What's the smallest possible input signal of an amplifier?

[windarray]

some questions about noise:
how low can the input signal of amp be?I mean,a very small signal (e.g 1uV) come into the amp,if it's smaller than the input referred noise,can it detected at the output?or depending on the dynamic range of output......
if the smallest input signal is referring with the input noise,is it the noise power or noise power density? ......
help!thank u very much!

 

[yibinhsieh]

Re: noise about amp

Yes, if it is smaller than the input refer noise in a speicified BW, you cant detect at the output. Usually the noise is in spectral noise density. But you should multiply the BW when you calculate the SNR.

 

[flatulent]

Re: noise about amp

To be numerical, you cannot get below the thermal noise of circuits at room temperature which is -174 dBm/Hz spectral density. Then the noise figure of the active devices makes it worse. For the lowest noise it is hard to beat a JFET with a large IDSS or Gm. Transformer couple the input signal so that its source impedance is equal to the ratio of JFET equivalent input noise voltage spectral density divided by the equivalent input noise current spectral density. These two parameters are shown on graphs in the data sheet as a function of frequency.