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Old 11th July 2013, 09:25 AM   #61
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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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Old 11th July 2013, 10:17 AM   #62
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Originally Posted by merlin el mago View Post
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
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Old 11th July 2013, 12:35 PM   #63
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db = 20 X Log Av
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Old 11th July 2013, 12:51 PM   #64
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Quote:
Originally Posted by ruffrecords View Post
x10 = 20dB
x21.28 = 26.6dB

Cheers

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db = 20 X Log Av
Thanks guys to teacher me
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Old 11th July 2013, 12:52 PM   #65
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How can calculate the new output impedance?
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Old 11th July 2013, 04:47 PM   #66
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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.

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Old 11th July 2013, 05:11 PM   #67
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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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Old 11th July 2013, 05:54 PM   #68
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Quote:
Originally Posted by merlin el mago View Post
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

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Old 11th July 2013, 08:42 PM   #69
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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.

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Old 12th July 2013, 03:39 PM   #70
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Thank you Ian
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