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I'm curious, with a transistor being equivalent to two diodes, could one wire a single PNP with a pull down diode to be an OR gate? My thinking is as follows:

enter image description here

Is this right, or wrong. Why?

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A tidy version of the OP's 'OR' gate.

It might work in certain circumstances. The problem is that the base current affects the emitter-collector resistance.

  • If A is high then an emitter-base current will flow.
  • This will turn on Q1 and provide a low-resistance path between A and B.
  • What happens next will depend on the output impedances of A and B. If they are the same then Q1 will tend to pull them together towards mid-supply voltage.

It's not going to work as you planned.

schematic

simulate this circuit

Figure 2. Simulation circuit.

Running a simulation using the CircuitLab tool results in the following readings for R1 = R2 at 1k and 100 Ω:

     1k         100 Ω
A    1.927 V    2.469 V
B    1.909 V    2.365 V
Q    1.163 V    1.663 V

Note that at lower source resistance the effect of R3 is less and A and B settle down close to mid-supply. Q is 0.7 V below the emitter voltage as would be expected.

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    \$\begingroup\$ +1 and you beat me by about 5 seconds. I'd add, if the OP is considering this for a quick and small fix that double diodes are available in the same sort of packages. \$\endgroup\$ – Trevor_G Sep 18 '17 at 16:09
  • \$\begingroup\$ @Trevor what might that ic be called? \$\endgroup\$ – tuskiomi Sep 18 '17 at 16:13
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    \$\begingroup\$ See diodes.com/assets/Datasheets/ds12003.pdf for an example. I've also updated my answer with sim results. \$\endgroup\$ – Transistor Sep 18 '17 at 16:16
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    \$\begingroup\$ @tuskiomi "Dual diode-common cathode" \$\endgroup\$ – Trevor_G Sep 18 '17 at 16:22

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