BJT Transistor Logic: 3 Input AND Gate

I am trying to couple 3(or more) transistors to use as a sort of logic gate but for some reason, if one of the transistors is closed and the other transistors are open the closed transistor still will still send power through the diode. Is there something I am missing? Does anyone have examples?

by sheonbanks
December 08, 2020

This is similar to the discussion here: which includes some working CircuitLab simulations.

by mrobbins
December 09, 2020

1 Answer

Answer by vanderghast

Your circuit has many flaws, see the comment of mrobbins here up.

Using PNP is quite easier: if one of their base is at 0V, that provides a path to ground with as low as 0.4 volt (in saturation, probably 0.8 in active mode), between the rail at the top and the rail at the bottom, which is NOT ENOUGH to light up the LED (1.6 V is generally required for a red one). So, all three have to be off (their base at a reasonably high voltage, 2.5 V seems to be ok, but that could depend on the exact PNP that you will use) in order that the LED turns on. Simple and reliable. Be sure that the resistor is ok, I haven't checked it ( I got timeout before I can fully check every cases).

+1 vote
by vanderghast
December 09, 2020

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