
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
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 
diyAudio Member
Join Date: Feb 2003
Location: ..

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 
diyAudio Member
Join Date: Apr 2002
Location: Prague

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
__________________
Whether we like to think of it this way or not, an audio engineer shares the professional goal of a magician  Richard Heyser Linear Audio Vol 12 is out! Check out my Autoranger and SilentSwitcher on Kickstarter! 
23rd August 2010, 05:39 PM  #5 
diyAudio Member
Join Date: Dec 2003
Location: Grand Rapids

thank you janneman  very succinct and it makes perfect sense

23rd August 2010, 06:12 PM  #6  
diyAudio Member
Join Date: Dec 2004
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
__________________
Whether we like to think of it this way or not, an audio engineer shares the professional goal of a magician  Richard Heyser Linear Audio Vol 12 is out! Check out my Autoranger and SilentSwitcher on Kickstarter! 

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

Really? It decreases output impedance at the plate/collector?
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." 
23rd August 2010, 07:40 PM  #10 
diyAudio Member
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.

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Zero Feedback Impedance Amplifiers  Susan_Parker  Solid State  1610  24th September 2016 04:19 PM 
New To Site?  Need Help? 