## Thursday, August 02, 2012

### Sinusoidal Oscillator

Block Diagram of Oscillator

Before explaining the block diagram of an oscillator, let us recall the concept of positive feedback amplifier. All of you know that positive feedback amplifier consists of amplifier having gain of ‘A’ and feedback circuit with gain of ‘Î²’. Here, a part of output is fed back to input through feedback circuit. The signal which is fed back is added to the input signal using summer ‘Î£ ’ and output of the summer acts as an actual input signal to the amplifier. The figure shows the block diagram of the oscillator. The difference between positive feedback amplifier and oscillator is that, in oscillator, there is no need of external input signal. To start the oscillations, output signal must be fed back in proper magnitude and phase.

Fig: Blockdiagram of Oscillator

Principle of Oscillators
An oscillator consists of an amplifier and a feedback network. Now, let us see which basic components are required to obtain oscillations.
'Active device' either Transistor or Op Amp is used as an amplifier.
'Feedback circuit' with passive components such as R-C or L-C combinations .

To start the oscillation with the constant amplitude, positive feedback is not the only sufficient condition. Oscillator circuit must satisfy the following two conditions known as Barkhausen conditions:

1. The first condition is that the magnitude of the loop gain (AÎ²) must be unity. This means the product of gain of amplifier 'A' and the gain of feedback network 'Î²' has to be unity.

2. The second condition is that the phase shift around the loop must be 360° or 0°. This means, the phase shift through the amplifier and feedback network has to be 360° or 0°.

How Oscillations are initiated at First?

We have stated that oscillators do not require any external input. This means an oscillator's output feeds its own input and it must satisfy the two essential conditions to start oscillations. Here, you might be eager to know, from where the starting voltage comes. All of you are aware that every resistor has certain free electrons. At room temperature these free electrons move randomly and generate a noise voltage across the resistor due to collisions. This voltage is also known as thermal noise voltage. Hence, the resistor acts as small ‘ac’ voltage source. When power supply is turned on, for the first time this small ac noise voltage gets amplified and appears at the output terminal. This amplified output is applied to feedback circuit and output of feedback circuit is fed back to the amplifier as an input. It is again amplified by amplifier and fed back to input through feedback circuit. This process is repeated and at one particular frequency, circuit satisfies the necessary conditions to start oscillation. Using proper feedback components, it is possible to select the particular frequency.

Fig: How Oscillations are initiated