What Are the Noise Parameters of a Very-Low-Noise Amplifier?

[kaancelebi]

We need a very-low-noise amplifier for a special application, and we buy such an amplifier with a noise bandwidth of 2.4 MHz and an amplification of 1000. To test it, we take a set of "perfectly" shielded standard resistors, connect each if these to the amplifier input, and measure the rms output noise voltage in each case. The data are given in the Table below. Calculate the following noise parameters of the amplifier: Equivalent noise input voltage, equivalent noise input current, optimum source resistance Ro, and noise factor F at Ro.
Table of output noise voltages:
R (Ω) ---- Urms (mV)
1 ----- ----- 2.0
10 -------- 2.0
10^2 ----- 2.4
10^3 ----- 4.6
10^4 ----- 15
10^5 ----- 90
10^6 ----- 8.1x10^2
10^7 ------ 8x10^3

i really don't understand what i will do. i know some formula Unoise=(4kBTf)^1/2 ,optimum source resistance , and noise factor but i don't know how i can do circuit , use formula or where can i use these table.
i need help !

 

[marcusl]

Amplifiers are modeled with two effective noise sources, a voltage and a current source. The current source is connected directly across the amplifier terminals. The voltage source in series with the input resistance is also across the terminals, in parallel with the current source. (It's hard to describe, easy to grasp if you see a diagram. Is it in your book?)

When the input R = 0, the current source develops no noise voltage drop so all noise comes from the voltage source. The opposite is true for R=infinity. Your data shows directly the noise voltage, and I think the last two terms give you the current noise. From there you can calculate the optimum source resistance, and the total noise there. That should give you noise factor.

 

[kaancelebi]

diagram mean =
www.resimupload.com/ds271573906_diagram.html[/URL]

i have a book its name is Principles-of-Measurement-Systems and i tried to find it but i didnt find :S

also i understood thanks a lot. it is really good information.
(((((((((When the input R = 0, the current source develops no noise voltage drop so all noise comes from the voltage source. The opposite is true for R=infinity.))))))))

i solved question .thanks again

 

[marcusl]

Yes, it looks like you got it exactly. Good job!