How to calculate the Noise Figure (NF) of a Low Noise Amplifier (LNA) ?

[monster33]

Hi everyone,

I want to purchase a new LNA but before ordering one, I want to compare its features with the old one.
Well the old one has a bandwidth of (0.1 MHz - 100 MHz), Gain = 30 dB and a NF = 4.5 dB. I used two amplifiers in cascade to get a gain of 60 dB.
Now I want to buy another one with bandwidth of (1 MHz - 10 MHz), Gain = 60 dB but in the datasheet there is no NF value only the noise input density is given = 0.9 nV / Hz

here the link of the datasheet : https://www.lasercomponents.com/de/...ets/femto/voltage/hva-10m-60-b.pdf&no_cache=1
There is also this additionnaly datasheet that gives the output noise (e_n) View attachment HVA-10M-60-B Output Noise.pdf where e_n = -109.38 dBm

I tried to calculate the NF using this equation :

NF = -174 dBm + Gain (dB) - 10log(Bandwidth) - e_n (dBm)
= -174 dBm + 60 dB - 6.95 dB - 109.38 dBm
= 2.33 dB :shock:
Am I right ?

Best regards,
Sana

 

[RCinFLA]

Specification of nV/rt-Hz is usually applied to op amp and refer to input referred noise.

Pure nV/rt-Hz can be converted knowing the impedance. Noise power is (nV/rt-Hz)^2 * BW(Hz) / R where R is the impedance at the circuit point. This is equivalent input referred noise similar to equivalent input referred noise temperature of an amplifier. The db ratio of KTB noise to equivalent referred noise power is noise figure.

Input referred noise power generated by a stage can be added as an idea power summer then gained by amp gain to find the noise output power. This can then be summed with next stage equivalent input referred noise power to get the next stage noise output. In typical system design it should be apparent the backend stages, although having poor stage noise figure, resulting in high input referred equivalent noise power, is small in contribution compared to the gained noise takeover from frontend stages.

However, in an op amp design, with feedback and input resistors, the noise generated by the resistors can add significantly to the overall result. Refer to OP amp manufactures sites for explanations on how to figure total equivalent noise with feedback and input resistor in op amp circuit.