Noise figure refers to a nominal source impedance rather than actual amplifier input impedance, first point to check if it's actually 200 ohms for the said VGA.
Sorry.@ KlausST: Unfortunatly your link redirects to this thread.
Not sure what the problem is. Noise figure versus VGA gain is specified in the datasheet, also NF versus frequency, so you can calculate input (or output referred) noise voltage density and noise voltage for any operation point.I still have problem to calculate the overall gain if the noise characteristics of the second stage is ONLY given by its noise figure.
An amplifier has both voltage and current input noise, in so far shorting the input won't give the complete amplifier noise. The referenced "noiseless amplifier output" is essentially a calculated quantity, it requires an accurate measurement of DUT gain.
Hi,
No, 2nV/sqrt(Hz) is the 200 Ohms reference noise. This is equals 0dB noise.
Now including the amplifier you get 2dB (or whatever) noise in total.
So the 2dB result in 2nV/sqrt(Hz) x 1.259 = 2.51nV/sqrt(Hz)
Now you just have to take your bandwidth into account.
Klaus
The datasheet specifies a noise figure with 200 ohm source impedance, you don't (and don't need to) know which part is caused by noise current and which by noise voltage. The question would be relevant for a variable source impedance.
No. I never said this.According to Klaus the output noise is direct proportinal to the VGA gain
The VGA noise is given by its noise figure NF in dB. This noise figure is also a function of the gain setting x and decreases with increasing gain.
How can I convert the noise figure into a rms voltage noise
So the output noise is given be N_out = 2 nV/sqrt(Hz) x 10^(NF/20)
Hi,
No, this is the equivalent input noise. "200 Ohms" is the source impedance = input impedance.
Multiply this with the VGA gain and get the output noise.
Klaus
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