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Old November 6th 19, 08:42 PM posted to rec.audio.pro
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On Tue, 5 Nov 2019 15:01:55 -0800 (PST), Phil Allison > wrote:

>
> wrote:
>
>------------------
>
>Bewa this troll is e REAL lunatic
>
>>
>> Please go into this in greater detail. My understanding is that when
>> using amplifiers, especially audio, the noise figure is a function of
>> the application - impedance levels, feeback and other external circuitry,
>> closed loop gain setting. etc. Therefore NF is usually not specified on
>> the data sheet
>>

>** Noise figure is widely used in electronics, predominately in RF circuits but also in low noise audio where connection to a transducer is is the game.
>
>It does appear on many data sheets for BJTs and JFETs.


This is widely known. It is also widely known that it is difficult to specify a NF for an amplifier, or even supply curves for NF, because of all the
different permutations of external circuitry for all the various applications. NF came up because you said you calculated a best source resistor for
NF for the OPA by dividing the input voltage noise density by the input current noise density. I asked you to explain this since these noise densities
are independent and are noise and current SOURCES and are modeled as such. They do not obey E=I x R and do not define a resistance.

And again, you snipped out my request for an explanation, shown below-

1) please explain the physical significance of dividing the input voltage noise density by the input current density.
2) please explain how the 3.3Kohm source resistor you arrive at by doing this improves the noise or any
other performance parameter. please show calculations showing lower numbers than those above
3) Since the SSM has a higher input noise current density and would therefore give a lower resistor for
this calculation, does this mean better amps have higher input noise current densities?
4) As mentioned earlier, if I use a Darlington follower on the OPA inputs and lower the input noise current
density to 5pa, would you now use a 3.3Mohm source resistor?

The ratio of an amp input voltage noise and current noise densities does not define a "input noise resistance" from which you can calculate anything.
They are independent sources and have to be used independently to calculate noise and NF. There is a concept of "equivalent input noise resistance",
but it is a much more complicated calculation invoiving other elements. The amp's input impedance for all AC signals including noise, as seen by the
source (i.e., at the amp input), is is given by the dynamic input impedance speced in all amp data sheets, and is given in the OPA case by Rb'+ (beta
x (kt/qIe) + RE) x2, (ignoring parasitic caps which are significant at higher frequencies) and it is this number you would use for determining the
loading of the source or, if needed, for impedance matching.

Putting in a 3.3K resistor as you want will not give best noise or NF but will increase noise several fold.

http://www.ti.com/lit/an/slyt470/slyt470.pdf see page 25 and 26


And once again you resort to name calling. However, I'm getting calibrated. Apparently this has been going on for decades and you just have a
non-magnetic personality, to say things in a nice way.


>https://pdf1.alldatasheet.com/datash...MI/2N4403.html
>
>See figure 9 showing a set of curves for Ic, resistance at the input and NF.
>
>For 1mA and 500ohms the NF is under 1dB at 1kHz and beyond.
>
>At lower currents, the best noise figure is at much higher resistances.


Expected. Input impedance is a function of transistor operating conditions and beta. The impedance
is higher at low currents and goes down as current is increased. It is easier to specify NF in the case
of a single transistor amplifier, but notice that even in this case, NF cannot be given as a single number,
it is given in a series of curves for different operating conditions.

>
>FFS go away dickhead.


There you go again. What's the deal? Did your mommy not hold you enough when you were a baby? You can't
get it up anymore? You can get it up but have no outlet because you drive off women within 10 minutes? Are you off your meds?
What??? You can't possibly be a happy person. It's sad you have to go through life this way.

Have you ever tried smiling and giving compliments? You might be surprised at how you feel afterwards.
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