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Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

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Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#11
post #3

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 troubl…

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)?

Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#12
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?

Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#13
post #3

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 troubl…

> 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.

Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#14
post #5

Earlier quoted context omitted.

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.

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.

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?

Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#17

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).

circuitlab.com is great for simple designs. Not as powerful as some SPICE simulators, but amazingly convenient.

Re: Variations of Two-Transistor Circuits: A Tribute to the Versatility of MOSFETs

#19
post #17

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).

circuitlab.com is great for simple designs. Not as powerful as some SPICE simulators, but amazingly convenient.

(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://electronics.stackexchange.com/questions/137161/why-a... and here https://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.

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