EMRFD Message Archive 7650

Message	Date	From	Subject
7650	2012-08-01 08:42:53	Tim BandTech.com	noise figure for small signal rf amp: bf998 mosfet versus IF140 jfet
Hi. I am slowly working on an experimental receiver. I am going to be building an rf preamp and am trying to compare the noise specs of two transistors that I happen to have on hand: bf998 dual gate mosfet : F = 0.6dB at 200 MHz; Gs=2mS IF140 jfet : 4 nV / (sqrt Hz) at 10 Hz; VDS=12V, VGS = 0V The symbol is theta bar sub N and means "Equivalent Short Circuit Input Noise Voltage" those units are nanovolts per square route hertz I believe. How do I compare these two? Let's say that I am building a narrow preamp for 14 MHz and let's not worry about atmospheric noise at the input. I do have a background in EE, but don't recall doing these sort of noise computations. I do get things like Y = G + jB I = Y V but I've forgotten what Gs above is. Theoretical and practical advice is welcome. Thanks. - Tim AB1AH
7651	2012-08-01 13:24:24	Gary, WB9JPS	Re: noise figure for small signal rf amp: bf998 mosfet versus IF140
The noise spec for your FET is stated at 10 Hz, not radio frequencies; extrapolation is very unreliable. It *will* be quieter at higher frequencies, but without knowing the 1/f corner, nobody knows for sure... Assuming you got that data, here goes: 0.9 nV/rtHz is the rms thermal noise of a 50 ohm resistor at room temp. That comes from the formula en = sqrt(4kTRB), which you can find in any applicati
7661	2012-08-05 06:43:03	Tim BandTech.com	Re: noise figure for small signal rf amp: bf998 mosfet versus IF140
Thanks so much Gary for covering the basics for me. I've read through your message several times, and it does take me several times to understand, or at least to partially understand.