[sdiy] What makes a transistor "low noise"?

[ASSI]

On Freitag, 5. November 2004 17:31, Metrophage wrote:
> I often see op-amps labelled as being less noisy than general-purpose
> types, but typically not for transistors.

If a transistor is low-noise, the datasheet usually proudly states that 
on page 1 and at least one of the application examples used to be for a 
turntable pickup preamp.

> What is it about transistors which would indicate that they would or
> would not distort signals somehow? How are such factors reflected in
> the specs which are usually offered?

Noise is rather complicated to model exactly and it depends on the 
actual circuit. So any noise figures given are for a specific test 
circuit and biasing conditions. Normally noise is only a very small 
part of the total signal which affords some shortcuts in the analysis 
and is also typically treated seperately from distortion. One of the 
more useful figures in circuit analysis is "input referred noise"; that 
is the noise is treated as if it were added to the input signal as a 
single lumped source.  The other way is to specify the total noise at 
the output. In both cases, you can then treat the resulting noise 
voltage source like any other signal source for analysis.

In data sheets for "low-noise" transistors you should find at least 
three parameters: the noise figure (NF) in dB plus the collector 
current and the frequency band for which that noise figure is 
specified. There is a minimum noise figure for a certain collector 
current and in the absence of any other information it is safe to 
assume that the specified collector current is the one that minimizes 
the noise figure. Noise is also a broadband signal, so if you measure 
it just in a small frequency band (i.e. you filter it), the noise 
figure drops significantly. You'll find that the frequency 
specification is often tailored to mask certain noise sources that are 
either hard to measure or hard to avoid, like very low frequency noise. 
If your application does not work in the frequency band specified (i.e. 
you are trying to build a very slow LFO, but the frequency band 
specified is 40Hz-20kHz), the noise figure does most likely not mean 
anything. When both the application and the specification data match, 
the noise figure tells you how much the transistor "amplifies" noise, 
for instance total noise current at the output vs. total noise voltage 
at the input. There are many possible definitions for noise figures, so 
comparisons between those are only meningful if the same definition and 
same method of measuring is used.


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46 Neuron microQkb Andromeda XTk sonic heaven]>+

SD adaptation for Waldorf rackAttack V1.04R1:
http://homepages.compuserve.de/Stromeko#WaldorfSDada