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Noise comparison of two transistors

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gag2000

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Hi there,

I would like to compare two transistors, to replace a nearly obsolete reference in a design.
The first one (the reference) has a noise density of 1nV/sqrt(Hz). We can consider that the noise density is flat on the usefull bandwith, i.e. 10Hz-> 15kHz (not true, but a worst case).
The second transistor has a noise figure of 2.8 dB on the same bandwidth.

How can I compare the noise on these two transistors ? I am quite stuck with the noise figure concept...

Regards,
RG
 

The lower is the noise figure then the lower is the noise. But transistors have a range of spec's, noise included. The graph in datasheets shows a "typical" device but the text shows maximums.
Maybe you are looking at the graphs but the transistor with a lower typical noise figure might have more actual noise that the transistor with a higher typical noise figure.
 

Audioguru :
There are unfortunately no graph for the transistor which has only "noise figure" in its datasheet...

Crutshow
The datasheet specify "noise figure" for given vce and Ic (and of course bandwidth). They also give a "Rs", which looks like a load resistor. I understand that noise figure is quadripole convention measurement. So manufacturer should give configuration, CB, CE or CC ,but they don't... :(
 

Most datasheets have graphs. You forgot to say which transistor and which manufacturer of it.
Rs is the source resistor of the input to a common emitter transistor.
Here is from the datasheet of a 2N5088 and 2n5089 transistor:
 

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Manufacturer is NXP and references are BCM847/857. Both are matched transistor pair (to replace ssm2xxx wich becomes hard to find)
If you are aware of another low noise bipolar matched pair in production, I would be thankfull if you could ive me some references.
 

The BCM847/857 transistors are ordinary duals, not low noise. Low noise transistors have the maximum noise listed that is allowed.
Why are you trying to make a low noise opamp instead of buying one?
Sorry, I have not seen European transistors for about 40 years. I have never used surface-mount dual transistors.
 

I am not trying to build an opamp, Just a differential pair. What make you saying that bcm847 is not low noise ? Back to my first question : How to compare noise figure to noise density ?
 

Noise figure and input noise voltage are related by the source impedance of the circuit under test (for the noise figure measurement). Without that, a comparison can't be made. The input noise current should be known as well.
 

The sales sheet and datasheet of the BCM847/857 do not say they are low noise. The datasheet shows a "typical" noise figure but not the guaranteed and not to be exceeded maximum. If you want a low noise one then the manufacturer will not give you one, you must buy thousands and test them all. You might find a low noise one.
 

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