TheFettler said:
Those of you 'in the know' will doubtless have already read this, but I thought it was of general interest:
Low Noise JFETS - Superior Performance to Bipolars?
http://www.vishay.com/document/70599/70599.pdf
ray
yes, I have seen similiar curves, but for older devices, in the big Siliconix books on JFETs. I recommend reading that book. It gives lots of insights in the various types of JFETs: long-channel, short-channel ...
To see better comparisons for JFETs and bipolars, noise-wise, see the famous Motchenbacher/Fitchen: Low Noise Electronics.
Only the Japanese are able to produce JFETs with noise as low as 1nV/Sqrrt(Hz), and these are only the 2SK170 and its siblings. On the other side there are hundreds of bipolars, who are able to, at 1mAmps, these are old like 2N4033, or standard, like 2SC2240, or high-tech, like MAT-02.
The main difference in noise is, that there is virtually no current noise in JFETs, making them optimally suited for high-impedance source. When designing a line stage with bipolars in the input, one should avoid pots over 10k, whereas with JFETs, one could go up to 100k and above. (considering only noise - bandwidth and miller caps are a different subject).
For moving magnet cartridge input stages (they normally have app. 300 Ohms resistance and app. 1k impedance), you can use both types equally good, if you confine yourself to 250uAmps collector current in the input stage, when using bipolars.
Even for moving coils, you can use both. The lowest noise MC phono stage, the Linn Linto with >80dB signal/noise ratio, has a bipolar input. But a 2SK170 would do also it in practical use.
Though, getting substantially below 1nV/Sqrrt(Hz) is difficult. For ultra low output moving coils with output at 0.1mV like the big Allaerts or the big Audio Note, only transformers will do.
regards,
Hartmut
