Hysteresis without Feedback

In general, you have to know the *history* of its input conditions to know its state.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin
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Only sequential logic. A Schmitt is purely combinatorial. It's outputs are a function of its input(s) - only. Need feedback to make it work.

Reply to
krw

Okay, so what's the state of a Schmitt trigger when its input lies between the rising and falling thresholds.

Which a Schmitt trigger has.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

Outputs are NOT a function of inputs alone when the input is in the hysteresis region.

There's stored charge, in the base of the turned-on transistor, that holds the state. That feedback loop is unstable without the stored charge limiting the slew rate. If you model the base Rbb and capacitance, you can initialize it so Spice always starts it up in your preferred state when the input is in the hysteresis region.

Reply to
whit3rd

In LTspice: .IC V(Q1B) = XX volts

Reply to
Steve Wilson

Wherever do you get that from?

I thought you were asking about a transformer core. Your question was excessively vague.

What is your point?

NT

Reply to
tabbypurr

Do pay attention.

Reply to
krw

Then it's not a Schmitt, it's a latch.

Don't be absurd.

Reply to
krw

I thought you were following the thread. I guess that's too much to expect.

Reply to
krw

Ach so!

I wanted something I could poke into white proto breadboard :)

piglet

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Reply to
Piglet

Precisely!

piglet

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Reply to
Piglet

OK, thanks. So it does come out of a diff pair like piglet said.

George h.

Reply to
George Herold

I am. But I don't read minds. Your question was so vague it was hopeless.

NT

Reply to
tabbypurr

Where did bird-brain krw get that idea? The 'subject at hand' is hysteresis. And he can't even spell 'Schmitt' ;-)

Yup. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

A latch (Eccles-Jordan bistable circuit) is a two-transistor circuit with positive feedback (collector-to-base coupling). A Schmitt is a two-transistor circuit with positive feedback (emitter coupling).

Both are bistable, both have hysteresis. The bistable nature implies an INTERNAL state, which is why hysteresis in a latch implements memory, as does hysteresis in a magnetic core or a Schmitt trigger.

The similarities are profound, the differences are... minor details.

Reply to
whit3rd

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