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Colpitts Oscillator

Colpitts oscillator

Colpitts oscillator

  • The LC tank circuit uses two capacitive reactance and one inductive reactance.
    1. X1=X2=C and X3=L
  • The amplifier stage uses an active as a transistor in common emmitterconfiguration.
  • The R1 and R2 are the biasing resistances. The RFC is the radio freqency chock. Its reactance value is very high for high frequencies, hence it can be treated as open circuit for a.c. While for d.c conditions, the reactance is zero hence causes no problem for d.c capacitors and biasing.
  • Re is also a biasing circuit resistance and CE is the emitter bypass capacitor.Cc1 and Cc2 are the coupling capacitors
  • The common emitter amplifier causes a phase shift of 180*, while the tank circuit adds further 180* phase shift. Thus the total phase shift around a loop is 360* which is necessary to satisfy Barkhausen condition .
  • To satisfy Aβ=>1, it is necessary that hfe of the transistor used in the amplifier stage must be greater than or equal to C2/C1.
  • The frequency of oscillations is desided by C1,C2 amd L

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