feedback factor beta

[nodame123]

Why can't beta(feedback factor) be greater than 1 (in negative feedback)? I have read it somewhere and understood long back.Something like what's the use of amplifying or feeding more than the output,and the input to the integrator can't be -ve or something. I have forgotten why exactly,can someone help fill the gap?

 

[LvW]

The feedback factor is larger than unity if it is not a PASSIVE but an ACTIVE feedback path (with another amplifying unit).
Example: Phase-lead integrator. In this circuit the feedback path consists of an inverting opamp amplifier in series with the integrating capacitor - fed back to the non-inv. opamp termnal.
Advantages: The parasitic phase shift of the feedback opamp can - partly - compensate/neutralize the parasitic phase shift of the main opamp.

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I forgot to mention: Of course, this is a non-inverting integrator. (Using the same principle we can create a phase-compensated non-inverting gain stage).

 

[nodame123]

okay. My question was about why do we mostly see passive feedback? Why can't we replace the passive fb with active,for example? Instead of feeding 20% of output why not 200% ? Is there any technical difficulty if there is an integrator in the forward path,as in the simple case.Can you answer it in that perspective?

 

[LvW]

okay. My question was about why do we mostly see passive feedback? Why can't we replace the passive fb with active,for example? Instead of feeding 20% of output why not 200% ?

Of course, we can. But why should we? Do YOU see any advantage for feeding back 200% ?

 

[nodame123]

No,I don't.The proper explanation for that is what I am looking for.
Different perceptions is what I need.

 

[LvW]

No,I don't.The proper explanation for that is what I am looking for.
Different perceptions is what I need.

Do you know the closed-loop gain value of an amplifier with a feedback factor larger than unity?

 

[nodame123]

No,I haven't studied such a circuit yet.