# loop gain in multivibrators

Discussion in 'The Projects Forum' started by anchal, May 6, 2012.

1. ### anchal Thread Starter New Member

May 6, 2012
1
0
How does loop gain in stable states of bistable multivibrators is less than unity whereas during unstable states , loop gain is more than unity?

2. ### DickCappels Moderator

Aug 21, 2008
2,651
632
I would guess that a loop gain of 1 or more is required for instability.

3. ### count_volta Active Member

Feb 4, 2009
435
24
I just took a course in control. Perhaps it has to do with the gain margin and phase margin? Each of those margins is a measure of stability. For an oscillator you want marginal stability. The poles should be located on the imaginary axis for sustained oscillation without growth or decay. The phase margin should be exactly zero degrees. The gain margin should be 1 or 0dB. Look up the Nyquist diagram on wikipedia for more information. I am guessing it has something to do with that. Not sure though.

Last edited: May 6, 2012
4. ### crutschow Expert

Mar 14, 2008
13,001
3,229
Typically for a non-linear bistable multivibrator you don't worry about loop gain and phase margin as such. That's generally used for linear feedback systems. You just design the circuit to have enough positive feedback to flip the state at the end of each half-cycle.