Difference between amplifier and oscillator : We know that amplifier is a device which amplifies the input signal . Means output voltage has maximum power level than input signal . This additional power is supplied by external D.C. source .On other hand , oscillator is a device which produces an output signal without any input signal .It keeps on producing output signal as long as D.C. power is supplied .
LC oscillator : LC oscillator is also known as tank oscillator , resonant circuit . As it can produce oscillations from 1 MHz to 500 MHz , hence they are also known as R.F. oscillators.With an amplifier and an LC circuit , we can feedback a signal with right amplitude and phase to maintain oscillations .
Depending on the type of feedback used , LC has following five types :
1. Tuned collector / drain oscillator or Armstrong oscillator
2. Tuned base oscillator .
3. Harley Oscillator .
4. Colpitts Oscillator .
5. Clap Oscillator .
Here , we will discuss more on FET colpitts oscillator.
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| FET Colpitt's Oscillator |
Working : When the circuit is energized by switching on the power supply , the capacitor C1 and C2 are charged . When these capacitors are fully charged , it starts discharging through inductor L producing magnetic field around the inductor . When capacitors are fully discharged , the magnetic filed collapses and charges capacitor in reverse direction . Then again capacitor discharges through reverse direction , this process continue to produce oscillations of frequency ,
fo = 1 / 2.𝜋√L.C
where C = C1.C2 / ( C1+C2)
Oscillations at C2 are fed back to gate terminal of FET . Since gate has high input resistance , therefore loading effect on the tank circuit is low than that of BJT transistor oscillator .