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Old 28th October 2013, 11:28 AM   #71
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I guess I don't understand well the gain reduction, if I use Rf 470k & Rin 47k I will get a gain reduction of -20dBs? If without NFB I have aprox. 35dBs -20dBs using example I will get 15dBs?
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Old 28th October 2013, 08:25 PM   #72
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Quote:
Originally Posted by merlin el mago View Post
I guess I don't understand well the gain reduction, if I use Rf 470k & Rin 47k I will get a gain reduction of -20dBs? If without NFB I have aprox. 35dBs -20dBs using example I will get 15dBs?
The gain with NFB is called the 'closed loop gain'

The gain with NO feedback is called the 'open loop gain'

The amount of feedback is open loop gain (expressed in dB) minus the closed loop gain (expressed in dB).

For this particular amplifer, the closed loop gain is very approximately Rf/Rin.

For more details of NFB in tube circuits check this out:

http://www.ianbell.ukfsn.org/data/nfb101.pdf

Cheers

Ian
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Old 29th October 2013, 12:27 PM   #73
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Thank you.
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Old 30th October 2013, 10:01 AM   #74
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So increasing Rf I increase NFB and the preamp will have less gain?
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Old 30th October 2013, 10:24 AM   #75
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No, opposite. Have you read Ian's note on feedback?
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Old 30th October 2013, 10:25 AM   #76
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My English level is very poor, thanks for help.
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Old 30th October 2013, 10:34 AM   #77
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So if I use 110K as Rf:

110/47 = 2.13 ratio
2.13 = 6.57 dBs

Amount of feedback 35dBs - 6.57dBs = 28.43dBs

35dBs (without feedback) -28.43 (amount of feedback) = 6,57dBs new gain

Last edited by merlin el mago; 30th October 2013 at 10:57 AM.
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Old 30th October 2013, 11:18 AM   #78
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So if I use 110K as Rf:

110/47 = 2.13 ratio
2.13 = 6.57 dBs

35dBs - 6.57dBs = 28.43dBs

35dBs (without feedback) -28.43 (amount of feedback) = 6,57dBs new gain
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Old 5th November 2013, 08:49 AM   #79
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Quote:
Originally Posted by merlin el mago View Post
So if I use 110K as Rf:

110/47 = 2.13 ratio
2.13 = 6.57 dBs

35dBs - 6.57dBs = 28.43dBs

35dBs (without feedback) -28.43 (amount of feedback) = 6,57dBs new gain
Correct.

Cheers

Ian
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Old 5th November 2013, 09:08 AM   #80
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Many thanks Ian.
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