I was curious about the noise figure of a standard 2N3904 BJT used in an RF preamp, so I built the circuit shown above. Using my home built NF measurement kit, I found the circuit has a noise figure of about 7.5 dB.

My interest was sparked by a YouTuber I really like named Charlie Morris (ZL2CTM): https://www.youtube.com/channel/UCSNPW3_gzuMJcX_ErBZTv2g. He makes the coolest radios using the construction technique you see in the picture. This is one of his designs for a preamp that works in the HF bands. If you are interested, the design for the circuit can be found here: https://www.youtube.com/watch?v=iRABXFKScqk&t=653s

A couple things to note about the measured 7.5 dB noise figure:

• No attempt was made to maximize NF in the design. It was simply constructed as a gain block to provide 10 dB of gain and match 50 ohms input and output impedances. The design will work just fine for Charlie’s needs in the 80m and 40m bands.
• I found a data sheet saying the maximum NF of the 2N3904 is 5 dB. This is consistent with my result, I’m sure the feedback used in the design increases the NF a couple dB.
• I’m still working out the kinks in my home made noise figure kit. I’m pretty confident right now that my result is correct to within plus or minus 0.5 dB.

Fun afternoon project!