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  1. #1
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    when the op unstable , what is its behavior?

    When OP is unstable ,and it connects as closeloop , what is its output and freq , or its relationship ? if the output is not VDD or GND , just like the osc application

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  2. #2
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    Re: when the op unstable , what is its behavior?

    When the opamp with feedback does not function as a stable amplifier - and when at the same time its output is neither at Vdd nor at Vcc (or ground in case of single supply) it probably will oscillate at a frequency fo. This frequency fo can be found in the BODE plot of the loop gain where the magnitude is 0 dB.



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  3. #3
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    Re: when the op unstable , what is its behavior?

    natch, as LvW says, the criteria for oscillation are:

    1. Gain greater than 1 at frequency f0
    2. positive feedback (i.e. 360 degree phase shift from input to output at frequency f0)

    congratulations! You arent stable! I think its called the Barkhausen criteria.

    again, as LvW points out, these can be taken from a properly constructed Bode plot (or more accurately stated, a properly loaded circuit)



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