sometimes I was thinking about feedback circuit, And got some questions about that
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So I am confused about feedback system,how dose it make system stable?Or am I thinking in a wrong way?
I think, your description of the sequence is realistic. The output pulse you have mentioned may, for example, occur as the result of an input signal.
In this case - as described by you - it will take some time until the negative feedback network does react and brings the output back to the linear region.
Examples:
* This can be observed for opamps with (negative) feedback. In case of a sufficient large input step the time needed for the feedback signal to arrive at the input results in a drastically increased rise time at the output known as "slew rate effect".
* For lower step amplitudes the amplifier remains within the linear range but may show a kind of "ringing (overshoot)" at the output. In the frequency domain this effect is quantified by a parameter called "phase margin". In the time domain this effect is caused simply by the delay of the feedback signal.
*
But note: In both cases we do not speak about "instability". If the system returns back to a stable operating point it is a stable system - perhaps with a stability margin that is too small for certain applications (where no overshoot is allowed). Thus, also the case as described by you does not indicate instability in general.
* However, this feedback delay is a major issue in control systems - in particular the delay caused by (perhaps) unknown "dead times" within the closed loop. Indeed, this may lead to instability of the whole system.
LvW
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Supplement:
Regarding your last sentence ("
how does it make system stable") - you must discriminate between static (dc) and dynamic stability.
Negative feedback always
improves static stability (operating point) but - at the same time -
decreases dynamic stability.