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why better use big switching types i.s.o. low-noise types
If you have a free choice of source impedance, wind to about 1K-10K impedance and use a small high-gain transistor. It is easier to get high Beta on a small die, the odds of a defective spot are lower, and small transistors are so cheap you can sort-out the noisy ones.
In this case we have a specified fixed source impedance: the 150-250Ω of a commercial "Lo-Z" microphone. If transformers were ideal and free, we would use a step-up transformer into a small transistor.
Transformers are not ideal or free. Transistors are very nearly free today, and you can find one with the several critical parameters very close to ideal.
However the "low noise" transistors were developed 30 years ago when there were a lot of non-ideal excess-noise transistors around. They have high Beta and selected processing and are still good choices for impedances like 1K-10K (though most any high-Beta transistor will work about the same).
These transistors, aimed for medium-Z sources, have base resistances (dead resistance in series with the Base) of 50-300Ω. This is "small" compared with 1K-10K sources. But it is "large" compared with a "lo-Z" microphone. So the noise resistance would be 200Ω unavoidable resistance in the mike, 200Ω dead resistance in the transistor. Total noise power is twice the noise of the mike alone. 3dB Noise Figure, where 0dB is perfection (no amplifier noise added to mike self-noise) and 1dB is possible with a transformer and good transistor or tube.
So we want a transistor with low base resistance and good Beta. At 100-200Ω the Beta is less important (it transforms current noise which tends to be a small problem); we want low Rb.
The 2N4401 is a "Switch". You would use it to turn-on lights, relays, other part-Amp loads. And designers often want a switch that is easy to drive, like a car with power steering, to lessen the load on whatever is driving it (may be a cheap CMOS gate or processor). So a good switch will have fairly high Beta, but also a low base resistance so that you can stuff excess current (for sure switching) into the Base without needing a high voltage.
I don't have the 2N4401 datasheet open. But I think it is rated for 500mA collector current with small Collector-Emitter voltage drop (for efficiency). And that Beta is over 50, so for solid switching you would force 50mA of base current to ensure solid 500mA load current, yet the 2N4401 will have less than 0.8V or 1V of Base voltage at this 50mA base current.
Hmmmm.... even a big-die device will have about 0.7V intrinsic Vbe at 50mA into the Base. If the specified Vbe is 0.8V at 50mA in the Base, then resistance is causing (0.8V-0.7V) 0.1V extra drop. 0.1V/0.050A= 2Ω of dead resistance in the Base. Actually the specs often say 1.0V Vbe at heavy drive, so it may be 5Ω-10Ω. Still this is much lower than the "low noise" parts, and lower than our 200Ω source, so the base resistance noise is low compared to mike self-noise.
{later...} Here's a snip of the 2N4401 data:
This is typical Vbe when you force Base current to be 10 times Collector current. I want to see Base current so I have re-labeled the bottom.
Note that from below 0.1mA to well past 1mA in the Base, the Vbe plotted on log-lin coordinates is a straight line, as we would expect from the exponential action of an ideal junction. At higher current it diverges, as we expect from the real-world fact that we can't make perfect contact to a junction. If you measured Vbe for base currents of 0.1 Amp to 1 Amp it would be curved on these coordinates but straight on lin-lin coordinates: very nearly a pure resistor. Except a real 2N4401 would melt very quickly at 1 Amp! So I've extended the "ideal junction" line from 1mA-10ma, where it does follow ideal action very nearly, up to 50mA where it is clearly non-idea. At 50mA we expect about 0.8V, we get about 1.0V. The difference is 0.2V. 0.2V/50mA= 4Ω. The transistor acts like an ideal device with 4Ω of dead resistance in the Base. (Some of this is really Emitter dead R transformed by Beta; we don't much care. Re is usually small, much smaller than Rb. And if it were large, this would be a lousy Switch transistor, so we know the maker kept Re as small as practical.)
This derivation is sloppy and unrealistic. It is based on data from soaking the junction with current. Noise resistance in the linear amplification range is higher, maybe much higher. For 2N5089 (classic "lo-noise" part) the forced-Beta curves suggest Rb is 15Ω but the measured noise curves say 800Ω in the audio band.
Still the idea is good. Pick a transistor that makes a good switch at currents 100 times higher than the proposed amplifier current (and be sure Beta holds up at your lower current), and non-ideal parasitic effects will be small. (This also assumes modern clean Silicon: 1967 Silicon and most Germanium has enough leakage current to complicate noise estimation.)
