Amplifiers
Amplification is process of increasing the strength of a signal. For instance, when you increase the volume setting on your stereo, you are taking a weak signal and increasing its power level (making it stronger). Circuits designed to provide amplification are known as amplifiers.
In "Electronics", signal amplifiers are frequently used devices as they have ability to amplify a relatively small input signal, for instance from a Sensor like a photo-device, into a much larger output signal to drive a Relay, lamp or loudspeaker for instance. There are a number of forms of electronic circuits classed as amplifiers, from Operational Amplifiers and Small Signal Amplifiers up to the Large Signal and Power Amplifiers. Amplifiers can be thought of as a box containing the amplifying device, like a Field Effect Transistor or Op-amp, Transistor, which has 2 input terminals and 2 output terminals (ground being common) with output signal being much greater than that of input signal as it has been "Amplified".
An ideal signal amplifier has 2 basic properties, Input Resistance or ( Rin ), Output Resistanceor ( Rout ) and of course amplification known as Gain or ( A ). Does not how complicated an amplifier circuit is, a general amplifier model can be used still to show the relationship of these 3 properties.
Oscillators
Oscillators are circuits which produce a repetitive waveform with only a DC voltage at output. The output waveform can be rectangular, sinusoidal, triangular, etc. At base of almost any oscillator there is an amplification stage with the positive feedback circuit which will produce a phase shift and an attenuation.
Positive feedback consists in redirecting of the output signal to the input stage of amplifier without a phase shift. This feedback signal is then amplified again generating output signal which generates the feedback signal. This phenomenon, in which output signal "takes care" of itself in order to generate continuum signal is called as oscillation.
Two conditions then should be fulfilled to have a stable oscillator:
1. The phase shift of feedback loop should be 0
2. The overall gain of feeddback loop should be 1
With the aim of coming to the stable regime of oscillator, during the starting period the gain of the feedback loop should be greater than 1, to allow the amplitude of output signal to achieve the desired level. Once this amplitude has been reached, overall gain should goes down again to one. The very 1st oscillations are generated by trasitory reactions and noises as the power is switched on. The feedback circuit should be designed to amplify a single frequency only, and the relatively faint signal picked up from transitories and noise is then used to startup the oscillator.
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