
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

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 
14th August 2005, 04:18 PM  #1 
diyAudio Member
Join Date: Nov 2002
Location: sg

a question about local feedback
i was pondering bout my next amp project and it hit me that i didn`t have an idea what local feedback did to the output impedance of the stage. and if implemented in an output stage, what kind change in output impedance would i see...? is it very much like global feedback?
thanks! 
14th August 2005, 04:48 PM  #2 
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ

Local or global, voltage NFB is voltage NFB. It raises I/P impedance and lowers O/P impedance. The math stays the same.
__________________
Eli D. 
14th August 2005, 04:53 PM  #3 
diyAudio Member
Join Date: Nov 2002
Location: sg

with 3db of feedback, how much change in the o/p Z am i expecting to see change?
thanks! 
14th August 2005, 10:50 PM  #4 
diyAudio Member

3 decibels (power) = x 2 = 6 decibelvolts.
dBV are double (duodecibels?) because doubling voltage doubles current into a resistance, so power actually goes up by a factor of four (i.e., +6dB). At any rate, linear negative feedback reduces Zo, distortion, gain and, if connected properly, increase Zin all by the same factor. This factor is the amount of feedback. Tim
__________________
Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation 
15th August 2005, 10:37 AM  #5  
diyAudio Member
Join Date: Jan 2003
Location: Sweden

Quote:
dB for voltage = 20 * LOG(V2/V1), so if V2/V1 is 2, (double voltage) it gives 6dB dB for Power = 10 * LOG(P2/P1), so if V2/V1 is 2, (double power) it gives 3dB However 3dB for voltage and 3dB for power is the same thing V2/V1 = 1.4142 gives 3dB and an increase of 1.4142 in voltage gives an increase of 2 times in power, (P ~ V^2) and 2 times in power is also 3dB. Back to the original question, 3dB feedback is 1.4142 times in voltage so the decrease in output impedance will be 1.4142 times, (I assume that what is meant with amount feedback here is how much the gain decreases) Regards Hans 

15th August 2005, 01:19 PM  #6 
diyAudio Member
Join Date: Nov 2002
Location: sg

thanks a million, now i`ve got an idea how my amp will turn out.

16th August 2005, 02:13 AM  #7  
diyAudio Member

Quote:
By numbers alone, 3dB (power) = factor of two = 6dBV. I noted lower that, into a resistance, 3dB = 6dBV. Tim
__________________
Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation 

16th August 2005, 06:31 AM  #8  
diyAudio Member
Join Date: Jan 2003
Location: Sweden

Quote:
"dB for voltage = 20 * LOG(V2/V1), so if V2/V1 is 2, (double voltage) it gives 6dB dB for Power = 10 * LOG(P2/P1), so if V2/V1 is 2, (double power) it gives 3dB" This is the definition of dB. What it means is that 3dB is 2 times for Power but only SQRT(2) for voltage. An example: an amplifier with 40dB gain has power gain of 1000 times but a voltage gain of only 100 times, (assuming same input and output impedance) In this case, (what Hacknet asked about) we had a feedback of 3dB which means that the power AND voltage gain decrease by 3dB i.e. SQRT(2) ~= 1.4142 times but a power gain decrease of 2 times. Regards Hans 

16th August 2005, 06:42 AM  #9 
diyAudio Moderator Emeritus
Join Date: Jan 2004
Location: Jakarta

This thread, which I strarted last January, discusses the effectiveness of partial/local feedback for improving damping factor. There were many helpful contributions from people with a lot of experience.

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Cascode with local feedback  croccodillo  Tubes / Valves  8  19th July 2007 03:30 PM 
How to accomplish local feedback in pp amp ?  Jaap  Tubes / Valves  11  7th December 2006 05:04 PM 
Long Tail and Local feedback  croccodillo  Tubes / Valves  19  26th October 2005 11:39 AM 
'Local Feedback and Nonlinearity of Multistage Feedback Amplifiers'  mikeks  Solid State  62  20th July 2005 08:03 AM 
New To Site?  Need Help? 