Available Noise Power and Amplifiers

In summary, the conversation discusses the calculation of noise in a resistor and its connection to an amplifier. The speaker has a question regarding the assumption of matched impedances and the importance of noise power in real world applications. They question why the book recommends matching impedances and argue that assuming a noiseless matching resistance is more realistic than assuming a noiseless infinite input impedance and load.
  • #1
Runei
193
17
Hello there,

I'm working with the textbook by Wim van Etten, "Introduction to Random Signals and Noise". And right now I'm preparing for an exam in a stochastic processes course.

I have a question regarding some noise calculations, that I just can't wrap my head around. I hope someone will be able to shed some light!

The situation
So we have a noisy resistor of value R. By using approximation we can say that the RMS voltage across this resistor due to thermal noise, is equal to (Vr is assumed to be the rms voltage).

[itex]V_r = \sqrt{4kTRB} [/itex]

And here B is of course the bandwidth of whatever measuring system you use to determine the voltage. So far so good.

The noise spectral density of the noise voltage is

[itex]S_r= 2kTR [/itex]

Now we connect this to to an amplifier (noiseless to begin with). The amplifier has an input resistance of Ri, and output resistance of Ro, a frequency response of H(ω) and connected to a load resistor of RL. All resistors other than R are assumed to be noiseless.

Now, what I would do then is to say:

The noise spectral density at the input is then given by

[itex]S_i = S_r \frac{R_i^2}{(R_i + R)^2}[/itex]

The output spectral density (not across the load by the voltage generated before the output resistance), is then given by

[itex]S_o = S_i \cdot |H(\omega)|^2[/itex]

Looking then at the spectrum seen across the load resistor we have

[itex]S_L = S_o \frac{R_L^2}{(R_L + R_o)^2}[/itex]

The totality of this becomes the following

[itex]S_L = S_r \frac{R_i^2}{(R_i + R)^2} \frac{R_L^2}{(R_L + R_o)^2} |H(\omega)|^2 [/itex]

This is all well and good, however, what I don't understand is why he goes ahead in the book and want to create the maximum power transfer (real power).

As I see it, if the impedances are matched, the spectral density at the loads go down, as opposed to the case where the input impedance was set to infinity, and the Load resistance was also set to infinity. This would give an indication of maximum possible noise voltage we could see at the output, due to that single resistor (or noisy circuit with an equivalent temperature of T and equivalent resistance of R).

I know that if we DO match the impedances, the R dissapears, but why is this a "smart" thing to do? Don't we want to analyze situations in which the noise signal we get out is the maximum, to see what the worst case scenario is? Or am I completely missing something?

I know it's nice that have the available noise power as

[itex]P_a = kT/2[/itex]

But this is only the case of matched impedances, which make the signal become smaller, and hence, create a smaller output noise than in the case of a larger impedance...

Am I ranting? Or am I making sense? o_O

My general problem is that I don't see why we go ahead and assume matched impedances.

Thank you for any help you might give!Rune
 
  • #3
I'm not sure, but I expect real world applications are generally more concerned with noise power than noise voltage. Assuming a noiseless matching resistance is less "unreal" than assuming a noiseless infinite input impedance and load.
 
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Likes Runei

What is available noise power?

Available noise power is the amount of noise power that can potentially be utilized by a device or system. It is typically measured in decibels (dB) and is related to the signal-to-noise ratio (SNR) of a system.

How is available noise power related to amplifiers?

Available noise power is directly related to the noise figure of an amplifier. Noise figure is a measure of how much the amplifier adds noise to the signal, and a lower noise figure indicates a better amplifier with less added noise. Therefore, a lower noise figure results in a higher available noise power for the system.

What factors affect available noise power?

The main factors that affect available noise power are the noise figure and gain of the amplifier, as well as the bandwidth of the system. Additionally, external factors such as temperature and interference can also impact available noise power.

How is available noise power calculated?

Available noise power can be calculated using the Friis formula, which takes into account the noise figure, gain, and bandwidth of the amplifier. It can also be calculated using the noise factor, which is the ratio of the output signal-to-noise ratio to the input signal-to-noise ratio.

Why is available noise power important?

Available noise power is important because it affects the overall performance and sensitivity of a system. A higher available noise power can result in a better signal-to-noise ratio and improved system performance, while a lower available noise power can lead to decreased sensitivity and potential signal loss.

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