I only have 5 PNP transistors and I want to build two AND gates. (for a T Flip-Flop) What would be the simplest way to make an AND gate using PNP transistors? (It can't use more than 2 transistors)

This is the only way I found, but it uses way too many transistors. enter image description here

  • \$\begingroup\$ I would ask instead "how to make a TFF with these components". As the final application matters, not the presumable subcomponents. It is so-called XY-problem. \$\endgroup\$ – Eugene Sh. Aug 19 '15 at 14:57
  • \$\begingroup\$ @EugeneSh. This appears to be a follow on from a previous question electronics.stackexchange.com/questions/185350/…. Now its being made more difficult by an aribtary PNP only transistors. \$\endgroup\$ – JIm Dearden Aug 19 '15 at 17:58

A good way to build gates with limited numbers of transistors is to use RTL (resistor-transistor logic) techniques. For example, here is a NAND gate, which can be converted into an AND gate with an inverter:


simulate this circuit – Schematic created using CircuitLab

The fan-in and fan-out capabilities of this type of logic are not great, which is why it was abandoned once the development of ICs made the incremental cost of transistors essentially negligible. But it can be used to build moderately complex logic circuits if you don't care too much about the speed.


You could do this for a simple gate:

Simple AND Gate

The function of this gate is LOW OR LOW = LOW. From DeMorgan's theorem, I can invert all inputs and outputs and change the gate type, so I get HIGH and HIGH = HIGH.

The image quality is not particularly good. Both collectors go to ground and I have not shown series input resistors. I would probably use about 100 ohms for starters. Both the pullup resistors go to the power rail.

  • \$\begingroup\$ This is actually a rather poor design for a gate. While it demonstrates the principle in isolation, it can't be used to build more complex circuits. An important property of a practical gate is that it must have gain -- if you're using voltage for signaling, it must have voltage gain, and if you're using current, it must have current gain. Unfortunately, you're using voltage for signaling, but you've selected a circuit configuration (emitter follower) that has less than unity voltage gain. In fact, the output can never be lower than one \$V_{BE}\$ drop above the lowest input. \$\endgroup\$ – Dave Tweed Aug 19 '15 at 15:13
  • \$\begingroup\$ @DaveTweed Yes, but the question didn't ask for good: it asked for simple. \$\endgroup\$ – Phil Frost Aug 19 '15 at 15:23
  • \$\begingroup\$ @PhilFrost As long as I can use it for a T Flip-Flop \$\endgroup\$ – Nicolas Aug 19 '15 at 15:25
  • 2
    \$\begingroup\$ @PhilFrost: The point is, a gate can be both simple AND good. \$\endgroup\$ – Dave Tweed Aug 19 '15 at 15:32
  • \$\begingroup\$ @Philfrost: Thank you indeed. Simple is what was asked for with 2 pnp devices. I think I provided that. \$\endgroup\$ – Peter Smith Aug 25 '15 at 19:03

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