Desperately Seeking Low Noise PNP

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JohnRoberts

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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.
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.

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.

JR
 

abbey road d enfer

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Ic is about 2.5mA, Rx is about 5K.
Way too much. Should be about 50-100uA.
A question for those familiar with the Schoeps topology, which looks to me like a couple emitter followers hung off pins 2 and 3 of the XLR -- a neat trick to harvest power from phantom. The circuit I started with had Vb (relative to gnd) about 40V with 48v phantom; too close to the rail for my taste, so I upped Ic significantly to bring Vb closer to the midpoint. It's now resting around 28 to 30V, but Ic is now more than doubled (but I can use 30V phantom, which is kind of important to me for one field mixer which is all it offers). Otherwise, Vb was sitting around 28V with 30V phantom. That did not seem anywhere near where I wanted to be.
Typically, teh transistors would run at about 1mA, so the voltage would indeed be about 40-42V. There is no value in reducing the voltage here. The transistors typically run with 2-5V Vce, which is largely enough for handling teh typical mic output voltage.
Back to the noise curves -- I see that as Ic increases, so does noise (ref 2N4403).
Also there is an ideal Rs, the farther you get from ideal, up or down, the worse NF gets. That one seems counter-intuitife. Can someone explain?
The input noise voltage decreases and the input noise current increases.
 

dyamakuchi

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larger emitter-base area
I've always heard, from folks that specialized in transistors, that that base-emitter junction area was the _key_ to low noise.

Also, a number of higher end designs I've looked at all used very similar 'compound' transistors, probably to get the input parameters of one transistor, and the output characteristics of another.
 
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