ECC81 mu-follower problem

[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?

 

[ruffrecords]

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

 

[TheGimp]

db = 20 X Log Av

 

[merlin el mago]

x10 = 20dB
x21.28 = 26.6dB

Cheers

Ian

db = 20 X Log Av

Thanks guys to teacher me🙂

 

[merlin el mago]

How can calculate the new output impedance?

 

[ruffrecords]

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

 

[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?

 

[ruffrecords]

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

 

[ruffrecords]

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

 

[merlin el mago]

Thank you Ian🙂

 

[merlin el mago]

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?

 

[ruffrecords]

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

 

[merlin el mago]

Thank you.

 

[merlin el mago]

So increasing Rf I increase NFB and the preamp will have less gain?

 

[DF96]

No, opposite. Have you read Ian's note on feedback?

 

[merlin el mago]

My English level is very poor, thanks for help.

 

[merlin el mago]

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

 

[merlin el mago]

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

 

[ruffrecords]

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

 

[merlin el mago]

Many thanks Ian.