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.
Best Answer
If you are making gates out of discrete transistors, diodes and resistors, you can make an inverter with one transistor, a NAND with two transistors, or with diodes.
If it is in a integrated circuit, where a resistor is more complex to make than a transistor, more transistors are used either for polarisation (for example ni old NMOS), or now as part of more complex circuits with complementary logic such as CMOS. A basic CMOS inverter uses 2 transistors. Inputs can be added by using transistors with several gate contacts.
It works when that gate is one among many others, driving a few similar gates. For large fan-out gates, or when these gates are sold as components, like a 7400 or a HC4000, there are additional transistors, with different geometries for conditioning the input, multiply the output current rating.