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Basic OperationAuthor: Leonard Krugman If the control of an oscillator can be maintained by simple high or low resistance values in the three transistor electrode arms, the substitution of series and parallel LC resonant circuits in their place is a natural step. The insertion of a parallel resonant circuit in the base lead will cause the circuit to oscillate at the resonant frequency because of the tank's high impedance at resonance. On the other hand, placing a series LC circuit in the emitter or collector arms will cause oscillation at the resonance frequency due to the tank's characteristic low impedance at that point. Fig. 66 illustrates the ac equivalent circuit of a negativeresistance oscillator that includes all three methods of controlling oscillation. Since LC resonant circuits produce sine waveforms, the oscillators using LC resonant tanks are generally referred to as sinewave oscillators. Fig. 66. Basic impedance controlled negative resistance oscillator.


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