AW: Re: Couple EE questions...

Magnus Danielson e93_mda at drum.it.kth.se
Thu Oct 24 13:59:55 CEST 1996


> 
> 
> > On should select Rn = en / in
> >
> > For a NE5532 that will turn out to be 5000/0.7 = 7.1 kohm
> >
> > Given this source impedance and full swing of the NE5532 (12.7 Vrms) you 
> get
> > close to 130 dB of signal/noise ratio....
> 
> Sorry for asking again, but I want to be sure:
> So at 7k the effect of inoise and enoise are equal.
> If I choose 3.5k for a 5532, will the SNR be worse, better or equal then?
> I'd say *slightly* better. Or will it really worsen below 7k ??

You have a noise level curve which looks like this:

^ Noise level
|
|              /
|             /
|            /
|           /
|          /
|         /
|________/
|
|
+---------------- > Ohm

When you select

     en 
Rs = --
     in

The above figure shows the noise of the amlifier itself (et^2 = en^2 + 
(in*R)^2).
The noise inside the amplifier itself does depend on the external resistance
(the current part).

However, with the added external source resistance you start to cut into the
noise with an addition of the Johnson noise which adds 4kTR to it.

When you set the R = Rs you will make the internal current noise balance with
the external Johnson noise... so a lower R will in fact make the amplifier
noisier!

Phew!

I guess I should have read more carefull before... it was just a faint memory
before I grabbed the book again :)

So, it is in fact three various types of noises, and one of them you can't
do anything about and the other two must be balanced to get the best result.

Picking an arbitrary low value will not generate the best noise.

Note however that shot noise, flicker noise is not included within this 
discussion. Metal film and wire wound resistors are quieter in general than
carbon composition and carbon film resistors when it comes to flicker noise 
(1/f noise).

Shot noise comes from the statistical distrobution of noise caused by the non
continous nature of a current and can be written In = (2qIdcB)^(1/2).
Where q is the electron charge, Idc is the DC component over a resistor and
B is the bandwidth. So, letting a DC currnet flow througth a resistor will
cause shot noise. The flicker noise comes from fluctuation in resistance.

Do you now grasp the noise problem a little better Juergen?

Let's make more noisemakers!
Magnus




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