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| Why do companies obsolete so many good transistors without substitutes? |
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| coppice:
--- Quote from: strawberry on September 04, 2022, 08:51:48 pm ---OP amps are not great for voltages above 35V --- End quote --- There are great op-amps above 35V. There are great op-amps up to hundreds of volts. The great price can be an issue, though. :) |
| Someone:
--- Quote from: matthuszagh on September 04, 2022, 04:18:29 pm ---Why did OnSemi discontinue these? --- End quote --- Did you order x00,000 units per year for the next n years? Well if no-one did, they aren't going to keep an effectively zero demand product in production (overheads of selling through small quantities, or would you like them to offer 10 units at $40,000 ?) |
| coppercone2:
i think you can tweak those numbers to be a little more realistic, its not a billion gate FPGA sometimes I feel like people start throwing scary numbers around based on some real edge case stuff to try to make a point.... |
| Someone:
--- Quote from: coppercone2 on September 04, 2022, 11:34:21 pm ---i think you can tweak those numbers to be a little more realistic, its not a billion gate FPGA sometimes I feel like people start throwing scary numbers around based on some real edge case stuff to try to make a point.... --- End quote --- Try getting a quote for a fresh batch of obsolete transistors run on a new fab. The figures above are nowhere close to a one off fab of a large ASIC, assuming you'd even take the first batch before they have stabilised the process... Having been in the meetings with suppliers working through costings for semi-custom diode and transistor parts right up to large ASICs. Setup and packaging usually dominates the cost of low volumes. Researchers take bare dies from pool services without testing, qualification, or tuning of process control, as the cheapest option: https://mycmp.fr/price-list/ Which is about the cheapest you can get a low volume set of bare die built for, $1000 per mm2 on a 0.35um BiCMOS fab. Compared to the retail of the largest production FPGAs: https://au.mouser.com/c/semiconductors/programmable-logic-ics/fpga-field-programmable-gate-array/?m=Intel&series=Stratix%2010%20GX $20,000 and up per unit |
| mawyatt:
--- Quote from: matthuszagh on September 04, 2022, 04:18:29 pm ---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. --- End quote --- Have you considered a SiGe BJT? They might be a suitable replacement, very high ft, low noise, low 1/f corner, low rbb, high Beta which is ~flat with Ic, however are low voltage devices. Mouser has a number of BFR, BFP and MT types in stock. Best, |
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