Transistor based logic gates - Page 1
Forum section: electronics / beginners

author: higginsdj
author_rank: Contributor
timestamp: March 04, 2023, 07:15:09 am
content: I see lots of examples of logic gates where the gate output is on the emitter of one of the transistors. ie AND gate herehttp://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/trangate.htmlI do not understand how people can think this can be valid since a high passed into the base of a transistor will always pass voltage out that transistors emitter. In the above example, if I put 6v into B I will get 3.68v on the output when A is low and 4.4v when A is High. That's sufficient voltage to act as a 'high' on a subsequent logic gate input. What am I missing here?

author: Slh
author_rank: Regular Contributor
timestamp: March 04, 2023, 07:45:43 am
content: You're missing the base resistors. In your example, the voltage on the input is 6V but the voltage at the base is the output voltage plus about 0.6V. The output voltage is going to be the emitter current * 4k7.When A is low, the only current is from the base so we're looking at around (6-0.6)*4k7/14k7 which is less than 2V. If you have the same circuit following then this shouldn't be enough to register as a high.When A is high, the current is hfe*ib so it's going to be higher. Probably 5 to 5.5V.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 04, 2023, 10:51:05 am
content: See attached. Worst case LO is 1.7V which may be fine if your thresholds and noise margin is defined sensibly.The big problem with these circuits is that the are linear. There is no set threshold.

author: golden_labels
author_rank: Super Contributor
timestamp: March 04, 2023, 11:26:35 am
content: People would believe it works, because they widely used it in 1960s and it worked. This is a pretty convincing argument, isn’t it? The technology was called resistor-transitor logic (RTL) and it worked well enough toland humans on the Moon.The very important part missing from the picture is: this circuit absolutely can not be considered in isolation. And it makes little sense from 2020s perspective, if someone thinks in terms of circuit design from digital chips.This is an early 1960s technology, developed by people with 1940s mindset and still using approaches dating back to 19th century. They did not know the concept of integrated circuits yet. Functional blocks, while present, were much bigger units and hardly universal.While looking at this circuit, do not see a 7400 or 4000 series gate. See it the same way as diode logic. A pattern, that is a part of a larger circuit. If an AND gate like this is used, it will not be blindly chained with a bunch of other RTL gates. Its output will be fed to something, that is tuned to accept that output.Equally, while calculating the values in such a gate, you must take into account both what controls the inputs (10 kΩ base resistor adds to impedance of whatever drives it) and what is the load (in your scenario, the output can deliver only very low current).For practical purposes, if good separation and good noise immunity was needed, NOR and NAND gates were used. They act as amplifiers, offering full and clear swing from Vee to Vcc. Though still poor current sourcing. Aforementioned Apollo computer used NORs.If you wish to experiment with that nowadays, pay attention to transistor parameters. Most transistors in early 1960s would have typical current gain in 2-digit region, the minimal value sometimes being 1-digit. Even relatively modern transistors, like BC547, are in the hundreds range. Current, that would hardly make a 1960 transistor do anything, will put a modern one into saturation. So the outcome may be drastically different. 2N2222, used by the linked page, is similar enough to old ones — but be aware of that issue.

author: higginsdj
author_rank: Contributor
timestamp: March 05, 2023, 05:07:09 am
content: Quote from: Slh on March 04, 2023, 07:45:43 amSorry I don't know what you mean - I built the circuit as shown with those same components. However I used a 5v VCC, passed 5v into B and held A to ground and measured 3.68v at the output.Edit - OK my bad - output measured 1.38v. Seems I had a 'dicky' breadboard that was doing weird things. I need to get my head around how these transistor logic circuits actually work.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 05, 2023, 09:21:34 am
content: Dicky Breadboard- Wasn't he David Attenborough's brother?