Ian Iveson wrote:
> 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.
No, that is series derived as you are connected in series with the
output and also called current derived because you are sensing current.
I agree it can be confusing.
> Another example is the use of an unbypassed cathode
> resistor, which is shunt derived, series applied.
>
Correct, shunt derived because we connect the feedback network directly
across the output (cathode resistor) and series applied because derived
voltage is applied in series with the input.
> 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 proportion 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.
>
Yes, I am still not sure about it. Needs further thought. At least it
seems my maths is OK.
Cheers
Ian
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