I'm currently playing around with discrete transistor logic gates and now want to create a tri-state buffer.

The only circuit that I found that kind-of works is this one, but it inverts my input:

inverting tristate buffer

Putting an inverter in front of it, will cause the T2 to be open, and therefore connect the output to the ground, which is not really what I want for a tristate buffer.

Putting an inverter behind the gate will cause the output to be connected to ground via the inverter-gate. Also not what I want.

Is there a non-inverting tristate circuit which I can build with NPN/PNP BJT transistors only?

  • \$\begingroup\$ well... you could always just use two of those inverters. \$\endgroup\$
    – Hearth
    Apr 29, 2017 at 23:40
  • 1
    \$\begingroup\$ Or could just run the inverter output to pin 2 and the data input to pin 6. \$\endgroup\$ Apr 29, 2017 at 23:42
  • \$\begingroup\$ @Felthry what would be the purpose of that? \$\endgroup\$
    – SkaveRat
    Apr 29, 2017 at 23:50
  • \$\begingroup\$ @WhatRoughBeast I'm not sure I understand. Can you draw me a circuit? \$\endgroup\$
    – SkaveRat
    Apr 29, 2017 at 23:52
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    \$\begingroup\$ Use two inverting buffers. That makes one non-inverting buffer, with twice the propagation delay. \$\endgroup\$
    – Hearth
    Apr 29, 2017 at 23:53

1 Answer 1


Your linked circuit shows what is essentially a 74125 buffer on the left side. TI's SN74125/126 datasheet shows how the TTL tri-state buffers are implemented (the only difference between '125 and '126 is the polarity of the tri-state input):

SN74125/SN75126 schematic

(TTL chips used only NPN transistors, so the output cannot go up VCC.)

  • \$\begingroup\$ so if I understand correctly, there is no simpler circuit with BJT transistors only? \$\endgroup\$
    – SkaveRat
    May 8, 2017 at 14:07
  • \$\begingroup\$ In the Good Old Times™, logic chips were made only with NPN transistors. I guess PNPs would make it easer. (An open-collector buffer would be trivial.) \$\endgroup\$
    – CL.
    May 8, 2017 at 14:43
  • \$\begingroup\$ PNP wouldn't be a problem. But as I want to create a bigger logic circuit, having several tristate buffers made from a dozend transistors each, would be quite a pain. That's why I'm looking for a simple NPN/PNP BJT cuircuit. Now looking into open collector stuff, to check if I can use that instead \$\endgroup\$
    – SkaveRat
    May 8, 2017 at 15:46

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