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I'm trying to simulate a NAND gate using transistors and I have remade the circuit as it was in the notebook, but the simulator shows me the error in the title. This is the circuit scheme:

enter image description here

What is the meaning of that error? What is not properly placed, even though this is the scheme from the notebook?

Kudor
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    Does this answer your question? [Modelling and simulating multiple-collector/emitter BJTs](https://electronics.stackexchange.com/questions/300979/modelling-and-simulating-multiple-collector-emitter-bjts) – Ferrybig Jan 17 '23 at 09:24

2 Answers2

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Your 0V and +5V sources are directly attached to each other at the NPN's emitter.

Falstad shows the error as a "path to ground" because one of the sources was 0V. If you had two sources with nonzero voltages it would show an error about exceeding max current.

Polynomial
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  • Yes that was the issue, but the scheme in my notebook shows the transistor with 2 emitter branches, each one of them connected to one of the sources. Was I not correct connecting both sources to the same emitter and should I find another transistor with 2 emitters? – Kudor Jan 16 '23 at 16:18
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    Yes. A [multi-emitter transistor](https://en.wikipedia.org/wiki/Multiple-emitter_transistor) is different to just connecting two wires to a single emitter. Falstad does not have multi-emitter bipolar transistor models; you'll need to switch to LTSpice or similar. – Polynomial Jan 16 '23 at 16:20
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    I think you could simulate the multi-emitter transistor with two diodes though? like make each emitter a diode coming out of the same actual emitter. It's not how it actually works but I think it still makes the circuit work. Or two transistors with the collectors and bases connected together. – user253751 Jan 16 '23 at 16:23
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    @user253751 2 transistors with the base and collector connected have an higher dissipation and an higher switching speed compared to using a proper multi-emitter transistor. But since they want to simulate a logic gate, it is a valid approach: https://www.falstad.com/circuit/circuitjs.html?startCircuit=ttlnand.txt – Ferrybig Jan 17 '23 at 08:53
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See the words I put in the cropped image below: -

enter image description here

Andy aka
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