Bipolar transistors are ANDN gates !

A project log for Hackaday TTLers

Where DIY DIP/SSI/MSI CPU makers meet and discuss other TLA (three letters acronyms)

yann-guidon-ygdesYann Guidon / YGDES 09/10/2018 at 12:413 Comments

In the log Bipolar XOR gate with only 2 transistors  we see the legendary interlocked transistor gate :

Then this  HAD post   describes a very clever driving scheme :

Then it became clear... Under the right driving situation, a transistor (with its base resistor) is a ANDN gate !

Why does it matter ? Because this could greatly help to implement circuits such as this 7-segments decoder:

This is a sort of "slow" circuit where parts count matters more than speed (hard saturation is not an issue) so hacks like this will help a lot :-)

For the above "arbitrary logic" array, wired-oring will also save parts. All we have to do is "solve/reduce with ANDN". Who wants to play that game with me ? :-D


Tim wrote 04/01/2020 at 06:27 point

For this it is important to keep in mind that bipolar transistors also work in reverse mode. So the input/output isolation of the ANDN gate is not always given. I almost fell for this several times, but luckily i used spice at first to simulate my designs.

  Are you sure? yes | no

roelh wrote 09/10/2018 at 15:24 point

In my project I use both: the 2-transistor XOR gate and the ANDN gates. Simply said, when used with base resistors it is RTL logic, and without base resistors it is DCTL logic. For 7-segments I would try DCTL logic because it hardly uses any resistors....

  Are you sure? yes | no

Yann Guidon / YGDES wrote 09/10/2018 at 15:45 point

but resistors are not a real problem, and it's good to keep the base current in check...

Anyway, I'm really looking forward to read your next logs about your cool ALU :-D

  Are you sure? yes | no