Even though I have a degree in electrical engineering, and have done logic design professionally, I have never designed or built my own gates. I never really saw the need to do so.
Well, about a month and a half ago, a friend approached me about coming up with a project for high school kids who were learning how computers work. He and a cohort got the idea in their heads that they wanted the kids to actually build a logic gate out of transistors.
Over lunch on Friday, we set about discussing how to do this. After explaining to these two fellows (both mathematicians, by the way) how transistors worked, we proceeded to come up with a couple of circuits. Surprisingly, they looked a lot like the circuits found in this exercise for a University of Vermont EE course.
This afternoon, these two guys came over and we actually built a NAND gate out of some 2N2222s that I recently bought. The cool thing is that the circuit actually worked.
The disadvantage of this type of circuit is that it requires a transistor for each input. To get around that you could build the diode-transistor logic circuits found in Basic Homebuilt Discrete DTL gate circuits (Diode Transistor Logic) by Richard Steven Walz.
One of the guys had done his homework and suggested that instead of 2N2222s, that we build the gates out of CMOS FETs. He pointed to the circuits found at the web page CMOS gates demonstration. This page has a cool Java-based demo of how CMOS logic circuits work.
One problem with actually building the circuits, though, is that the circuits really weren’t meant to be built using discrete transistors. Because of this, the web page does not specify any part numbers. I suppose for the N-channel transistor you could use the MPF102, but I’m at a loss for suggesting a P-channel equivalent. I just don’t have enough experience with FETs. Anyone have any suggestions??
The paper Oscillations and Regenerative Amplification using Negative Resistance Devices suggests that maybe the 2N3820 might be used. This paper is interesting because it describes regeneration and includes a simple circuit for a regen broadcast band receiver. I have some MPF102s on the way–I guess I’ll have to buy some 2N3820s now as well.
Jon KG6URI says
Your problem isn’t unique, I remember my transistor circuits professor saying that he avoided circuits with FETs in them because it’s impossible to find a good discrete FET that’s not for power switching.
The closest was the VN0104 and the VP0104. They seemed to work fine to tens of MHz and for small-signal apps as well, and you can find a pretty good matched pair in a handful. I recall having to use a curve tracer to get the characteristics for milliamp level gain because they’re not on the datasheet, though it’s probably not very important for a digital gate.
I also have a few unused CD4007C, but looking at the datasheet, I can see why. They’re three complementary pair, one of which is wired as an inverter in an IC, so don’t bother if you can find the 0104s
Ronny, KC5EES says
This project is outstanding. If you can get it to work and write up a nice, easy to follow description, and provide a parts list (and place to find them), this could easily become a basic part of teaching kids about electronics/amateur radio. Too many old-timers gloss over the fact that kids today use computers, but don’t have a clue how they operate. In order to truly understand and progress in the amateur radio ‘tinkering’ category, they need to be taught hands-on electronics. I have not met a single ham yet that is willing to sit down and write out an A-Z project-based curriculum to teach ‘basic’ electronics necessary to understand amateur radio circuits. Most just want kids to jump in and build something so complicated (MOS-FETS/OP-AMPS/complicated discussions of reactance) that the kids just go ‘glazey-eyed’ about 2 minutes into the discussion. If only more Elmers would take your (and your friends’) view and think about keeping it simple (at least in the beginning). I have gone through an old 100-in-1 kit project book and marked about 50 projects that teach basics. Start with capacitors, resistance, coils, relays, and move on to transistors, etc… That is my beginning, but there are probably better projects that could build toward a better understanding of radio circuits specifically. Unfortunately, I am a social studies major, and not an EE. Anyway, I applaud your efforts. Keep us updated on your progress.
Ronny