Oscillations and Regenerative Amplification using Negative Resistance


Posted on Feb 6, 2014

The usual procedure for the production of sustained oscillations in tuned L-C networks is to overcome circuit losses through the use of designed-in positive feedback or regeneration. Then, during the circuits on-transient oscillations will build up triggered by the thermal noise of circuit elements. Two possible situations may arise for a potentia


Oscillations and Regenerative Amplification using Negative Resistance
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l oscillator. The tuned active network may have excess loss, in which case the circuit will refuse to oscillate (Fig. 1-a). Nevertheless, it may still work as a regenerative or high-gain narrow-band tuned amplifier. The second situation that may arise is that of a successful oscillator. In this case, excess loss will have been compensated for (Fig. 1-b) and a sustained oscillation is obtained at the circuits output. In the analog realm, two sine-wave oscillator types are distinguished: the feedback and the negative-resistance oscillator. Regarding cancellation of resistive losses, both types may be shown to be equivalent, losses being effectively cancelled out by the negative resistance contributed by the active device and associated reactive components. In Fig. 2-a the negative resistance is series connected with the L-C tuned network. Rs represents the tuned circuits series loss. The net resistive loss is RT = Rs - r, and clearly, it may be nulled out. and it also may be nulled out. In this particular case, the net parallel resistive loss would be infinity, meaning zero power losses in the tuned L-C circuit. If loss cancellation is incomplete, the loop-gain of a sine-wave oscillator will be less than unity and oscillations will not start building up. On the other hand, if the gain is close to unity the circuit will behave as a regenerative or high-gain narrow-band tuned amplifier. There are a number of single and compound...




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