Transistor Logic
Monday, April 5th, 2010There are a few things in the world which are being lost on new hobbyists, one being the importance of knowing why an IC does what it does. I am going to risk sounding rude and arrogant, but any fool can strap a couple of resistors to an IC and make an audio amplifier of marginal (mobile device) quality. Logic, too, has gone this direction, where no real knowledge of the internals is necessary, only a basic understanding of the truth tables. For debugging a circuit, this is fine. But for building a new one, it is important to understand that a NAND gate is NOT an AND gate with an inverter after it. It is, actually, an AND gate where your transistors are switching ground. As it is later than I thought, I will leave you with the there fundamental logic circuits:
It is important to note that these circuits can be used like building blocks, just like the conventional symbols shown below them. However, it will be demonstrated that a lot of parts and time can be saved by using much more efficient means of achieving the same result. Your power supply will also be grateful (This is, after all, TTL logic). Also, you will not see the OR gate drawn that way again, as it was merely to demonstrate the current paths that made it an OR.