Author Topic: What's 50 fF?  (Read 2416 times)

0 Members and 1 Guest are viewing this topic.

Offline ezalys

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
What's 50 fF?
« on: January 26, 2022, 10:17:29 pm »
I'm working on an amplifier that needs 50 fF of input capacitance -- which is an easy number to write on paper but extremely difficult to achieve in practice. In the interest of not spoiling the input characteristics of my FET -- what's 50 fF look like? If I take a 2 mm metal sphere, that's (4*pi*e0)*(1 mm)=100 fF, but I don't know how to extrapolate a sphere to a wire bond or what have you. Just looking for your experience!
 

Offline Alex Eisenhut

  • Super Contributor
  • ***
  • Posts: 3013
  • Country: ca
  • Place text here.
Re: What's 50 fF?
« Reply #1 on: January 26, 2022, 10:27:20 pm »
It would like about half of one of these

https://www.digikey.ca/en/products/detail/murata-electronics/GCQ1555C1HR10BB01D/7803156

I'm only kinda kidding, I don't have a good idea of what capacitance "means" outside of a physical part on a board, you know?

But this sounds like some Ka stuff. This is where you need Southwest Microwave connectors that need screwing down to the board because the quality of the solder joint from the SMA to a microstrip determines the return loss so just don't put solder.
Hoarder of 8-bit Commodore relics and 1960s Tektronix 500-series stuff.
 
The following users thanked this post: RVergo

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 19444
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: What's 50 fF?
« Reply #2 on: January 26, 2022, 10:29:15 pm »
Nothing achievable over any kind of size, that's for sure.

Is this going to be a bootstrap application?  (Sounds like it!)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline ezalys

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
Re: What's 50 fF?
« Reply #3 on: January 26, 2022, 10:34:29 pm »
It's not. It's a HEMT or JFET being wire bonded directly onto the island whose charge I'm trying to sense -- it should all fit inside a few square mm.
 

Offline ezalys

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
Re: What's 50 fF?
« Reply #4 on: January 26, 2022, 10:37:11 pm »
And that capacitor is a bit funny. I believe there's 0.1 pF between the two leads, it's just that I imagine the self capacitance of each is probably a bit bigger -- but I don't know.
 

Offline MK14

  • Super Contributor
  • ***
  • Posts: 3617
  • Country: gb
Re: What's 50 fF?
« Reply #5 on: January 26, 2022, 11:08:37 pm »
Is this going to be a bootstrap application?  (Sounds like it!)

What exactly do you mean by bootstrap application, in this case ?

Do you mean, allow the device to start running, by creating an initial voltage or something ?
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 4792
  • Country: us
  • Retired, now restoring antique test equipment
Re: What's 50 fF?
« Reply #6 on: January 26, 2022, 11:29:13 pm »
"Bootstrapping" is careful application of positive feedback to a guard node or similar to effectively remove some capacitance from the circuit.
 
The following users thanked this post: MK14

Offline inse

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
Re: What's 50 fF?
« Reply #7 on: January 27, 2022, 05:34:59 am »
What's 50 fF?
Not much!

Sorry, could not resist
 

Online ejeffrey

  • Super Contributor
  • ***
  • Posts: 2869
  • Country: us
Re: What's 50 fF?
« Reply #8 on: January 27, 2022, 05:47:44 am »
50 femtofarad looks like 1.5 mm of 100 ohm transmission line.  A wirebond of that length will have enough a significant fraction of your capacitance.  It will also have inductance you probably can't ignore.

50 fF is a pretty easy capacitance to produce on chip, islands of with sizes in the ~100 of um on a side will have capacitance to ground in the ~10s of fF (depending on a lot of factors), and for series capacitance you can use interdigitated capacitors to get similar values relatively easily.

These are just approximate scales to get a starting point, if you want something accurate you probably need to simulate it.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 11226
  • Country: de
Re: What's 50 fF?
« Reply #9 on: January 27, 2022, 09:57:05 am »
Is this going to be a bootstrap application?  (Sounds like it!)

What exactly do you mean by bootstrap application, in this case ?

Do you mean, allow the device to start running, by creating an initial voltage or something ?

Bootstrapping can drive parts of the circuit (e.g. the fet drain or some shields) to approximately the level of the input. This can reduce the effectice capacitance. So there is still a physical capacitance from the input to the driven parts, but it is not longer effective as the potential moves in parallel.
It depends on the application if this trick helps: it helps when it comes to the frequency response, e.g. from a very high impedance source, but it does not really help with some noise effects.
 
The following users thanked this post: MK14

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 19444
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: What's 50 fF?
« Reply #10 on: January 27, 2022, 10:05:27 am »
Phil Hobbs had an application something like that, as I recall, for a wideband wafer probe sort of thing.  So, a probe head that hovers over the target, and needs minuscule capacitance to avoid loading the target (which being in the middle of a chip, isn't low impedance).  He used a bootstrapped JFET source follower, so that both drain is bootstrapped (coupled to same as output AC voltage), and guards/shields around the input (gate).  I forget if that was using ordinary parts (e.g. BF862) or some combination of PHEMT and HBT.  (The two are apparently a great combination, because one has quite high output conductance -- thus getting extra value from the drain bootstrap -- while the other has poor noise at low frequencies, or something like that.  So, also good when the signal is taken from the collector of the stack, i.e., regular cascode.)

(Note that a bootstrapped-drain source follower, is the same circuit as a cascode, except the source is allowed to move (CCS for bias, output taken from here), the drain/collector is grounded, and the cascode transistor is (at least) AC-coupled to the output.  Whereas if the source is (AC-)grounded, the bootstrap voltage is also zero so we don't have to worry about any other changes there, and just add a load resistor and output connection at the top (+V end).)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: MK14

Offline MK14

  • Super Contributor
  • ***
  • Posts: 3617
  • Country: gb
Re: What's 50 fF?
« Reply #11 on: January 27, 2022, 10:26:19 am »
Thanks for all the explanations, about bootstraping (application).

It's making a reasonable amount of sense to me now. As I see it, the transistors involved, are able to perform at incredibly high frequencies (>>GHz), but the laws of Physics, would still leave an immovable (but small amount of) capacitance at the 'input' (gate or whatever). So by introducing positive feedback, from the appropriate part of the circuit ('output'). It is making the ultra fast transistor, actually considerably remove the 'input' capacitance (effects/drag), and hence allow much high bandwidths (by virtually removing much of the inputs capacitance).
It reminds me of how car power steering systems work, for removing the efforts of steering the wheels manually. Where the drag/friction/stiction/mass of the steering/wheels are analogous to a capacitance.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 19444
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: What's 50 fF?
« Reply #12 on: January 27, 2022, 12:30:13 pm »
Note that TANSTAAFL; you're improving the input, at expense to output loading, and eventually, stability (at some length [of the input structure], the input and bootstrap/shield will look like a 1/4 wave open stub transmission line, and you have an oscillator).  A similar phenomenon is attempting to use active negative capacitance elements to improve a circuit: as it turns out, the impedance converter just can't run as fast as you want, and the problem ultimately reduces to any old peaking / feedback solution, with respect to the ultimate limits of node impedance and device capability.

So, you can extend the envelope, maybe increasing bandwidth (at same input loading, flatness, etc.), or maybe you dramatically reduce input loading but sacrifice BW to do it (IIRC, Dr. Hobbs' example rolled off around 100MHz, despite using either low-GHz, or 10s-GHz, transistors -- depending on which thing I'm [mis]remembering, alas).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: MK14

Offline MK14

  • Super Contributor
  • ***
  • Posts: 3617
  • Country: gb
Re: What's 50 fF?
« Reply #13 on: January 27, 2022, 01:59:37 pm »
Note that TANSTAAFL; you're improving the input, at expense to output loading, and eventually, stability (at some length [of the input structure], the input and bootstrap/shield will look like a 1/4 wave open stub transmission line, and you have an oscillator).  A similar phenomenon is attempting to use active negative capacitance elements to improve a circuit: as it turns out, the impedance converter just can't run as fast as you want, and the problem ultimately reduces to any old peaking / feedback solution, with respect to the ultimate limits of node impedance and device capability.

So, you can extend the envelope, maybe increasing bandwidth (at same input loading, flatness, etc.), or maybe you dramatically reduce input loading but sacrifice BW to do it (IIRC, Dr. Hobbs' example rolled off around 100MHz, despite using either low-GHz, or 10s-GHz, transistors -- depending on which thing I'm [mis]remembering, alas).

Tim

While I was digesting your lovely post, I kept a reminder in my head to look up the technical term, which I'd never (remembered) hearing before 'TANSTAAFL', but it makes a lot of sense, now I looked it up.   :-DD

These explanations, have explained a number of puzzles I've had over a long period of time, as to why certain things couldn't/didn't exist in practice. I've sometimes seen these amazing looking transistors (a while ago, many seemed to have 4 leads coming out at 90 degrees between the leads). These days, especially (ignoring component shortages), mouthwatering specifications, such as 35GHz, yet not too expensive, perhaps $0.40 to $1.25 or so, depending on the exact part, and the quantity, bought.

So, why the heck can't we go and make cheap/small, discrete part, 2GHz NE555 timers, comparators, and so forth. But as I can see from your post, and the other explanations. Some circuits, can practicably made, such as microwave amplifiers, radar detectors (as used in cheap person detectors, e.g. burglar alarms), and certain other microwave things. But, the other things I listed, (presumably/intuitively now I understand it better), can't realistically be made at such high frequencies.
So, it is like going into an electronics lab, and them saying diagnose this very high frequency circuit for us. Feel free to use the 40GHz Oscilloscope on the bench, but we can only let you use this old/battered 50MHz probe with it, sorry.
So on the one hand, I'd be excited by the 40GHz Oscilloscope, but the 50MHz scope probe, with its (highly likely), very significant probe capacitance, would mean that I would have a job, using the scope beyond, 20MHz or so (ignoring trick question cheats).
So these 35GHz (or whatever they are these days) transistors are so tempting, yet would probably struggle to even reach the speeds of a normal (relatively slow) bjt, in most normal circuits. Apart from where those circuits which do allow extreme performance from them are used, along with the appropriate construction techniques, to go with it.

Even with ECL logic, many of which can do ridiculously high speeds, compared to stuff like TTL/HC and faster logic series (discrete), with some exceptions. The very vast bulk of the time, no one, uses ECL for very high speed logic. They usually stick to 74LS/74S (in the past, somewhat obsolete now), and some of the later, and sometimes faster logic series. Because of all the (rumored and probably true really), difficulties with making transmission lines all over the place, relative incompatibility with normal logic voltages, tricky power supply requirements and other possible difficulties.
So, the rare exceptions when ECL was used were things like very high frequency meters (a long time ago, higher freq meters used mixers/rf technology, to get into the many GHz bands, even if the ECL counters can't get anywhere near that high a freq, at that time), in the early divider stages, super high speed computers (e.g. Cray1), when that was the best/fastest technology available, in its day. But not really much else.
So these amazing 35GHz (or whatever) transistors, seem such a waste, if they can only be used in so few applications, and even then, it is very tricky to design with them, it seems. Otherwise, we could be using 10GHz or 20GHz single thread cpus, by now.
 

Offline fcb

  • Super Contributor
  • ***
  • Posts: 2103
  • Country: gb
  • Test instrument designer/G1YWC
    • Electron Plus
Re: What's 50 fF?
« Reply #14 on: January 27, 2022, 02:10:26 pm »
It would like about half of one of these

https://www.digikey.ca/en/products/detail/murata-electronics/GCQ1555C1HR10BB01D/7803156

I'm only kinda kidding, I don't have a good idea of what capacitance "means" outside of a physical part on a board, you know?

But this sounds like some Ka stuff. This is where you need Southwest Microwave connectors that need screwing down to the board because the quality of the solder joint from the SMA to a microstrip determines the return loss so just don't put solder.
0.1pF +/- 0.1pF  |O
https://electron.plus Power Analysers, VI Signature Testers, Voltage References, Picoammeters, Curve Tracers.
 

Offline MK14

  • Super Contributor
  • ***
  • Posts: 3617
  • Country: gb
Re: What's 50 fF?
« Reply #15 on: January 27, 2022, 02:24:01 pm »
0.1pF +/- 0.1pF  |O

It's easy. Just bin the value on the production line, and then unchangingly fix/freeze, the other parameters, such as the weather, humidity, precise temperature, stop all aging, use magic to fix drift and always keep it orientated in exactly the same place.
 

Offline mawyatt

  • Super Contributor
  • ***
  • Posts: 1739
  • Country: us
Re: What's 50 fF?
« Reply #16 on: January 27, 2022, 02:58:06 pm »
A similar phenomenon is attempting to use active negative capacitance elements to improve a circuit: as it turns out, the impedance converter just can't run as fast as you want, and the problem ultimately reduces to any old peaking / feedback solution, with respect to the ultimate limits of node impedance and device capability.
Tim

We did some work awhile back with Non-Foster Networks and using NICs to create negative capacitances, also had the benefit of the key research principles behind this employed within the same company (their company was acquired, like ours). Anyway, we had the benefit of fractional THz SiGe transistors, which let us push this much higher than was open published, the main use was for very broadband physically small antennas.

Interesting stuff indeed!!!

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
The following users thanked this post: RoGeorge, MK14

Offline mawyatt

  • Super Contributor
  • ***
  • Posts: 1739
  • Country: us
Re: What's 50 fF?
« Reply #17 on: January 27, 2022, 03:46:38 pm »

These explanations, have explained a number of puzzles I've had over a long period of time, as to why certain things couldn't/didn't exist in practice. I've sometimes seen these amazing looking transistors (a while ago, many seemed to have 4 leads coming out at 90 degrees between the leads). These days, especially (ignoring component shortages), mouthwatering specifications, such as 35GHz, yet not too expensive, perhaps $0.40 to $1.25 or so, depending on the exact part, and the quantity, bought.

The problem with very high speed packaged transistors is the package almost dictates the ultimate performance. Here's where an IC solution allows much better performance because of the physical dimensions involved.

Quote
So, why the heck can't we go and make cheap/small, discrete part, 2GHz NE555 timers, comparators, and so forth. But as I can see from your post, and the other explanations. Some circuits, can practicably made, such as microwave amplifiers, radar detectors (as used in cheap person detectors, e.g. burglar alarms), and certain other microwave things. But, the other things I listed, (presumably/intuitively now I understand it better), can't realistically be made at such high frequencies.
So, it is like going into an electronics lab, and them saying diagnose this very high frequency circuit for us. Feel free to use the 40GHz Oscilloscope on the bench, but we can only let you use this old/battered 50MHz probe with it, sorry.
So on the one hand, I'd be excited by the 40GHz Oscilloscope, but the 50MHz scope probe, with its (highly likely), very significant probe capacitance, would mean that I would have a job, using the scope beyond, 20MHz or so (ignoring trick question cheats).
So these 35GHz (or whatever they are these days) transistors are so tempting, yet would probably struggle to even reach the speeds of a normal (relatively slow) bjt, in most normal circuits. Apart from where those circuits which do allow extreme performance from them are used, along with the appropriate construction techniques, to go with it.

One could indeed make a 555 with some fast transistors, however the performance would be limited by the package, and with other methods to achieve nanosecond or faster pulses likely not of much practical use. However, this would be an interesting project of a EE student!!
Quote
Even with ECL logic, many of which can do ridiculously high speeds, compared to stuff like TTL/HC and faster logic series (discrete), with some exceptions. The very vast bulk of the time, no one, uses ECL for very high speed logic. They usually stick to 74LS/74S (in the past, somewhat obsolete now), and some of the later, and sometimes faster logic series. Because of all the (rumored and probably true really), difficulties with making transmission lines all over the place, relative incompatibility with normal logic voltages, tricky power supply requirements and other possible difficulties.
So, the rare exceptions when ECL was used were things like very high frequency meters (a long time ago, higher freq meters used mixers/rf technology, to get into the many GHz bands, even if the ECL counters can't get anywhere near that high a freq, at that time), in the early divider stages, super high speed computers (e.g. Cray1), when that was the best/fastest technology available, in its day. But not really much else.
So these amazing 35GHz (or whatever) transistors, seem such a waste, if they can only be used in so few applications, and even then, it is very tricky to design with them, it seems. Otherwise, we could be using 10GHz or 20GHz single thread cpus, by now.

Yes ECL and CML (also SCL) is a pain to work with, unless you really need the speed. CMOS 74AC and ACT logic is pretty easy to use for moderately high speeds and a good replacement for the older 74LS and S Logic.

An interesting story behind the development of Silicon Germanium HBT bipolar technology goes back to IBM in the 80s, they wanted to get a faster IBM360 Mainframe which was done with ECL and CML, thus wanted a faster bipolar device. Drs Bernard Meyerson and David Harame were tasked with developing a faster bipolar transistor, and they developed the UHVCD technique for depositing the Ge implants in a Si process, thus creating a SiGe HBT. These SiGe devices were considerably faster than equivalent Si devices, but IBM had decided to go with a parallel CMOS CPU solution rather than a single fast CPU and thus looked for another market for these very fast HBT devices. That market turned out to be in the RF/MW/MMW and other very fast analog uses, as well as the orginial intended ECL/CML use. Various flavors of the BiCMOS SiGe process evolved over the decades producing HBT devices with ~500GHz Ft.

Long ago we began taking advantage of faster devices employing older analog techniques with newer faster transistors. DARPA picked up on this and funded the development of an InP 100GHz Op-Amp where one could use active filter techniques at GHz frequencies and so on. We utilized the old vacuum tube technology Cherry-Hooper amplifier topology in various flavors of SiGe and InP HBTs and also the 1970s Caprio Cell for ultra-low distortion use, combining both in InP with a new type mixer with a simulated ~50dBm OIP.

So lots of interest in employing these fast HBTs devices in some of the older techniques, if one can get around the package limitations where an integrated solution benefits!!

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
The following users thanked this post: RoGeorge, MK14

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 19444
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: What's 50 fF?
« Reply #18 on: January 27, 2022, 04:20:52 pm »
These explanations, have explained a number of puzzles I've had over a long period of time, as to why certain things couldn't/didn't exist in practice. I've sometimes seen these amazing looking transistors (a while ago, many seemed to have 4 leads coming out at 90 degrees between the leads). These days, especially (ignoring component shortages), mouthwatering specifications, such as 35GHz, yet not too expensive, perhaps $0.40 to $1.25 or so, depending on the exact part, and the quantity, bought.

So, why the heck can't we go and make cheap/small, discrete part, 2GHz NE555 timers, comparators, and so forth. But as I can see from your post, and the other explanations. Some circuits, can practicably made, such as microwave amplifiers, radar detectors (as used in cheap person detectors, e.g. burglar alarms), and certain other microwave things. But, the other things I listed, (presumably/intuitively now I understand it better), can't realistically be made at such high frequencies.
So, it is like going into an electronics lab, and them saying diagnose this very high frequency circuit for us. Feel free to use the 40GHz Oscilloscope on the bench, but we can only let you use this old/battered 50MHz probe with it, sorry.

Keep in mind, what's possible is far broader than what's available.  What's available, is possible, AND has been developed into a product, AND is a commercially successful product (at least, more or less, depending on how much investment or loss-leading the manufacturer might be using with that product/line..).  If you want just the first one, pick up some time at a research lab, and bright a hearty budget; the second, move it out of the research lab into a commercial development lab; only the last one is available from suppliers, and at that, not necessarily at any kind of affordable cost still.  There's a lot of hurdles to pass before something winds up on Digi-Key!

So, 2GHz 555s are perfectly feasible, I'm pretty sure -- but no one wants them.  You can get close right now, with an equivalent made from ECL or what have you -- check out what kinds of timers, one-shots, etc. are available.  I don't know much about ECL families; I'm guessing there are some?

I know, it was just an example of course.  But, that said, it can be interesting to do a design experiment like that -- follow through and see how close you can actually get, off-the-shelf, and what the performance specs would be.  (For my part, I've designed a current-limiting switch using discrete transistors, with about 3 times lower power consumption than any building-block solution -- trouble is, the sheer parts count means it costs more.  It's not that such a function is impossible in IC, it would trivially do 10x better than my version; there just doesn't happen to be a commercial offering of it!)

And there's lots of reasons besides profitability:
- Such a fast timer is a liability, not an asset, in most applications -- you need to manage the sub-100ps risetime, somehow not letting it radiate, while also hopefully getting all your timing components close enough to the poor chip that stray inductances (transmission line lengths, really) don't completely smash the response, making it some mixture of LC oscillator rather than just the RC that was intended.
- The operating voltage must be quite low, or the power dissipation will be extraordinary (I'd guess most such circuits run at 2.5V or below, if CMOS, and ECL a bit higher).  If you wanted say 5V of output swing, or even 15V, at 2GHz -- you'll be paying for it with a whole chain of power amplifiers!
- And again, it's about transmission lines -- their use is an unavoidable necessity.  You can't drive a net and wait for it to settle, like you can with CMOS or TTL.  Practical nets are some cm long, while the 1/4 wave is some mm (depending on whether you're counting just the edge (harmonics), or the whole waveform as such).  It's always waves in flight.  So you can't make an assumption like, "this pin will draw some current during the edge, but it'll eventually settle down, so we don't need to waste much space on power dissipation for these output transistors" -- instead, those waves need to dump into a (termination) resistor eventually, lest they come back around and dirty up the signal.
- And who needs an RC timer?  Most applications up there are radio-related, so, very precise frequency control is required.  (Not all of them; spread spectrum might be a virtue for CPU clocking for example!)  You're only going to do that with a quartz reference and PLL multiplier.

But outside board-level components -- there are applications on-chip.  Most PLLs, AFAIK, are some form of this.  Greatly simplified, since they don't need the versatile trigger/threshold function, but in the general sense that it's an RC circuit, yeah.  Typical example is a ring oscillator, where the resistance is some combination of device transconductance and Rds(on), and capacitance is node / body C.  By varying the bias current into the amp stages / inverters, the frequency is controlled.  Hook that to an error amp and phase detector, and you've got yourself a PLL.

So, in a sense, you're not wrong, about thinking about something like that -- it's just not something you'll see in catalogs, but very much out of sight, tossed in a boring corner of many ICs, just another piece of invisible fabric supporting a vastly more complex whole.


Regarding clock speeds in general -- fT can be seen as an upper limit for edge rates or propagation delays; only the simplest circuits can run at some fraction of fT, or indeed above it (usually using distributed amplifiers).  CPUs have to run a small fraction of this rate, due to the complexity of the gate chains, and bus lengths, used to implement them.  Only very simple CPUs could run that fast (sub-100GHz say), and they'd be utterly useless for any kind of real work (an 8051 running this fast, is absolutely no competition against a modern 64-bit machine at 4GHz).

Kind of a small example of this, and heh, a bit less useless, actually a practical example.  Compare PIC to AVR for example: PIC is an ancient 8-bit (give or take extensions) architecture, based on a 4-clock (mostly?) instruction cycle.  AVR is (mostly) single-cycle.  A lot of PICs run up to 64MHz, for 16MIPS performance; a lot of AVRs run up to 20MHz, for ~20MIPS performance.  Both are 8-bit, both have various peripherals, memory (and give or take penalties for accessing certain memory spaces, or competing with DMA bus access, etc.), and, they're fairly close, considering.  Mind, these are 3.3-5V fabbed parts.

Or compare further to non-pipelined ARMs (Cortex M0 something or other).  Typically topping out at 30-60MHz, 3.3V operation.  A bit finer fab scale, but much wider buses, far more powerful as a result.  (Adding in pipelines and caches, operation blasts up to 200MHz or so; although some/most of these may be internal low-voltage core designs, I'm not sure.)


Or taking things in yet another direction -- there was a fairly reasonable op-amp I was looking at some years ago, I forget what part, something Analog Devices.  Somewhat over 20MHz GBW, precision inputs, RRIO, 16V supply limit or something like that, I think -- competitive price too.  The hook is this: supply current is just a bit lower than everyone else's, oh and by the way they're using their proprietary SiGe process to do it (XFCB I think?).  Bwha!?  Yes indeed, using seemingly RF-geared tech just to get a higher figure of merit (namely GBW/Iq: in most things, opamps, comparators, etc., you pay for bandwidth with bias current) than its conventional relatives.  And unlike CMOS which has to run at lower voltages for greater speed (there are indeed many fantastic <=5V or <=3.3V amps, comparators, etc. out there in CMOS), HBTs (SiGe) are perfectly comfortable mixed with conventional BiCMOS designs.

So they can sneak in that high technology, right under your nose, in other ways too. :D


Quote
So on the one hand, I'd be excited by the 40GHz Oscilloscope, but the 50MHz scope probe, with its (highly likely), very significant probe capacitance, would mean that I would have a job, using the scope beyond, 20MHz or so (ignoring trick question cheats).
So these 35GHz (or whatever they are these days) transistors are so tempting, yet would probably struggle to even reach the speeds of a normal (relatively slow) bjt, in most normal circuits. Apart from where those circuits which do allow extreme performance from them are used, along with the appropriate construction techniques, to go with it.

Yeah -- the probe itself wouldn't be necessarily a problem for sheer frequency response, but flatness along with it, good luck.  That's why e.g. 500MHz probes have smaller ground leads and tips, and pretty much anything above that you need either a special probe, or just route the damn signal into a coax connector and view it directly.

And, again about feasibility: 40GHz scopes do indeed exist; they're very complex to make, and though there isn't much demand for them -- that's paid for by the serious need customers have for such equipment.  And yeah, you're definitely using the right hardware to hook up one of those beasts, probes are irrelevant up there. :D

Also, oh good, mawyatt's joined the thread.  Definitely take his real IC experience over my mere suppositions of the market. :)

Tim
« Last Edit: January 27, 2022, 04:22:26 pm by T3sl4co1l »
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: MK14

Offline Bud

  • Super Contributor
  • ***
  • Posts: 5509
  • Country: ca
Re: What's 50 fF?
« Reply #19 on: January 27, 2022, 04:53:18 pm »
50 fF is close to fringing capacitance of an open SMA connector. Do not know how easy that value of capacitance can be realized on a chip.
Facebook-free life and Rigol-free shack.
 

Offline mawyatt

  • Super Contributor
  • ***
  • Posts: 1739
  • Country: us
Re: What's 50 fF?
« Reply #20 on: January 27, 2022, 05:02:58 pm »
Another important aspect of dealing with higher speed devices is the model. For SiGe HBT this started out with VBIC which proved inadequate in representing the device at high current densities at high speeds, MEXTRAM and HICUM HBT models have better fidelity under these conditions. For CMOS the BSIM4 model had a fatal flaw in the channel conductance, and later Berkley developed the BSIM6 model to address this.

Was told by some IBM folks that a good SiGe HBT device model in their BiCMOS process cost ~$1M to develop, so actual usage must be high to support this model investment. Keysight wasn't satisfied with the standard HBT device model with their InP process, so they developed a custom HBT model. You can only imagine how much investment this required.

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 

Offline mawyatt

  • Super Contributor
  • ***
  • Posts: 1739
  • Country: us
Re: What's 50 fF?
« Reply #21 on: January 27, 2022, 05:04:40 pm »
50 fF is close to fringing capacitance of an open SMA connector. Do not know how easy that value of capacitance can be realized on a chip.

On chip this can be easily achieved, the issue is getting on/off the chip, even flip-chip with ball bonds.

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 

Offline SiliconWizard

  • Super Contributor
  • ***
  • Posts: 9494
  • Country: fr
Re: What's 50 fF?
« Reply #22 on: January 27, 2022, 05:49:01 pm »
Yep. But what kind of application would require such a low capacitance *off-chip*?
 

Offline ezalys

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
Re: What's 50 fF?
« Reply #23 on: January 27, 2022, 09:56:53 pm »
This is for research on van der waals heterostructures. I can’t put transistors on the chip containing the sample. Whatever I make has to be on the tip of a probe, or glued on top of the sample I’m provided.
 

Offline ezalys

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
Re: What's 50 fF?
« Reply #24 on: January 27, 2022, 10:01:23 pm »
And yes, we’ve considered bump bonding. It’s just that I don’t know where I can get discrete transistors that have been bumped. We also have particular HEMTs we like. We might try an SET, which we can make and bump.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf