# Can a single bjt PNP be an or gate?

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:

Is this right, or wrong. Why?

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.

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.

• +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. – Trevor_G Sep 18 '17 at 16:09
• @Trevor what might that ic be called? – tuskiomi Sep 18 '17 at 16:13
• See diodes.com/assets/Datasheets/ds12003.pdf for an example. I've also updated my answer with sim results. – Transistor Sep 18 '17 at 16:16
• @tuskiomi "Dual diode-common cathode" – Trevor_G Sep 18 '17 at 16:22