difference between noise voltage and noise current

when to use noise voltage and when to use noise current? for example for a resistance <in^2>=4ktB/R while <vn^2>=4ktBR. So does noise increases with resitance or decreases with resistance?

thanks

Noise alone, has no meaning. In electronics circuits, you are measuring some quantities, voltage or current. So the SNR is the important value. If you have a noise of 1v it seams a lot, but compared to 100v signal - it insignificant.
About current and voltage noise, for a specific component as a resistor, those quantities are connected by ohm's law. So it is true that increasing resistor will increase Vn^2, but decrease in^2.
Where to use each of them: Vn^2 usually used in a cases with small impedances for example terminations in RF... also Vn^2 used as input noise representation of opamps...
Current noise is used with high impedance sources (current sources) like : high impedance detectors (photo/radiation detectors).
Basically it is the same and we use it in a way that it reduce our efford of calculation and better understanding of circuit...

See my appends in The Designer's Guide Community Forum - RF circuit Dc bias question

mona123 said:

when to use noise voltage and when to use noise current? for example for a resistance <in^2>=4ktB/R while <vn^2>=4ktBR. So does noise increases with resitance or decreases with resistance?

Click to expand...

It is just about thevenin and norton modeling of one thing.

About your second question, If the resistor is in series with the signal, then you should use 4KTR and means that you should decrease the series resistance to reduce the noise and when the resistor is in parallel with the output node, you like to increase this resistor as much as possible to reduce the noise.

so if i have a resistance in series with a current source, should i increase or decrease it to get lower noise current?

---------- Post added at 02:45 ---------- Previous post was at 02:41 ----------

small impedance looking towards looking towards source or load?

mona123 said:

so if i have a resistance in series with a current source, should i increase or decrease it to get lower noise current?

Click to expand...

Theoretically, if you have anything in series with current source, you can ignore it, or in other words , the noise of it will not influence the circuit.

could you please explain why?

If you have constant current source, it will force a constant current source in IT'S BRANCH!!!
If you have a resister in series with current source: lets define each pin of resistor as node, so according to Kirchoff, sum of the currents entering the node, equals sum of the currents exiting the node. so sum of the currents entering the resistor equals to sum of current exiting it.So everything in series with current source will have it current flowing through.
This is more or less the explanation.