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Ordinarily I think most people would build a multiplexer out of a NOT gate, two AND gates, and an OR gate, which would involve seven transistors, wouldn't it? I want to see if I can do a multiplexer with just four transistors. I've got three bit inputs, A, B, and Ua; and one bit output Out. I want Out to take the value of A if Ua is high, and I want Out to take the value of B if Ua is low.

I don't know how to create an image of a circuit diagram, so I'm going to have to just explain it. In addition to the three input bits and the one output bit mentioned, I have one input bit Vcc, which is a high voltage, and output bit Gnd, which is just ground, three transistors, Toa, Tob, Taa, and Tab, and a resistor Res. All transistors are the type where the base activates them if it is high except Tab; Tab's base activates that transistor when it is low.

I connect Vcc to both the collector of Toa and the collector of Tob. I connect A to the collector of Taa. I connect B to the collector of Tab. I connect Ua to both the base of Taa and the base of Tab. I connect the emitter of Taa to the base of Toa. I connect the emitter of Tab to the base of Tob. Then, I connect together the emitter of Toa, the emitter of Tob, the resister Res, and Out. Finally, I connect Res to Gnd.

If anyone cares to make a circuit diagram of this, that would be much appreciated. I have a diagram drawn in Notepad right here, but I suspect that if I tried to copy that into my article for posting it would get garbled.

All that said, would this work? I realize that the outputs of Taa and Tab (which are the emitters of those two transistors) are not getting amplified, but the output of the OR gate (made up of transistors Toa and Tob) is getting amplified, isn't it? So isn't that okay? If anybody has any input on this description, I would really like to hear it.

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  • \$\begingroup\$ You can add a schematic using this button in the editor; you will be taken to a tool where you can place, connect, and annotate components on a schematic page. Once you click the "save and insert" schematic, the code for the image will automatically be added to the editor; save the post itself and we will be able to see the schematic. Thanks! \$\endgroup\$ – Andrey Akhmetov Mar 23 at 23:03
  • \$\begingroup\$ This actually sounds like homework. If it weren't, you would specify \$V_\text{IH}\$, \$V_\text{IL}\$, \$V_\text{OH}\$, and \$V_\text{OL}\$ or some equivalent. Probably also output sink/source current compliances for the outputs and input current maximum sink/source for the inputs. Do you have any specifications? \$\endgroup\$ – jonk Mar 24 at 4:26
  • \$\begingroup\$ Andrey Akhmetov posted, "you will be taken to a tool where you can place, connect, and annotate components on a schematic page." I've seen schematics that have the symbol for a transistor but they have a little circle right where the wire connects with the base of the transistor. I'm pretty sure that means the resistance from the collector to the emitter goes low when the value applied to the base is low. I didn't see that transistor in the choices of transistor in the editor. Is there a way to generate such a transistor? \$\endgroup\$ – KevinSim Mar 29 at 0:22

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