Good evening, this turns out to be a good discussion... Any more inputs ? 73 de Ulrich In a message dated 7/19/2015 7:13:39 P.M. Eastern Daylight Time, rich...@karlquist.com writes:
On 7/18/2015 2:16 AM, Attila Kinali wrote: > > I always wonder how you figure out whether a transistor is low noise > or not. What part of the datasheet hints at which transistors have low > noise and which have not? Even if it's just try and measure, how > do you find good candidates to measure? > > Attila Kinali > For a BJT operating above the 1/f noise corner, and at non-microwave frequencies, the noise properties depend only on RF current gain and base spreading resistance. See "Low noise electronic design" by Motchenbacher and Fitchen. RF (not DC) current gain can be measured the usual ways, but base spreading resistance has to be inferred from noise figure measurements made with low source resistance. The RF current gain is the real fundamental noise property of the device that you cannot change. Fortunately, it can be determined from the data sheet, if not directly, then by calculating it from DC current gain and F-sub-t, based on the operating frequency. The low frequency current noise (above the 1/f corner) is simply equal to the shot noise of the DC base current. The low frequency voltage noise is the sum of the Johnson noise that a resistor would have if its value were the sum of the base spreading resistance and half of r-sub-e. Where r-sub-e is the "emitter resistance",IE the effective on resistance of the transistor. Base spreading resistance can be overcome by using a sufficiently high source impedance and/or paralleling devices (if you can tolerate the additional capacitance). At frequencies such as 100 kHz and 10 MHz, it is very easy to get a noise figure well below 1 dB with a BJT, so it should be no great problem to find a suitable device. Even lower noise figures are available with JFET's, which have noise current equal to the shot noise of gate current, which is specified. The resulting noise current is negligible for most devices. This leaves the noise voltage, which is just the Johnson noise of a resistor equal to the channel resistance. By scaling to larger devices and/or paralleling devices, this can be reduced to arbitrarily low values. The limiting factor is the substantial capacitance of JFET's. This limits them to about 1 to 10 MHz, before high beta BJT's dominate. I have observed noise figure of below 0.2 dB in JFET's at 2 MHz. Below 50 to 100 MHz, MOSFET's and ePHEMT's have excessive 1/f noise and are a non starter. Above the 1/f corner, it is easy to get noise figures of a few tenths of a dB with ePHEMT's. All of this discussion doesn't address 1/f noise, which could be an issue in oscillators and low phase noise amplifiers. For that purpose, you are back to characterizing devices yourself. Putting negative feedback around the transistor can alleviate this by reducing upconversion of noise. Rick Karlquist N6RK _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
