# loop gain in multivibrators

Discussion in 'General Electronics Chat' started by vigi04, Sep 21, 2009.

1. ### vigi04 Thread Starter New Member

Sep 9, 2009
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i am not understanding the concept of loop gain in a bistable multivibrator. can anyone explain it. how do u visualize loop gain in a bistable multivibrator?

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I don't think there is such a thing. By definition, it is a digital circuit, it's trigger levels are defined by the logic family used to make it.

Another name for bistable multivibrator is a set/reset flip flop.

Here are a couple of examples.

3. ### vigi04 Thread Starter New Member

Sep 9, 2009
12
0
this is based on the working of the circuit.
at some places they say tat loop gain has to be more for the working of a bistable multivibrator, at others, they say that it has to less.
which is the right sentence and why???

4. ### vigi04 Thread Starter New Member

Sep 9, 2009
12
0
i mean the internal circuit which has transistors.

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Show the source. Like I said, I do not believe there is such a thing as loop gain. When talking about digital circuits gain is not a factor. It is a 1, or a 0.

6. ### vigi04 Thread Starter New Member

Sep 9, 2009
12
0
i dont have the ebook, but it is mentioned in 'pulse and digital circuits' by millman and taub under the topic ' bistable multivibrators'

7. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
There are simple logics where a pure 1 and 0 can be a problem, such as diode logic. It works, but there are losses because there are no active components. With IC's this doesn't exist. The main thing for any logic family is to hit a "pure" 1 or 0 for the input, generally this does take some gain, but very little. The one thing a logic family should never do is easily operate in the linear region.

One the basic 1 or 0 requirement is met, it no longer matters, especially with bistable circuits. It sounds like you're text book is approaching it from the perspective of analog gain.

Just for laughs I typed the term loop gain in Wikipedia, here is what I got...

http://en.wikipedia.org/wiki/Loop_gain

There the concept deals with the feedback loops used in amplifiers and other physical processes that use feedback loops.

I can see where such a concept might apply for a multivibrator of any sort. In the world of digital though, it is harder to apply. I'll step aside and see if anyone has better input.

8. ### vigi04 Thread Starter New Member

Sep 9, 2009
12
0
can some one please help me, i'm not able to understand this concept of loop gain in a bistable multivibrator. wen its above 1, wat happens and how?? and the same for below 1.

9. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Wouldn't the loop gain be a determining factor in rise/fall time of a particular device?? True, it would be internal to the device, but none the less it requires gain to perform the switching.