Design and Technology
My Own Personal Yellow Brick Road
Logic Families
Categories: Explorations

Digital ICs are classified into logic families depending on how they’re constructed on the inside. Some construction methods make the circuits run faster or work using lower currents or just by being able to fit more gates in.

There are loads of logic families but the two most famous (and the ones I’ll likely be using) are TTL and CMOS.

TTL stands for transistor transistor logic, and uses bipolar transistors to construct gates and amplifiers. You can fit more gates in and they’re cheaper to make but they draw a lot more power and need a specific 5V power supply to work. Most TTL IC chips use the 74xx or 74xxx part numbers e.g. the 7400 is a quad input NAND gate (there are 4 NAND gates contained withing the chip).

CMOS stands for complementary metal oxide semiconductor, this is a type of technology used to make MOSFETs (metal oxide semiconductor field effect transistors, what a mouthful, you can see why they shortened the name to CMOS!). They’re a bit more expensive to make but they draw less power are operate over a wider range of voltages. One problem with them is that they are very susceptible to static electricity so you have to be careful when handling them (I’ve already had one issue with static messing up my circuit). Chips in the 40xx series are part of the CMOS family.

TTL chips are not really used anywhere outside of schools now as CMOS chips can do the same jobs without the drawbacks.

Note (07/03/11) – B series CMOS chips have ‘buffered’ outputs to increase the voltage gain from input to output (Have a look at buffers here), this means that they have a faster response to input signal changes. 

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