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 11th July 2013, 09:25 AM #61 diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe Thanks for help & explanation Ian, I learned the lesson how to calculate the NFB 470/47 = 10? not 20dBs? 1000/47 = 21.28 not 25dBs? Last edited by merlin el mago; 11th July 2013 at 09:28 AM.
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Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
 Originally Posted by merlin el mago Thanks for help & explanation Ian, I learned the lesson how to calculate the NFB 470/47 = 10? not 20dBs? 1000/47 = 21.28 not 25dBs?
x10 = 20dB
x21.28 = 26.6dB

Cheers

Ian

 11th July 2013, 12:35 PM #63 diyAudio Member     Join Date: Aug 2009 Location: Johnson City, TN db = 20 X Log Av
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Join Date: Sep 2009
Location: Catalonia - Europe
Quote:
 Originally Posted by ruffrecords x10 = 20dB x21.28 = 26.6dB Cheers Ian
Quote:
 Originally Posted by TheGimp db = 20 X Log Av
Thanks guys to teacher me

 11th July 2013, 12:52 PM #65 diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe How can calculate the new output impedance?
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Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
 Originally Posted by merlin el mago How can calculate the new output impedance?
The output impedance is reduce by the amount of NFB. So if you have 6dB of NFB the output impedance will be halved. If you have 20dB of NFB the output impedance will be reduced to one tenth.

The amount of NFB equals open loop gain in dB minus closed loop gain in dB.

Cheers

Ian

 11th July 2013, 05:11 PM #67 diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe Gain without NFB 58.8dBs Output impedance 267 ohms Gain with NFB 58.8dBs - 32.2dBs (NFB = 26.6dBs? Output impedance 267 ohms : 10 = 26,7 ohms?
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Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
 Originally Posted by merlin el mago Gain without NFB 58.8dBs Output impedance 267 ohms Gain with NFB 58.8dBs - 32.2dBs (NFB = 26.6dBs? Output impedance 267 ohms : 10 = 26,7 ohms?
Not quite. Gain without feedback = x58.5 = 35.4dB

Cheers

Ian

diyAudio Member

Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
 Originally Posted by ruffrecords Not quite. Gain without feedback = x58.5 = 35.4dB Cheers Ian

Gain with NFB = 1000/47 = x21.3 = 26.6dB

So NFB = 35.4 - 26.6 = 8.8dB = x2.75

If output impedance without NFB = 267 ohms then output impedance with NFB = 267/2.75 = 97.1 ohms.

Cheers

Ian

 12th July 2013, 03:39 PM #70 diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe Thank you Ian

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