General > General Technical Chat
MOSFETs versus BJT in discrete designs
tszaboo:
Always? I use a mix of P and N, PNP and NPN. Turning on a MOSFET on a 1.8V MCU is more difficult. An always on NPN will use more power than an NFET. A PNP transistor could be more suitable to dissipate some power because the power pad of it is connected to the ground and not the series resistor. There are different requirements, different design goals, and I don't know why would a good engineer, who has an understanding on how these 4 devices work would just use one type.
Alex Nikitin:
I like FETs and made many FET-only and even MOSFET-only analogue circuits, mostly for audio applications, from phono stages to power amplifiers. The lack of proper P-ch complements is not necessarily an obstacle. I've designed several amplifiers with N-ch only MOSFET output devices, these amplifiers were manufactured for many years and thousands of units were produced, some receiving top awards, including Stereophile Class A recommendation. I would say that every type of an active device has some advantages and some problems, and it is up to us to use them wisely.
Cheers
Alex
mawyatt:
Having a career that spanned over half a century we've been involved 1st hand from discrete Bipolar, MOS, SiGe, GaAs and GaN design and ICs in CMOS, BiCMOS, SiGe and InP design.
Simple fact is the semiconductor technology is driven by Digital CMOS, nobody is building new fabs for anything other than Digital CMOS (pushing feature size limits), outside a few maybe for GaN or SiC, and boutique fabs for exotic types like InP.
We've utilized the early bipolar from Fairchild, first BiCMOS analog from RCA, their 4000 series CMOS (we actually used this for analog as well), National's bipolar and JFET and so on and discovered the advantages and disadvantages of each technology for analog use.
A fundamental fact about comparing MOS with Bipolar is, Bipolar has more transconductance (for given area), better matching and temperature tracking than MOS, and somewhat faster to a limit (3nm CMOS is really really fast, but not as fast a bipolar devices in InP), MOS has lower Ron (for given area) and no saturation voltage and obviously no gate current (other than leakage).
However the advantages of bipolar have been displaced over time by digital techniques and folks being forced to design in a Digital Friendly CMOS process as things became more integrated with Mixed Signal over time.
Chopper Stabilized and Commutating Auto-Zero techniques allowed pure CMOS to surpass bipolar designs in offset and 1/f noise regions and fully integrable with Digital CMOS. Almost all new ADCs are processed in Digital CMOS, same for DACs and some of the best performing Op-Amps are pure CMOS.
The MOS switch parameters can't be emphasized enough, they are so powerful and allow digital signal processing techniques to be directly applied for analog use. Even displacing the last "Analog" holdouts, the RF and Microwave worlds, with things like Software Defined Radios, pushing the high speed CMOS ADCs right up to the antenna and with Dr Joe Mitola's Cognitive Radio concepts from the 90s, and the PPM or N-Path Mixer, where traditional analog techniques are yielding to more digital like with MOS switches and producing results not possible with pure analog old school techniques (like ~1dB NF passive microwave mixers).
Ironically the much touted BiCMOS technology in it's day originated from the Digital world, as the need for higher output drive capability (weak PMOS) which the bipolar emitter follower buffered CMOS provided until the smaller feature size CMOS eliminated the need for the bipolar buffer and replaced with a pure CMOS buffer.
Even SiGe bipolar has it's modern origins based upon Digital CMOS, IBM was developing SiGe bipolar technology for a new faster ECL/CML logic for the next generation IBM360 mainframe. During the SiGe development IBM decided on pure CMOS multi-core chip sets instead of a fast single core SiGe bipolar design, but allowed the SiGe development to continue with analog emphasis which ended up in a SiGe BiCMOS process.
Anyway (can't believe I'm saying this being an old analog type), but Digital CMOS has actually been highly beneficial to the analog world and likely ending the era of the bipolar transistor and designs with such. No one is designing new chips or circuits based upon bipolar technology today (maybe a few small areas like voltage references), and no new bipolar fabs are being built, everything is moving or already has to CMOS.
Hopefully we'll still be able to get 2N3904 and 2N3906s in the future as some ancient bipolar fab in someone's garage will still be cranking these out, maybe we all should get a stash now while we can just in case ???
Best,
coppercone2:
what we need is cheaper fabs so we can get what we want instead of relying on piggyback crap
things may not be as grim as you imagine if fab tech gets economized or new construction methods for transistors occur.
Some kind of new manufacturing machine. The 3d printer was amazing, maybe there is something that can be made for larger transistors. Years ago I heard the same thing about plastic parts that you will never be able to get high quality complicated plastic parts without a tool and die/mold guy, its only kind of true, even for high quality metal parts.
FAB are vastly overpriced because of the secrecy too. I know that. even adding useless steps to throw off the competition or obsfurate manufacturing technique. and its pretty easy to find someone that says they were ignored about process improvements. All the fab stuff is so tight budget its all based on probability of projections coming through to put ANY money into improving ANYTHING. This is mostly based on hearsay amongst businessmen.... you might consider alot of tech being left behind at a expensive legacy level simply because the bosses think demand might decrease making it unprofitable based on the company history not the cost of the capital (true for any technology outside of the say middle 25% of the bell curve in terms of actual demand and projections). That is, the line runs with the upmost priority being lowest possible severance costs (meaning absolutely bare minimum input towards upkeep and zero input for improvement, for instance something as simple as replacing some manual pressure gauges and valves with a PLC that automates part of some process at costs under 10k dollars when its not even the cost of a months return). this kind of stuff exists! It sometimes even happen because there is no growth expected, even if the demand is not expected to change, if the business strategy chosen happens to be "only put money into things that are likely to grow! we are not even interested in reducing day to day and manufacturing cost because we might grow elsewhere, its what the board wants this quarter!!!!!" this results in something being unbelievably bad but IT EXISTS!!!!!! and if someone wants to do a external industry-info cost study of such a thing, you will get a fucking stone wall bullshitter that makes you think its all running at peak fucking efficiency and its been studied to death, your talkin north korea levels of honesty here
and then there is the geopolitical side of things. that can make it extra stupid. like mega. when you involve washington, and the pentagon, into costs, the ramifications are only the limits of peoples imaginations.
David Hess:
--- Quote from: mawyatt on November 22, 2023, 04:29:09 pm ---Chopper Stabilized and Commutating Auto-Zero techniques allowed pure CMOS to surpass bipolar designs in offset and 1/f noise regions and fully integrable with Digital CMOS.
--- End quote ---
But only for low source impedances and low bandwidths where their high current noise from charge injection is not a problem. Maybe they now make some parts which solve this, but I have not found them yet.
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