what's the relationship between amplifier's linearity and NF

Hi,everyone. the past days, I've an argument with my workmates. I think there's

big relationship between NF and linearity(such as IP3) of an amplifier,but they

say the NF basically has nothing to do with linearity. and factly, we've find no book

or articles about this topic,could someone kindly enough to illustrate it to me,Thanks

a lot!
l.j.zheng

I think that what they mean is that amplifiers with heavy feedback from resistors have high IP3 but the resistors add noise. Usually such amplifiers have a 6-8 dB noise figure. There are several methods of using directional coupler feedback to improve the IP3 without ruining the noise figure, but these are usually less than an octave bandwidth.

Is no direct relation between Noise Figure and Linearity.
But if is looking for Output Noise Power it is. Actually is a test for measuring the Conversion Gain (and indirect the Output Noise Power) that use the same setup as IP3 Two-Tone test, but using unequal input levels.

Thank you for replies! I'm sorry I don't describe my question clearly.

The main point I wonder if there is relationship between noise figure and linearity

in physical essence of transistors or FETs. I find some facts that amplifiers with

very good linearity almost always with good NF, so I have the doubt.

Best Regards

You mean simple devices?
I think in ICs there is a tradeoff between NF and linearity.

Hi,

I agree that there is not direct relation between NF and linearity.

NF is defined by dissipative components of a transistor. And it is very important that because of low level of noise signal any nonlinearity is not considered. Nonlinearity in turns provides distortion of high level input signal.

And now let's image that there are two dissipative components. One is reffered to the input and another one is refered to the output (like gate resistance and channel resistance in MOSFET). If we want to decrease contribution of drain noise current to the output current we have to increase gain.

If you design single-stage amplifier it seems OK. But in multistage amplifier increasing gain of the first stage results in deterioration of IIP3.

So, to provide better performance we have to look for compromise. And it is exactly we do (I mean RF IC designers) to meet spec.

Rgds

Usually , if you increase current of BJT or FET this will improve linearity and increase noise figure .
You can see this effect in the datasheets.

2 divan:

If NF and IIP3 are defined by bias condition it does not mean that there is direct relation between these parameters (it is the topic of this thread).

Moreover as to MOSFETs at some bias current we can get maximum IIP3, i.e. maximum linearity. Further increasing bias current results in deterioration IIP3 and only at very high bias current (with respect to usually used one) IIP3 decreases again. So, it is not the rule: increasing the bias current results in improving IIP3 and in NF deterioration.



Rgds