- In the general form of a oscillator, any of the active devices such as Vacuum tube, transistor, FET, Operational amplifier may be used in the amplifier section.
- Z1, and Z2 are reactive elements constituting the feedback tank circuit which determines the frequency of oscillations.
also provides temperature stabilization.
- The radio frequency choke (R.F.C) offers very high impedance to high frequency currents i.e., acts like a d.c short and open.
- Thus it provide d.c load for collector and keeps a.c. currents out of d.c. s source.
- The function of Cc and Cb to block d.c. and to provide an a.c. path.
- Frequency determining network is a parallel resonant circuit consisting of inductors L1 and L2 and a variable capacitor C, the junction of L1 and L2 is earthed.
- One side of L is connected to base via C and the other to emitter via C So, L the input circuit.
- Similarly one end of L is connected to collector via C and other end id connected to emitter via Ce.
- So, L is in the output circuit.
- The two! are inductively coupled and form an auto ttansformer.
WORKING OF THE CIRCUIT:
- When the collector supply voltage is switched on, a transient current produced in the tank circuit.
- The oscillatory current in the tank circuit produces voltage across L in this way a feedback between output and input circuit accomplished through auto transformer action.
- So there is a phase reversal of 1800 between output and input.
- The common-emitter amplifier also produces a further 180° phase shift between input and output voltages.
- Thus total phase shift becomes 360°.
- This makes the feedback positive which is the essential condition for oscillations.
- When the loop gain | β | of the amplifier is greater than one, oscillations are sustained in the circuit.
FREQUENCY OF OSCILLATIONS:
The general equation for the oscillator is given by,