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Why my VCO amplitude not constant?

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tigonzn

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When I decrease the tail current, the amplitude becomes constant. Very bewildering, what I used is a common negative gm LC oscillator. Only NMOS transistors are used.
Thanks!
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Any oscillator need some time to stabilize its amplitude. Some need more, some need less time, depending by multiple factors, but mainly by the bias current (open-loop gain) and by the feedback.
If you go further in time with your simulation you will see that at some point should be stabilized.
 

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