noise understanding of resistor in a circuit

[mona123]

Hi, Can you please help me understand the noise of a resistor so that I can analyze noise behavior in a circuit? An example would be helpful.

The noise voltage is directly proportional to resistance R in a resistor but noise current is inversely proportional to R. So should one increase a resistor or decrease a resistor to reduce the noise? Thanks.

 

[goldsmith]

Hi, Can you please help me understand the noise of a resistor so that I can analyze noise behavior in a circuit? An example would be helpful.

The noise voltage is directly proportional to resistance R in a resistor but noise current is inversely proportional to R. So should one increase a resistor or decrease a resistor to reduce the noise? Thanks.

Hi mona123
I suppose you are asking about Johnson-Nyquist noise . isn't it ? it is because of effect of temperature on free electrons of the material which the resistor is based on .
These electrons can go around the resistor so we'll see something like a noise voltage across it ( but it is accidentally and it's exact shape is not clear because it is variable ) .
It will delivered by : Vn=sqrt(4*k*T*BW*R ) Where t is temperature in kelvin . K is boltzman coefficient which is 1.38*10^-23 . BW is bandwidth . R is value of resistor .


Best Wishes
Goldsmith

 

[mona123]

Sorry GoldSmith, I am not asking the definition noise. i am asking voltage noise will increase with resistance but current noise will decrease with resistance, so in designing circuit what should one do, increase or decrease the resistance? Thx.

Hi mona123
I suppose you are asking about Johnson-Nyquist noise . isn't it ? it is because of effect of temperature on free electrons of the material which the resistor is based on .
These electrons can go around the resistor so we'll see something like a noise voltage across it ( but it is accidentally and it's exact shape is not clear because it is variable ) .
It will delivered by : Vn=sqrt(4*k*T*BW*R ) Where t is temperature in kelvin . K is boltzman coefficient which is 1.38*10^-23 . BW is bandwidth . R is value of resistor .


Best Wishes
Goldsmith

 

[goldsmith]

Sorry GoldSmith, I am not asking the definition noise. i am asking voltage noise will increase with resistance but current noise will decrease with resistance, so in designing circuit what should one do, increase or decrease the resistance? Thx.

Mona ! Of course i know what you're asking and what you're asking means that you didn't understand that what is that noise and how to measure it ! because if you knew you wouldn't ask like this . Current of noise is not important in resistors . the resistance depends on your application . what kind of application you're referring to ? Microwave Preamplifier design ? usual narrow band amplifiers ? or ... etc ? you simply have asked a question without any information ! i hope you know why we calculate the noise amplitude for a resistance . do you ?

Best Luck
Goldsmith

 

[mona123]

I don't know thats why I am asking in this forum. Application is low noise amplifier design, using BJT or CMOS, whats the effect of collector resistance or drain resistance on noise . No book tells me when to consider current and when to consider voltage noise of a resistor while analyizing circuit. By definition. vn=sqrt(4*K*T*B*R) and in=sqrt(4*K*T*B/R). One is directly proportional and other inversely proportional to R. Sorry, I don't understand your second question. "why we calculate calculate noise amplitude"..what amplitude, voltage amplitude or current amplitude? In general we calculate noise to find out signal to noise ratio. Thx.

Mona ! Of course i know what you're asking and what you're asking means that you didn't understand that what is that noise and how to measure it ! because if you knew you wouldn't ask like this . Current of noise is not important in resistors . the resistance depends on your application . what kind of application you're referring to ? Microwave Preamplifier design ? usual narrow band amplifiers ? or ... etc ? you simply have asked a question without any information ! i hope you know why we calculate the noise amplitude for a resistance . do you ?

Best Luck
Goldsmith

 

[crutschow]

Whether you use current noise or voltage noise for determining system noise depends upon which is critical in the circuit. The two noises are really the flip side of the same coin. If you divide the calculated resistor voltage noise by the resistor value you will get the resistor current noise. If you multiply the current noise by the resistor value you get the voltage noise.

The resistor noise power is constant at a given temperature, independent of the resistor value. Since resistor power = V²/R = I²R you can see why the noise voltage goes up with resistor value but the noise current goes down for a constant noise power.

Typically voltage noise is used in noise analysis since that's generally the circuit noise you are interested in. On that basis you typically try to keep the resistor values as low as possible, especially those in the first amplifier stage input, since that noise is amplified the most by all the following stages. With sufficient first-stage amplification, the noise from the following stages usually adds little to the total noise.

 

[goldsmith]

I don't know thats why I am asking in this forum. Application is low noise amplifier design, using BJT or CMOS, whats the effect of collector resistance or drain resistance on noise . No book tells me when to consider current and when to consider voltage noise of a resistor while analyizing circuit

Hi Mona
Are you sure no book ? you'd better to mention non of the books which you have read . for understanding these things you must have a look into the telecommunication engineering and RF engineering books like telecommunications by George kennedy . or solid state radio engineering by herbert L Krauss .

I can give you some explanations to make it a bit clear . in non of the designs you are going through the noise current in resistor ( approx ) .
In amplifier design an example : we want design an amplifier which it's inlet is from 500 MHZ to 5GHZ . the input impedance must be 1 mega ohm . is that possible that our amplifier be able to amplify the signals with magnitude of 2 mili volt without any problem ? . consider the temperature of the area which our device being used is about 40 degree centigrade .
solution :
T in kelvin = 273+40=313 BW=5-0.5=4.5GHZ
So vn = sqrt(4*(1.38*10^-23)*(4.5*10^9)*313*10^6))=8.81 mv so our amplifier can not amplify our signal because of the awful value of SNR at this condition .

Any question ?
Best Luck
Goldsmith