- Nov 30, 2006
- Hickory, MS
To answer a question that hasn't been explicitly asked, many legacy low noise transistors were relatively low noise for back then and for the typical applications. Many supposedly low noise transistors don't hold a candle to modern stuff for different reasons. Another consideration a transistor that is low noise at RF may be inappropriate for audio use and vice versa.Thanks @sdimond for the 4403 suggestion. I guess I have to re-learn how to read data sheets. In my searches I never went beyond the tables; one would think if a transistor were low noise the mfr would tout it as a defining characteristic, but then there are a number of variables as @JR pointed out, so maybe it is hard to do. <sigh> Even the lowly 3906 is not that bad when I dig into the datasheet a bit deeper.
The THAT chip may be a bit big for the present application but i will keep it in mind for future. An array would be ideal but those things have gotten really expensive, I suppose due to integrated solutions.
As Abbey mentioned low input noise voltage is a function of low Rbb (base spreading resistance). Naturally higher current devices will have lower Rbb, but it isn't quite as simple as just using power transistors because process impurities also add noise. Then we find that input noise current is a function of HFE or beta (current gain), so higher beta means lower current noise. The transistors used in Paul Buffs low noise mic preamps (transamps) and VCA (EGC101) were medium power bipolar transistors, but also selected for good beta and low process noise.
The first text I read on the subject was, "low noise electronic design" by motchenbacher and Fitchen (C. 1973). For a historical footnote he presented low noise circuits based on the common 2n4403 transistor, not really considered a low noise device. They showed how you could parallel devices to lower noise voltage (while increasing noise current). Back in the 70s I discovered that Prof Fitchen was teaching at a college literally a couple towns away from me. So of course I called him up on the phone to pick his brain and get more information about low noise design. He asked me to consider writing a chapter for his next book but I declined (I knew that I didn't know enough). I never saw his next book but who knows?
Now we have some pretty respectable low noise JFETs and even IC preamps that make the exercise academic.