Non-regenerative feedback to an Oscillator

[santom]

hi

Can anyone tell me,what would happen if I try to maintain a non-regenerative feedback for an oscillator...



Cheers
Charles

 

[raghuram_msc]

Hi Friend,

what so you mean by "maintain" non regenerative feedback? I mean cn yu explain a little more please?

Sai

 

[santom]

Yups..I will


Normally oscillator by principle should be given a regenerative feedback.

But I just want to know what it would be its behaviour,if it is given a non-regenerative feedback.

Cheers
santom

 

[A.Anand Srinivasan]

non regenerative feedback means that the feedback happens such that it would act such as to change the output to something new rather than the old value... the outcome depends on the system... it may saturate or goto zero....

 

[santom]

Hi ,

Thanks for the reply.So,we can design oscillator with non regenerative feedback a lso?

Is that depends on the application..Correct me if I am wrong..

 

[A.Anand Srinivasan]

it can be done provided you can give that kind of feedback without the output reaching a steady state....

 

[raghuram_msc]

Hi Friend,

Feedback is the basic difference between an amplifier and an oscillator. An amplifier employs what is called "-ve" or Degenerative feedback, where as an Oscillator employs "+ve" or Regerative feed back.

The reason is: - An amplifier is expected to "Amplify" the given input signal without "Consuming Off" the given signal, I mean it should use only part of it (and therefore -ve Feedback), whereas an Oscillator has to "Reinforce" the oscillations generated by the "Tank Circuit" and also sustain those oscillations (therefore +ve Feedback).

I thing one cannot "oscillate" an Oscillator with degerative (-ve) Feedback.
Some one correct me if Iam wrong.

Sai

 

[lladnar23]

I think the question is: can we apply a signal to a resonant tank and get a steady-state oscillatory response at the output. (With the signal not being derived from the output of the circuit in question.) The answer, of course, is yes. All you need to do is provide an input signal of the correct frequency, which will produce an output signal of the same frequency at a reduced amplitude due to the finite Q of the tank.
This is equivalent to a bandpass filter.

 

[FvM]

Hello,

I also see an oscillator strongly connected to positive or "regenerative" feedback. Proceeding from oszillators, that have an amplifier as recognizable building block, e. g. a typical crystal oszillator, you can define a simple oscillator condition: The loop gain for the oscillation frequency must be +1. Different formula signs are usual for the parameters, e. g. A•β = 1. A is the amplifier gain, β is the feedback in this formula, the relation applies to the vectorial quantities.

It's not said, if the resonant element is contained in the A or the β term. Furthermore if A has negative sign, β would be negative too. You could call this an oscillator with negative feedback, but not in the usual meaning. Steady state is provided for this consideration. Otherwise, oscillation amplitude would either decrease and quickly die down or increase and reach steady state at the amplifier's apmplitude limits.

Non-steady state phenomena, e. g. cyclic oszillations can't be described by simple linear circuit theory, therefore the term feedback looses it's unequivocal meaning in this connection. You can find theoretical descriptions of such phenomena in nonlinear control theory as well as in chaos theory.

Regards,
Frank

 

[santom]

Hi Friend,

Feedback is the basic difference between an amplifier and an oscillator. An amplifier employs what is called "-ve" or Degenerative feedback, where as an Oscillator employs "+ve" or Regerative feed back.

The reason is: - An amplifier is expected to "Amplify" the given input signal without "Consuming Off" the given signal, I mean it should use only part of it (and therefore -ve Feedback), whereas an Oscillator has to "Reinforce" the oscillations generated by the "Tank Circuit" and also sustain those oscillations (therefore +ve Feedback).

I thing one cannot "oscillate" an Oscillator with degerative (-ve) Feedback.
Some one correct me if Iam wrong.

Sai

HI friend,
Thank u very much for the reply.By the way what exactly u meant by the term "it should use only part of it (and therefore -ve Feedback)".I am not able to understand it.Can u help me out in it..

Cheers
santom

Added after 2 minutes:

Hello,

I also see an oscillator strongly connected to positive or "regenerative" feedback. Proceeding from oszillators, that have an amplifier as recognizable building block, e. g. a typical crystal oszillator, you can define a simple oscillator condition: The loop gain for the oscillation frequency must be +1. Different formula signs are usual for the parameters, e. g. A•β = 1. A is the amplifier gain, β is the feedback in this formula, the relation applies to the vectorial quantities.

It's not said, if the resonant element is contained in the A or the β term. Furthermore if A has negative sign, β would be negative too. You could call this an oscillator with negative feedback, but not in the usual meaning. Steady state is provided for this consideration. Otherwise, oscillation amplitude would either decrease and quickly die down or increase and reach steady state at the amplifier's apmplitude limits.

Non-steady state phenomena, e. g. cyclic oszillations can't be described by simple linear circuit theory, therefore the term feedback looses it's unequivocal meaning in this connection. You can find theoretical descriptions of such phenomena in nonlinear control theory as well as in chaos theory.

Regards,
Frank

HI
Thanks for the reply.I am not able to understand ur phrase " You could call this an oscillator with negative feedback, but not in the usual meaning.".It will really be helpful,if u help me to know abt this further.

Thanks
Cheers
santom

Added after 2 minutes:

it can be done provided you can give that kind of feedback without the output reaching a steady state....

Thank u very much Anand as usual for ur proper reply .By the way,some of our friends have given some more detailed explanations on it and I was not able to understand some of their phrases.I asked them sincerely to clarify about it.Hope that will be useful for you too to know about it if u want to..

Cheers
Santom

 

[FvM]

Hello,

regarding the A*β=1 oscillation condition. Considering an amplifier, that has negative feedback to stabilize closed loop gain, as an example β = -0.1, A » 1. It could be, that at a certain frequency, A sign becomes negative due to a > 90 degree phase shift. With sufficiant magnitude of A at this frequency, the circuit could perform as an (almost unwanted) oscillator, although the external defined feedback is negative, actually a phenomenon well-known to circuit designers.

But I wouldn't describe it as negative feedback oscillator, cause the changing sign of A has effectively reversed feedback operation to positive.

Regards,
Frank

 

[raghuram_msc]

Dear Friends,

Firstly, I would say...

By the way what exactly u meant by the term "it should use only part of it (and therefore -ve Feedback)".I am not able to understand it.Can u help me out in it..

Is mean it (the Amplifier) should use only a small part of the Energy of the signal it needs to amplify. (If the Amplifier doesen't have its' own (dc) biasing it will end-up "consuming" the very signal that it had to amplify...!!).

Coming to Fvm's part:

It's not said, if the resonant element is contained in the A or the ß term. Furthermore if A has negative sign, ß would be negative too. You could call this an oscillator with negative feedback, but not in the usual meaning. Steady state is provided for this consideration. Otherwise, oscillation amplitude would either decrease and quickly die down or increase and reach steady state at the amplifier's apmplitude limits.

Dear Fvm,

I guess the Brakhausan's criteria cannot be viewed in this light. Aß=1 would mean that the Gain and Feedback ratio should always be one. I am not sure if we can apply the sign arguments (your earlier post) here. Moreso because Barkhausan's criteria is necessary and sufficeint and therefore, in my opinion, once criteria is met itself means steady state is achieved.

Please offer your arguments on this.

Sai

 

[subharpe]

No, Brakhausan's criteria doesn't ensure stabilityof the system. To justify simply, consider where A of the system (open loop gain) is not ∞ but Aβ = 1 with the phase relation required for oscillation. In that case the overall gain of the system is ∞ which means even if there is no signal input there will be output. But with every cycle of feedback the gain of the system increases and hence output reaches ∞ or saturation in the steady state. Therefore to ensure stability there needs to be negative feedback which will reduce the closed loop gain to finite value and hence stable oscilation takes place. In fact there also the system is marginally stable. I will give the example of Wien Bridge oscillator. The RC bridge is the path which makes the system oscillatory, but the output will be saturated. For stable oscillation negative feedback of proper value is given to the system. Brakhausan's criteria is only a necessar condition for oscillation, but reduction of the gain through the negative path is the also necessary condition.