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23rd August 2010, 12:21 PM  #1 
diyAudio Member
Join Date: Dec 2003
Location: Grand Rapids

Feedback and output impedance
I understand how loop/global negative feedback reduces distortion, but how exactly does it reduce the output impedance? I've searched around here and on google without a definitive answer  just some general hand waving about the reduction of distortion, increase in bandwidth and reduction in output impedance.

23rd August 2010, 01:07 PM  #2 
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Join Date: Feb 2003
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if you really know how output distortion is reduced by negative feedback then you already know how output impedance is reduced  feedback isn't "smart" enough to know if it is correcting a nonlinear distortion or a linear voltage drop from load current flowing in device impedance  both result in a measured error at the summing input and the error is amplified by the excess loop gain (feedback factor)
Blackmans Theorem is a keystone in feedback theory  look for it in your textbook index to determine if the book is useful in really explaining feedback Last edited by jcx; 23rd August 2010 at 01:16 PM. 
23rd August 2010, 01:13 PM  #3 
Design engineer, consultant
diyAudio Member

Very high feedback = necessity to use output coil = higher output impedance with frequency
Quite funny. Take a look at John Curl JC1. This power amp does really have LOW OUTPUT IMPEDANCE. 
23rd August 2010, 03:04 PM  #4 
diyAudio Member

The amp Zout is not a single physical impedance, but rather the effect of a sagging output level with increasing load. If, say, with a load increase of 5 amps, the output level drops 1V, we say (with a wink to Mr. Ohm) that Zout = 1/5 ohms = 200milli ohms.
If you use neg feedback, the drop of the output level is 'counteracted' by the feedback (or if you prefer, compensated). So now you only have a level drop of say 100mV with a 5A output load increase. Now Zout = 0.1/5 = 20 milliohms. BTW That's also an easy way to measure Zout. Measure the output level drop for a given output load increase, and divide. Be aware that it is frequency dependent and possibly level dependent. jd
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23rd August 2010, 05:39 PM  #5 
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thank you janneman  very succinct and it makes perfect sense

23rd August 2010, 06:12 PM  #6  
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Location: Next door

Quote:
All the effects you describe are only consequences of this stabilisation of the gain, which is set by two resistors. 

23rd August 2010, 07:17 PM  #7 
diyAudio Member
Join Date: May 2007

Negative feedback only lowers output impedance if it senses the output voltage, and tries to make this a defined multiple of the input voltage. You can arrange negative feedback to sense the output current instead, and then it raises output impedance. This is how an unbypassed emitter/source/cathode resistor works. You can even have a combination of the two extremes, and use feedback to set a particular impedance.
This may be why you could not find a simple explanation of why NFB reduces output impedance. Sometimes it doesn't! 
23rd August 2010, 07:20 PM  #8  
diyAudio Member

Quote:
jd
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An engineer designing a feedback amp is a mixed emotions creature  Hendrik Bode Check out Linear Audio! 

23rd August 2010, 07:26 PM  #9 
diyAudio Moderator

Really? It decreases output impedance at the plate/collector?
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23rd August 2010, 07:40 PM  #10 
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Join Date: May 2007

I suspect janneman may be confusing emitter unbypassing with emitter following. It all depends on where you take your output from.

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