COLPITT'S OSCILLATOR






  •     The figure shows the circuit diagram of colpitt’s oscillator.
  •      The circuit is same as that of Hartley oscillator except that the emitter tap is connected between the capacitance’s C1 and C2.
  •     The resistors R and Re provide the necessary bias conditions for the circuit.
  •     The parallel combination of Re and C in the emitter circuit is the stabilizing circuit.
  •     The function of Ce and Cb is to block d.c. and to provide an a.c. path.
  •      The radio frequency choke offers very high impedance to high frequency currents.
  •     Thus it prevents radio frequency currents from reaching source of collector supply voltage and prevents this source from short-circuiting the alternating output voltage.
  •     The frequency determining network is a parallel resonant circuit consisting f capacitors C and C and the inductor L.
  •      The junction of C and C is grounded. e voltage developed across provides the regenerative feedback required for the stained oscillations.

CIRCUIT OPERATION:

  •     When the collector supply voltage is switched on, a transient current is produced in the tank circuit.
  •      So damped harmonic oscillations are produced in the circuit.
  •     The oscillations across C are applied to the base emitter junction and ax in the amplified form in the collector circuit and supply losses to the tank circuit.
  •     If terminal 1 is at positive potential with respect to terminal 3 at any instant, terminal 2 will be at negative potential with respect to 3 at that instant because terminal  3 is grounded.
  •      Therefore points 1 and 2 are 1800 out of phase.
  •     A further shift of 180° is produced by the transistor.
  •     In this way, feedback is properly phased to produce continuous undamped oscillations.
  •      In other words, energy is supplied to the tank circuit in phase with the oscillations and if βA is greater than one, oscillations are sustained in the circuit.

Equation (3) gives the condition for maintenance of oscillations.

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