# can I build a multiplexer with just four transistors?

by KevinSim   Last Updated May 15, 2019 23:25 PM

Some time back I found a circuit on the Internet that professed to build a logical OR gate with two transistors; accompanying it was a circuit that professed to build a logical AND gate, also with just two transistors.

But then I got to thinking that one transistor by itself is practically a logical AND gate, isn't it? If the voltage at an NPN transistor's base is high, then there will be no resistance between the collector and the emitter, so the voltage at the emitter will be high if the collector is high, and low if the collector is low. And if the voltage at the base is low, the voltage at the emitter will be low regardless of the voltage at the collector.

Similarly, if the voltage at a PNP transistor's base is low, then the voltage at the emitter will be roughly the voltage at the collector, while if the voltage at the base is high, then the voltage at the emitter will be low regardless of the voltage at the collector.

That being the case, could I build a multiplexer with just four transistors via the following circuit? The inputs are bits Co (for control), Il, and Ih, and the output is bit Ou. The semantics are that Ou will take the value of Il if Co is low, and the value of Ih if Co is high. Vcc is high voltage and Gnd is ground.

simulate this circuit – Schematic created using CircuitLab

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#### Answers 1

There are a lot of "tricks" to get a single stage non-inverting logic gate to work, but a lot of times they will suffer from some input or output characteristic that is undesirable. In this circuit's case, I can identify the following issues you may see (in no particular order, not exhaustive).

• When `Ih = Co = '1'`, the output voltage will be two Vbe drops lower than `Co`
• When `Il = '1', Co = '0'`, the output voltage will be one Vbe drop and Vsat lower than `Il`
• Output voltage is not restorative, meaning you couldn't chain circuits using this technique one after the other forever
• Output voltages differ for various conditions of `Ou = 1`
• When `Il = '1', Co = '0'`, `Il` will be shorted to `Co` through the base-emitter junction of Q2
• When `Ih = '0', Co = '1'`, `Ih` will be shorted to `Co` through the base-collector junction of Q1
• Input impedance depends on states of other inputs (and changes dramatically)
• Reliable operation may require pull-down resistors on the base of Q3 and Q4
• Depending on voltage levels, reverse Vbe may be exceeded (typically -5V)

So it can probably be made to work, but there's a lot of issues with using it as a normal logic gate. You would likely need to test circuit behavior in every state of this gate, monitoring voltages and currents. If I grabbed a standard logic gate (TTL, CMOS, etc...), then I would generally expect none of these issues to really cause problems.

W5VO
May 15, 2019 23:13 PM

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