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Sinewave OscillatorsAuthor: Leonard Krugman
Fig. 69. (A) Basecontrolled negative resistance oscillator and idealized characteristic. (B) Emittercontrolled negative resistance oscillator and idealized characteristic. (C) Collectorcontrolled negative resistance oscillator and idealized characteristic. Figure 69 (B) represents the basic emitter controlled negative resistance oscillator and its idealized currentvoltage characteristics. Fig. 69 (C) is the basic collector controlled type. The fundamental operation of both is essentially the same. Oscillation occurs at the series resonant frequency of the LC combination because at this point the effective resistance in either the emitter or collector arm is at minimum. The base resistance must be large enough to furnish positive feedback in order to sustain oscillation. The base resistance r_{b} is generally large enough to cause instability when either the emitter or collector is shorted to ground, on the basis of equation 61. In practical circuits, however, r_{b} alone is rarely enough for dependable operation. An external resistor R_{B} equal to at least 2,000 ohms is generally added.


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