How many transistors are there in a logic gate?
If anybody asks me, I tell them:
- A NOT gate is 1 transistor.
- A NAND gate is 1 transistor per input.
- A NOR gate is 1 transistor per input.
- An AND gate is basically a NAND gate + a NOT gate, so it takes 1 transistor more than a NAND gate.
- Same for OR vs NOR.
- An XOR gate is built from multiple other gates, typically about ~4.
Sounds pretty reasonable, right? Thing is, I just realised... I have no idea why I think these are the correct numbers. I don't remember reading that somewhere or anything. I'm beginning to think maybe I just made it up. Sure, it sounds convincing; but that doesn't make it correct!
So what is the actual number of transistors per gate? I imagine it's different depending on which logic family we're discussing. (My brain is telling me that the numbers above are for TTL, and CMOS is exactly 2× the that, but again I don't know if there's actually a shred of truth to that.) If it does make a difference, I'm most interested in TTL and CMOS.