Ian Thompson-Bell wrote
>>>>> I have just been working through the math for shunt
>>>>> derived shunt applied NFB around an amp and Ican't get
>>>>> the expected result.
>>>> Is that the same as voltage derived, current applied? I
>>>> can never remember this stuff, sigh.
>>>>
>>> No, it's voltage derived, voltage applied. As a rule:
>>>
>>> Shunt = Voltage
>>> Series = Current
>>
>>
>> Thanks. Then how come your circuit employs "shunt
>> derived" feedback?
>>
>
> Sorry, don't understand - the feedback network is fed from
> the output voltage i.e. across or shunting the output. Is
> that what you mean?
>
>> And perhaps you could answer the question you cut out
>> from my post, considering I was good enough to answer
>> yours:
>>
>
> My apologies, I must have missed it.
>
>> "Your maths is correct or we're both wrong.
>>
>> "I don't know if it helps to see it as Ao/(1+ß.(1+Ao))
>>
>
> Yes, I had it as that to start with but changed it to
> 'show' the anomoly.
>
>> "You haven't explained why you believe there is an
>> anomaly.
>> What led you to your erroneous expectation?"
>>
>
> I had expected *all* NFB forms (however derived or
> applied) to reduce to Ao/(1+ß.Ao) but this one does not.
I find the four possible combinations of shunt/series
feedback can be difficult to interpret, so it may be just
me.
For shunt derived feedback I would expect to see a load and
means of sensing the current through that load. An example
would be the use of a small current-sensing resistor on the
ground side of a loudspeaker to derive the feedback signal.
Another example is the use of an unbypassed cathode
resistor, which is shunt derived, series applied.
If the load is purely resistive, I can't see the difference
between the two methods of derivation, because the current
will always be in fixed pro****tion to the voltage.
Where the only load is the feedback path itself, which in
your case is purely resistive, I can't see the difference
either. The most obvious way to me of seeing your circuit,
however, is that the feedback signal is derived from a
voltage divider between output and input. It seems to me
that is where the mysterious "1+" comes from...the fact that
the feedback is not derived from a voltage divider between
the output and ground...there's an extra Vin with respect to
that.
Ian
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