.The phase shift around the feedback loop must be zero degree.

2.The voltage gain Acl,around the closed feedback loop (loop gain) must equal 1 (unity).

The voltage gain around the closed feedback loop(Acl) ia the product of the amplifier gain (Av) and the attenuation (B) of the feedback circuit.

Acl=Av.b

For example,if the amplifier has a gain of 100,the feedback circuit must have a attenuation of 0.01 to make the loop gain equal to 1( that is Av.B=100*0.01=1).These conditions of oscillation are illustrated in figure.

Start-Up Conditions:

So far,you have seen what it takes for an oscillator to produce a continous sine wave output.Now let's examine the requirements for the oscillation to start when the dc supply voltage is turned on.As you know,the unity-gain condition must be met for oscillation to be sustained.For oscillation to begin,the voltage gain around the positive feesback loop must be greater than 1 ,so that the amplitude of the output can be build up to a desired level.The gain must then decrease to 1 so that the output stays at the desired level