4QD-TEC
Circuits Archive

4QD-TEC: Electronics Circuits Reference Archive
Some logic circuits
using discrete components
There have been many times when I've been
designing machinery controls where I have needed a logic function but only one
so it's not worth using a quad logic gate. Here are some of the ways I use. Look
carefully at them and you may gain a new perspective on logic and truth tables!

The first thing that may strike you are that the circuits are all in pairs
(except for the EXOR functions). The funny thing is that the pairs are pretty
nearly identical. The NAND gate is exactly the same configuration as the NOR
function! The same for the other pairs.

Don't tell me you thought NAND was a different function from NOR! Have a look
at these circuits: the pairs work in exactly the same way, except that one uses
a positive logic and the other a negative logic!

The pairs are exactly the same - if you invert all polarities. I'll bet this
is something they didn't teach you in college!

Of course there are limitations to these circuits: in a lot of them the
output is high in the appropriate circumstances, but not as high as the inputs.
They are lossy, so you cannot go stacking them instead of IC logic.

You can't expect to mix them with TTL logic either, because diodes drop 1/2
volt so they will degrade the noise immunity.

My favourite is the one-transistor EXOR gate!

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Page's Author: Richard Torrens

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