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
x10 = 20dB
x21.28 = 26.6dB
Cheers
Ian
db = 20 X Log Av
Thanks guys to teacher me
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
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
Not quite. Gain without feedback = x58.5 = 35.4dB
Cheers
Ian
And I should have added:
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
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
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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