Figure shows a block diagram of an amplifier and an oscillator.
An amplifier is a device, which produces an output signal with similar waveform as that of the input. But its power level is generally high. This additional power is supplied by an external D.C. source. Thus an amplifier is essentially an energy convection device I.e. a device, which gets energy from the D.C. source and converts it into an a.c. energy at the same frequency as that of the input signal. The D.C. to A.C.. conversion is controlled by the input signal. It means that if there is no input signal then no energy conversion take place. Thus there is no output signal. An oscillator is a device, which produces an output signal, without any input signal of any desired frequency. It keeps producing an output signal, so long as the D.C. power is supplied. An oscillator does not require any external signal to start or maintain energy conversion process.