RL, remember that all replies have no "context", unless you provide one.? When you wrote, "That's the interesting thing", we don't know WHAT is the interesting thing, and we don't know which message you replied to.

(I think your reply is about why the capacitor made a difference here.? I don't think there is any really compelling reason; just that the circuit might go either way.? Adding the capacitor changes the network equations which can have a subtle effect.)

Andy

That's the interesting thing.

In this sim schematic, 1pF or a megohm output contact stops
the output at start-up.

RL

Switched load circuit is part of the issue. Disconnect it - it goes.

RL

Can't help you there. Works out of box with no alterations on this older
and one completely up to date LTspice install. Same behavior for normal
and alt solver.

RL

The non-starting state is also avoided if the switched load circuit is
disconnected.

I recognize that biasing the reference from the output is a bit iffy,
but it's not my circuit. I'm just? interested in the sim's behavior around
these apparently trifling differences and wondering what relationship
it has to reality.

RL

Yes, I do see that.? Connect R5 to VCC instead of the source of the MOSFET and it will work fine.? The circuit isn't "starting up" because it is in a stable state where the output of the op-amp is zero and both inputs are zero,? I understand that you want to provide current for the reference diode, D1, from a regulated source to minimize ripple, but you need to provide some startup current into D1 to get the whole circuit off dead zero,??

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I just found it. I don't know why I couldn't before. Its Rdson is much too high to drive those big caps The circuit works with the? AO6408

John,

2N7002 is a small N-channel MOSFET.

Andy

RL,

It would probably help to get rid of all the switching stuff (V3, R7, S1, V2) and concentrate on just the core circuit without the other stuff dirtying the signals.

Andy

John,

2N7002 is a small N-channel MOSFET.

Andy

I think it is a start-up problem, and that the feedback loop may have more than one stable operating point.? Without the capacitor, it comes up driving the MOSFET ON.? With the capacitor C4, it seems to start up with the MOSFET OFF (I don't know why the cap should make any difference for DC), and finds a stable operating point, with nothing to bring the MOSFET ON.? The op-amp's inverting input pin has the extra 10K resistor, and the input bias current of the op-amp seems to go up quite a lot when its inputs are both low, and this drives the op-amp's output low so it never turns ON the MOSFET.

I can't think of an obvious reason why C4 would affect that.? Maybe things are so very close to teetering either one way or the other, that the slightest change alters which way it goes.

You can change which "state" it comes up in, by adding an .IC statement.? That's why I think the circuit has multiple stable operating points.

Andy

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I can't imagine what this is supposed to do, even if it would run. Did you mean R7 to be 100 megohms?

A filter circuit simulation was offered in ABSE, uploaded as 45216607d4_puzzle.zip

If an output transient load section, or a component C4 is connected, the circuit
won't start. Gives miraculous ac filtering, when off, but curious to know the
simulator mechanics behind this odd effect in sim schematic.

RL

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Where is the 2N7002 in LTspice? I can't find it.

RL,

Please make sure to upload complete schematics including custom symbols.? Yours is missing a special custom symbol for the 2N7002.? (Why, I don't know.)

Andy

I think the "won't start" problem that RL refers to (without actually saying it), is that the output DC voltage does not come up.? The simulation runs, but the output voltage stays low (<1mV).

Roy, that is not what you see?

That is what I see when i used the 2N7002 symbol that comes with LTspice.? When C4 is removed, the output pops up to about 6V.

Andy

I replaced the missing Mosfet with another NMOS part.? The circuit starts up fine for me.

RL,

Please make sure to upload complete schematics including custom symbols.? Yours is missing a special custom symbol for the 2N7002.? (Why, I don't know.)

Andy

A filter circuit simulation was offered in ABSE, uploaded as 45216607d4_puzzle.zip

If an output transient load section, or a component C4 is connected, the circuit
won't start. Gives miraculous ac filtering, when off, but curious to know the
simulator mechanics behind this odd effect in sim schematic.

RL

On 02/03/2021 19:16, LV wrote:

I know about possibility for logic modelling in LTSpice...
But what if you need model 20000 gates?
Draw schematic in LTSpice?

I know this is an LTspice group, but I can't help thinking that for purely digital designs, you might be better off using a tool intended for such, like Logisim for example.

--
Regards,
Tony

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If you have a number of identical blocks of gates, there are two ways of defining one and automagically creating from it as many as you want, without drawing them all.

Hi Vlad
I know about possibility for logic modelling in LTSpice...
But what if you need model 20000 gates?
Draw schematic in LTSpice?

--
Best Regards
Victor

Victor wrote:

? ? "But what if you need model 20000 gates?
? ? ?Draw schematic in LTSpice?"

Is this still a question about reversible logic?? (If it isn't, please start a new Topic.)

LTspice can simulate many thousands of gates.? It has been done with tens of thousands, if not hundreds of thousands.? It is limited by your computer's memory, not by LTspice.? Using LTspice's built-in A-devices instead of full transistor-level circuits should make it both more compact and faster.

Obviously, one may want to use hierarchy (subcircuits) when doing thousands of gates.? Right?

Andy

Hi Vlad
I know about possibility for logic modelling in LTSpice...
But what if you need model 20000 gates?
Draw schematic in LTSpice?

--
Best Regards
Victor