we sit down and think about it—all we are saying is that the kick furnished by the amplifier must be in phase with the oscillator ouput. Since there is a 180° phase shift through the transistor from base to collector, the tuned feedback circuit must provide another 180° phase shift so that the output signal appears in phase with the input- In summary then? to function as an oscillator, the amplifier and frequency selective network must exhibit a total gain of unity and a phase shift of zero (or 360) degrees,

In the two most popular oscillator crcuits. the Colpitis and Hartley, the frequency selective feedback path consists of three reactances denoted as Xa, Xb and Xc in Fig* 313, In the Colpitis oscillator, Xa is an inductor and Xb and Xc are capacitors. In the Hartley circuit, Xa is a capacitor and Xb and Xc are inductors.

Colpitis oscillator

In the transistor version of tlie popular Colpitis oscillator in Fig, 4 and capacitors Ci and Ca form a resonant tank circuit with the inductance L, A small fraction of the current flowing in the tank circuit is fed back to the base of the transistor through C3. Although the oscillation frequency is determined primarily by t fie tank components Gi, Ca, and L, the transistor input impedance (hi* and output impedance (hob) affect it ci i

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