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Why do companies obsolete so many good transistors without substitutes?
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matthuszagh:
I'm designing a low residual phase noise buffer amplifier for a 10MHz clock that uses an NPN transistor as the gain element. The NPN ends up being the most significant contributor to the noise performance of this circuit. Using the noise analysis in the Art of Electronics, the input-referred noise ends up being determined by the base-spreading resistance, DC current gain, 1/f corner frequency, and transition frequency (ignoring other important parameters such as collector current, temperature and source impedance). The Art of Electronics provides a really nice table of low-noise BJTs. Several standouts (among others) are the Sanyo 2SC3601 and 2SC3955, which have very low base-spreading resistance and good high-frequency operation (low input capacitance and high transition frequency). Unfortunately, these, along with many other good transistors are obsolete.

Why did OnSemi discontinue these? Same question probably applies to other companies and transistors. I've noticed a similar trend for good RF transistors. Is the market for high-quality discrete transistors too small? Do companies not use discrete transistors for low-noise applications anymore? Why is this? Is it because they can build their own ICs with the transistors they want, or are happy with the good, but not quite as good, performance of modern opamps? Any other reasons? I know a lot of these are through-hole parts, but presumably it isn't hard to place the die in a SMT package if the problem is that people aren't buying through-hole anymore.

I'm sure I'm not the only one who feels this way, but this is incredibly frustrating as a designer without the capabilities to fabricate custom transistors. Also, I know this isn't a particularly good reason, but I like being able to use discrete transistors in designs, rather than having a lot of the interesting work done for me with opamps.

I should also mention that the ZTX618 and ZTX851 are often used as replacements in applications where low rbb is desired, but they have pretty horrible input capacitance, so the high-frequency performance is significantly worse than the Sanyo NPNs mentioned above.
wraper:
Old equipment gets decommissioned for example, factory burns down, whatever. And then there is no new equipment for the same manufacturing process, or it's simply not viable to buy it just for some old parts with not that much market demand. Not to say there is no Sanyo anymore as it's gone for more than a decade. Something spun off, such as capacitor production which are now Suncon, other parts were bought by other companies such as Panasonic and Fujitsu. Obviously not all product lines survived this.
Benta:
30+ years of semiconductor marketing has taken its toll. It's all about MOSFETs, MOSFETs, MOSFETs and MOSFETs. BJTs are out of fashion with young engineers, although they are superior in many ways.
Just look around this forum: the knee-jerk solution to any problem is a MOSFET. People seek workarounds to the problems surrounding MOSFETs instead of looking at, eg, BJTs or other devices.

Only older, experienced engineers and audio people look a BJTs today. The same can sadly also be said for JFETs.

Sign of the times.
coppercone2:

--- Quote from: Benta on September 04, 2022, 05:53:00 pm ---30+ years of semiconductor marketing has taken its toll. It's all about MOSFETs, MOSFETs, MOSFETs and MOSFETs. BJTs are out of fashion with young engineers, although they are superior in many ways.
Just look around this forum: the knee-jerk solution to any problem is a MOSFET. People seek workarounds to the problems surrounding MOSFETs instead of looking at, eg, BJTs or other devices.

Only older, experienced engineers and audio people look a BJTs today. The same can sadly also be said for JFETs.

Sign of the times.

--- End quote ---

well it has to do with education, you need someone to in a class teach the differences and explain the nuances of where they actually make your life easier. The problem is it would need to be senior level class that you take right before your first job. How many universities teach with the idea that 95% of the students are going to go into making ASICs and the other 5% into code? But I agree it seems like at some point mosfets are 'hacked in' to a circuit to make it work and it looks like some kind of desperate bootleg substitution if you know the alternative.

Does it feel like 'oh, mosfet is the one they used in a processor, it MUST be better' and 'i feel comfortable and trendy because mosfet based device lets me browse tik tok on my cell phone, lets use it for everything, that way I don't risk being confused by a new concept'
tggzzz:
One explanation I've heard, but cannot verify, is that they make a large number of wafers, and store them until needed. When the wafers run out,the parts become obsolete.

That relies on there being a large number of very cheap devices on a wafer; it won't work with, say, large processors.
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