Re: NFB101 part 3

Patrick Turner wrote:

A cathpode follower is a case of local series voltage NFB.
That is shunt (voltage) derived, series (voltage applied)applied. Again
you need to differentiate between how NFB is derived and applied.



A cathode follwer is NOT shunt NFB.

This is because the output voltage at the cathode is all fed back and is
the SAME
phase as the grid input.


You cannot just say shunt NFB or series NFB, you *must* make the distinction between how the NFB is derived and applied.


In a CF the feedback is taken from directly across the load and is therefore shunt derived. Remember shunt derived feedback lowers output impedance which is why a CF has a low output impedance.

100% of the output voltage is fed bacl back so ß is 1.

The signal input is connected between g and ground, the amp input is between g and k so the feedback is connected to the amp input in series with the signal input in such a way that the feedback subtracts from the input. The feedback is therefore negative and series applied. Remember series applied feedback can increase input impedance which does happens in some CF versions.


All follower circuits are like this even opamp followers.

Agreed, and emitter followers too.


Shunt FB is where the Vo is the opposite phase to the input signal.


Again, you cannot just say shunt feedback, you need to distinguish between how the feedback is derived and applied.

Local series **current** NFB is where you have an unbypassed Rk.


In that case the feedback is series derived because the feedback is sensed by the cathode resistor which is in series with the anode load resistor. The feedback voltage is in the ratio of the cathode to anode resistors because they share the output current.

The feedback is applied in series with the input in the same way as a CF. So and unbypassed Rk has swries derived, series applied NFB.

That is series (current) derived, shunt (voltage) applied.


No it is series (current) derived, series (voltage) applied.

There isn't anything in shunt.


Correct.

Its just a form of series FB, but current derived.


Again, you must distinguish between how the NFB is derived and applied.

Series derived feedback is current derived.
Shunt derived feedback is voltage derived.
Series applied feedback is voltage applied.
Shunt applied feedback is current applied.

RHD4 explains it all, and categorizes the types of NFB and PFB and
gives the effects of each type of FB in a table.


Unfortunately RDH4 does not make the distintion between how NFB is derived and applied which seems to have confused some people.

It does but I don't think it makes a clear distinction between how
feedback is derives and then applied which is a failing of many books of
that era. I think separating and understanding the two is essential to
understanding NFB.

RDH4 does make a range of clear distinctions IMHO.


It does, but it does not distinguish between how NFB is derived and applied, and indeed many modern introductory test do not either.

Its obvious how all the FB circuits work, both positive and negative and either current or voltage if you analyse properly.

That is true.

The figure for gain used in Rout calculations after Global NFB FB is
applied
is the ( open loop gain of input and driver stages of an amp )
all multiplied x ( µ of the output tubes ) all divided by
the turn ratio of the OPT.
The figure used for gain for Rout calculations should be the open loop
gain. I don't know enough about PP tube stages to know if that is the
same as the formula you gave above.
The load affects the output tube open loop gain, so
as load reduces, so does gain.

Got it. Basically we are talking about load vs unloaded open loop gain.

No, we are not talking about load versus gain.

What you need to realise is that an amp's Rout is independant of the
load.


We need to distinguish between the Rout without NFB and then with is. I agree the Rout without feedback is independent of load. But if the load reduces the open loop gain then the effective Rout *with* NFB is not as low as it would be with NFB unloaded.


Cheers

Ian
.

Relevant Pages

  • Re: Explanation still required for triode superiority
    ... He does not call it NFB probably because it would have confused ppl at the time ... The observation of a triode as a mystery 3 terminal device could be ... But where one maintains the grid bias voltage and fixes the cathode voltage, ... One simply cannot deny that a huge anode feedback effect exists, ...
    (rec.audio.tubes)
  • Re: Explanation still required for triode superiority
    ... He does not call it NFB probably because it would have confused ppl at the time ... The observation of a triode as a mystery 3 terminal device could be ... But where one maintains the grid bias voltage and fixes the cathode voltage, ... One simply cannot deny that a huge anode feedback effect exists, ...
    (rec.audio.tubes)
  • Re: NFB101 part 3
    ... That is shunt derived, ... A cathode follwer is NOT shunt NFB. ... In a CF the feedback is taken from directly across the load and is ... feedback is derives and then applied which is a failing of many books of ...
    (rec.audio.tubes)
  • Re: NFB101 part 3
    ... the FB voltage is from a simple resistance divider, ... as the gain reduction. ... The applied NFB is "in series ... It does but I don't think it makes a clear distinction between how feedback is derives and then applied which is a failing of many books of that era. ...
    (rec.audio.tubes)
  • Re: NFB101 part 3
    ... That is shunt derived, ... A cathode follwer is NOT shunt NFB. ... In a CF the feedback is taken from directly across the load and is ... feedback is derives and then applied which is a failing of many books of ...
    (rec.audio.tubes)