[HN Gopher] Variations of Two-Transistor Circuits: A Tribute to ...
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Variations of Two-Transistor Circuits: A Tribute to the Versatility
of MOSFETs
Author : jrwan
Score : 71 points
Date : 2022-06-21 15:15 UTC (7 hours ago)
(HTM) web link (www.researchgate.net)
(TXT) w3m dump (www.researchgate.net)
| bombela wrote:
| If you happen to play with electronics, most circuits in the
| paper cannot be built at home realistically. Those circuits work
| best with MOSFETs built in silicon, with very precise tolerances.
|
| For example the first circuit; the inverter; when Vin is low, the
| bottom NMOS is off, the top PMOS is on. And vice versa when Vin
| is at the high voltage suitable for the devices.
|
| Statically this works pretty well. But the trouble appear during
| the transition of Vin between the two state.
|
| Un the middle of the range both MOSFETs are likely conducting,
| effectively producing a dead short!
|
| On silicon they will design both MOSFETs to control and reduce
| this effect as much as possible. If you attempt to reproduce
| this, you might be in for a magic smoke release moment.
|
| I did manage to somewhat reproduce this without a constant
| meltdown with tiny discrete MOSFETs by using two tiny ones with
| the most complementary specifications as possible. The goal was
| to have them with the least amount of overlap in conduction with
| regard to their respective inputs.
| amelius wrote:
| You can buy push-pull configured transistor pairs in a package.
| Gordonjcp wrote:
| > Un the middle of the range both MOSFETs are likely
| conducting, effectively producing a dead short!
|
| This is exactly the configuration described in the CD4069UBE
| datasheet, where single stages of the hex inverter are biased
| to the midpoint with a feedback resistor. This turns them into
| a surprisingly good programmable inverting amplifier, rather
| like an opamp without a non-inverting input.
| kayson wrote:
| The same thing would happen on silicon. It's just a question of
| transition time. I've seen power mosfet inverters on some board
| designs, but you're probably better off using a discrete logic
| inverter because it would be cheaper...
| Zondartul wrote:
| Can you use non-complimentary MOSFETS if they have a dead zone
| (i.e. both are closed for some middle voltage, resultig in
| high-z output)?
| bombela wrote:
| Yes, I was alluding to it in my post. I did successfully
| build a MOSFET driver (to drive a big MOSFET for a DC-DC
| converter) using two tiny MOSFETs (tiny as in sot23 package)
| in push-pull configuration. I found two MOSFETs that did
| barely overlap. It seems to work well. And the power
| dissipation was very reasonable at 42kHz. Note that I am
| merely a hobbyist.
|
| Using non well matched MOSFETs resulted in a rapid
| uncontrolled atomization of the MOSFETs.
| buescher wrote:
| I could have sworn I'd seen a reference to someone working out or
| cataloging all known-to-be-useful two-transistor circuits with
| bipolar transistors but I can't seem to find it now. Anyone know?
| kayson wrote:
| Some of these are standard, a few are genuinely clever, but many
| are bad designs and I'd never let someone tape them out in
| production.
|
| 2. Connecting the body terminal to something other than the
| highest voltage is dangerous and we rarely do it except in
| switches and diff pairs. We'd never do something like this. You
| can't even connect the nmos body to the gate unless you're using
| "deep" nwells which is an expensive process step that most people
| seem to avoid these days
|
| 3. I hate it when foundries do this in their standard cell
| libraries. Its an extremely weak pulldown and I've seen problems
| in production caused by using this structure. You can make a much
| better structure with a third transistor and positive feedback
| where devices still aren't connected to the supplies, but you get
| a stronger pull.
|
| 4-6. These are current-mode logic, but you never see them made
| this way because they're slow and high power. Instead they're
| usually made with a tail current source on the "bottom" and
| resistors on top, which keeps all transistors in a faster
| operating region. They get used often in RF clock dividers.
|
| 16. Again, connecting the body is dangerous, especially when the
| potential is somewhat unknown, but this does get used in
| extremely low power/low voltage applications.
|
| I didn't get through all of them, but it made me wonder - with
| two 4-terminal devices, how many possible configurations can you
| actually make?
| amelius wrote:
| > a few are genuinely clever
|
| Another perspective is that it would take a computer only a few
| minutes to generate all these circuits and test them for
| usefulness in a simulator.
| kayson wrote:
| That's harder than you might think. How would you test for
| all possible uses? Even if you looked at something
| fundamental like an N-port, you'd still have to define
| metrics for usefulness and many of the topologies in the
| paper would evade most obvious circuit metrics.
| mzs wrote:
| Which are clever?
| kayson wrote:
| 18 is cool. Folding is a really under-rated and under-
| utilized technique. Common-gate impedance matching is
| "standard", but 22 uses it to also generate differential
| current outputs which is cool - I imagine it sees more use in
| RF design. 23 is also interesting. 27 blew my mind. I'm not
| familiar with ultra low power techniques so that was cool to
| see.
| yababa_y wrote:
| The paper cites
| https://ieeexplore.ieee.org/abstract/document/8506644, which
| enumerates 582 possible topologies for two transistors, and
| 56280 for three. I imagine most are useless and/or dangerous.
| dekhn wrote:
| I like how I've been able to use MOSFETs as switches for the past
| 5 years while being almost completley unaware of the other things
| it can do. To me, it's a component that uses a small (low-
| voltage, low-current, low-resistance) signal to switch on a much
| larger path. Anything that makes it not behave like that idea is
| just problems for me (I spent a better part of a day debugging a
| circuit that works fine with a 5V arduino but not a 3.3V ESP32. I
| don't think I've even built a circuit with two transistors
| (multistage amp, flipflops...)
| aidenn0 wrote:
| Discrete power MOSFETs are only switches. Discrete linear-
| stable MOSFETs are also usable as linear amplifiers. TFA is
| about mosfets on ICs. When making your own ICs, you have
| precise control over a lot of the variables, _and_ the
| variables you have less precise control over are often going to
| be very similar for adjacent FETs on the same wafer, so you can
| do things with them that you can 't do with discrete MOSFETs.
| MisterTea wrote:
| What stinks is that linear use has waned so much that most
| lateral mosfets are no longer produced. Everything is
| vertical switching for PWM control.
| aidenn0 wrote:
| Wow, you aren't kidding. I just checked Onsemi and Vishay
| and neither makes lateral mosfets anymore. I'm surprised
| because many audio amplifiers still have a linear output
| stage, are those just always BJTs now?
| MisterTea wrote:
| Most of the linear amps these days are integrated devices
| that need a few supporting components and you feed a line
| level audio signal. Though you can still get decently
| sized BJT's and you can build your own amps.
| foobarian wrote:
| What layperson-friendly tools are out there to simulate simple
| analog circuits? For years now I've been going to the ancient app
| at https://www.falstad.com/circuit/ but wondering that surely
| there must be something that Real Electronics Builders use. (I
| intentionally did not say Engineers because I assume that would
| be covered by the pricy and/or UI-challenged tools like spice and
| company).
| drhodes wrote:
| There is JADE (for education), which can be integrated into a
| blog or for teaching on the web. It supports hierarchical
| custom analog and digital modules, a convenient way to create
| test vectors, (docs for that are floating around) This tool was
| used by students to build and grade the BETA CPU.
|
| https://computationstructures.org/exercises/sandboxes/jade.h...
| achr2 wrote:
| circuitlab.com is great for simple designs. Not as powerful as
| some SPICE simulators, but amazingly convenient.
| compumike wrote:
| (CircuitLab developer here.) Thank you for the kind words! :)
| Clickable link: https://www.circuitlab.com/ if anyone wants
| to give it a try.
|
| As far as the linked article "Fifty Nifty Variations of Two-
| Transistor Circuits: A tribute to the versatility of
| MOSFETs", I will mention that CircuitLab does NOT currently
| provide a MOSFET model that includes a separate body
| terminal.
|
| Instead, the body (also sometimes called the "back gate") and
| source terminals are assumed to always be internally
| connected in the CircuitLab MOSFET models, resulting in a
| three-terminal device.
|
| Almost all real-world discrete MOSFETs you can buy are also
| three-terminal (gate/drain/source), not four
| (gate/drain/source/body). Some discussion here https://electr
| onics.stackexchange.com/questions/137161/why-a... and here ht
| tps://electronics.stackexchange.com/questions/185109/mosfe...
| but not especially satisfying in my opinion.
|
| As a result, Figures 2, 16, 27, 29, 46, 49, and 50 can NOT
| currently be realized within CircuitLab.
| dhdc wrote:
| I'm afraid there really isn't any hobbiest simulators out
| there. A lot of these arrangements rely on the intrinsic
| characteristics and geometry of the MOSFETs used.
| nomel wrote:
| For simple circuits, LTspice: https://www.analog.com/en/design-
| center/design-tools-and-cal...
|
| And it's definitely used by real engineers. It's for making
| switching power supplies, so it can handle fairly complex
| circuits. I wouldn't venture into RF/high speed circuits with
| it though, since the included parasitics aren't sufficient.
|
| And, custom functions are supported, so, I believe, you can put
| whatever maths in a component (I've never ventured this far):
| https://ltwiki.org/index.php?title=B_sources_(complete_refer...
| achr2 wrote:
| This is a fun reminder that a MOSFET is useful as a transistor, a
| diode, and a capacitor.
| eternauta3k wrote:
| Also various types of sensors (magnetic, chemical,
| radiation...)
| hulitu wrote:
| And sometimes, as a fuse.
| jesuslop wrote:
| And resistor. And if etched in spiral the should've inductance,
| so also coils.
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