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Old 11th June 2011, 05:03 PM   #1881
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Old 11th June 2011, 08:24 PM   #1882
davada is offline davada  Canada
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Originally Posted by janneman View Post
Andrew and David,

Feedback is using a sample of the (an) output, sending it back to the (an) input to make the output a desired value.
A servo has to be negative feedback, otherwise the offset would be increased by the servo.
Whether you use opamps, chopper-stabilized, tubes or a fast operator with a potmeter, is immaterial. A servo is feedback and negative.
Does NOT need to be global. I have designed servo loops that are strictly local to the output stage. Works like a charm.

Edit: David I think I read your post wrong. I thought you meant a chopper-stabilized amp as a servo; you probably mean the process of chopper-stabilizing an amp. But, you bet it is (negative) feedback. With all the same stability issues a continuous servo has. Although, come to think of it, you probably can also use feedforward.

jan didden
Hi Jan,

Yes the edit is correct.

There seems to be an ongoing debate on this forum that pops up from time to time whether degeneration is negative feedback or not.

I don't really see how the two compare since they operate differently. Degeneration is a process of making, for a BJT, the effect of the intrinsic non-linear Re less significant by placing a linear resistance in series with it. If this is done with a 10:1 ratio of resistor to Re then isn't the effect of Re on the gain of an amplifier reduced to near one tenth?

This is very different than what negative feed back does so where does negative feedback come into degeneration?

Additionally, negative feed back has no effect on IPS gm or reduction of BJT beta sensitivity.

There is a large distinction between the two linearizing techniques.

David.
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Old 11th June 2011, 08:34 PM   #1883
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Originally Posted by davada View Post
Hi Jan,

Yes the edit is correct.

There seems to be an ongoing debate on this forum that pops up from time to time whether degeneration is negative feedback or not.

I don't really see how the two compare since they operate differently. Degeneration is a process of making, for a BJT, the effect of the intrinsic non-linear Re less significant by placing a linear resistance in series with it. If this is done with a 10:1 ratio of resistor to Re then isn't the effect of Re on the gain of an amplifier reduced to near one tenth?

This is very different than what negative feed back does so where does negative feedback come into degeneration?

Additionally, negative feed back has no effect on IPS gm or reduction of BJT beta sensitivity.

There is a large distinction between the two linearizing techniques.

David.
If you have followed the last few pages, you can see that some members regard emitter degeneration as local negative feedback, some do not.
You CAN make a case for it. The emitter resistor makes the output voltage 'work against' the input voltage: if you have 1V input, the emitter will be at 0.999 volt so the effective input voltage is reduced to 0.001V. Exactly the same mechanism as any other negative feedback configuration. That's why the max gain can never be more than one, btw. That is how negative feedback works: use (part of) the output voltage to 'work against' the input voltage. Same diference

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Old 11th June 2011, 08:49 PM   #1884
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Originally Posted by janneman View Post
If you have followed the last few pages, you can see that some members regard emitter degeneration as local negative feedback, some do not.
You CAN make a case for it. The emitter resistor makes the output voltage 'work against' the input voltage: if you have 1V input, the emitter will be at 0.999 volt so the effective input voltage is reduced to 0.001V. Exactly the same mechanism as any other negative feedback configuration. That's why the max gain can never be more than one, btw. That is how negative feedback works: use (part of) the output voltage to 'work against' the input voltage. Same diference

jan didden
Hi Jan
Are you trying to say that local feedback like emitter degeneration should be looked at in the same manner as global feedback?
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Old 11th June 2011, 09:06 PM   #1885
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Originally Posted by stinius View Post
Hi Jan
Are you trying to say that local feedback like emitter degeneration should be looked at in the same manner as global feedback?
I think Bob has demonstrated that effects on harmonics contents are not very different between local and global negative feedback.
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Old 11th June 2011, 09:12 PM   #1886
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Originally Posted by stinius View Post
Hi Jan
Are you trying to say that local feedback like emitter degeneration should be looked at in the same manner as global feedback?
Let me say this. If you look at the effects of global feedback and local (emitter degeneration) feedback, they look a lot alike:

- lower Zout (debatable for emitter degeneration)
- flatter freq response
- less distortion
- lower gain
- more stable operating point
- maybe more that I forget.

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Old 11th June 2011, 09:23 PM   #1887
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Quote:
Originally Posted by janneman View Post
Let me say this. If you look at the effects of global feedback and local (emitter degeneration) feedback, they look a lot alike:

- lower Zout (debatable for emitter degeneration)
- flatter freq response
- less distortion
- lower gain
- more stable operating point
- maybe more that I forget.

jan didden
Increase Zinput.
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Old 11th June 2011, 09:39 PM   #1888
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Originally Posted by wahab View Post
Increase Zinput.
Only with shunt feedback I think, with series fb Zin decreases, no?

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Old 11th June 2011, 11:27 PM   #1889
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- lower Zout (debatable for emitter degeneration)
I think that's the one to drop. What example do we have of degeneration
that has the other characteristics and also lowers Zout?

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Old 12th June 2011, 12:39 AM   #1890
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Cdom (miller capacitance) is both a local FB loop and lowers Z.

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