If there are two negative feedback in one circuit

[twonsr]

Hi:

If there are two negative feedback in one circuit, and the two feedback are not independent, how should we decide which one will survive ?

compare the loop gain AB or just compare the feedback factor B ?

Thank you.

 

[Borber]

What do you mean by survive?
Two different feedback usually means two feedback loops and the result is caused by action of both loops together.

 

[flatulent]

It is common to have loops inside loops. One example is a several stage amplifier. Each stage can have its own feedback, such as an emitter or source resistor, and then there is an overall loop from the output to the input of the whole works.

There are cases of more sophisticated systems where an inner loop has positive feedback and the outer negative. The whole system is stable. The inner positive loop increases some parameter of its area such as bandwidth.

 

[twonsr]

Hi :

please see the circuit, there are 2 negative feedback, but just one will work.
How to caculate or analysis to find the work one ?

Thanks

 

[kienhuy]

You can use Mason's Gain Rule to formulate a transfer function for systems with dependent feedback.

 

[willyboy19]

As far as I see from the post of your circuit, this is a pure academic question and does not lead to any pratical meaning. The actual operating point of the circuit will be dependent on many factors, including the two opamp's gain and output impedance level. I would not suggest waste too much time on this kind of pure 'academic' questions.

 

[flatulent]

The first problem is that you have two ideal voltage source outputs connected together. The other is that the two inputs can be set to values that require vastly different currents in the right side transistor-resistor network.

 

[JPR]

I see three possibilities for this scenario:

1- The amplifiers have a current limited pull-up and pull-down with similar current limitations. In this case, they are essentially OTA's (operational transconductance amplifiers), and the output will be defined by the differential current output. In this case, the output will be at a point somewhere in between each individual output (if all is equal, it will be at the mid-point)

2- The amplifiers have a difference between pull up capability and pull down capability (classic 2-stage miller amp, for example). Assuming the output stage has a pull-down current source and a pull-up miller compensated gain stage, the amplifier that is trying to pull up will win, and the result will essentially be a "smaller current" selector (opposite if the current mirror is the pull-up, and will be a "greater current" selector) There is likely to be a range where the two currents are similar where there this will act like #1.

3- The amplifiers have a strong pull up and pull down capability. In this case, the output stages will fight with high current. Part damage is a possibility in this case, and the output will be determined by the part that is not damaged.

Of course, all of this assumes that the amplifiers are the same. If they are different, you may get different results (strongest output will win)

 

[sanwolf]

agree with JPR, my viewpoint is this circuit is not simple feedback circuit. I have a question:since you think it's a feedback circuit, where is the output point.