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Topic: Understanding NAND (Read 3755 times) previous topic - next topic

Grumpy_Mike

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and below the transistors in AND circuit?
It's not.

p1ne

#16
Jan 20, 2017, 08:48 pm Last Edit: Jan 20, 2017, 08:58 pm by p1ne
Check Crossroads diagrams. NAND output is above transistors and AND output is below. More examples attached.

Grumpy_Mike

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Check Crossroads diagrams.
OK.

No special reason it is just the way it is implemented. It is the simplest implementation using the fewest transistors but there is nothing significant about the positioning.

westfw

#18
Jan 21, 2017, 10:29 am Last Edit: Jan 21, 2017, 10:30 am by westfw
Understand that the circuits you are showing are simplified, not-very-good, logic gates.  In order to understand them, you should assume that the transistors are perfect switches: an NPN transistor is turned on by a positive voltage (logic 1, usually), and a PNP is turned on by a GND voltage (logic 0)
Um.  That means I have doubts that the PNP circuit you showed actually implements an AND gate.  Looks to me like the output will only be 1 if both inputs are 0 (which makes it a NOR gate.)  Similar circuits (http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/and.html) make an AND gate with similar topology and NPN transistors...  (As in your second link)


p1ne

Thanks to all for weighing in. A key point to my understanding has come about with learning that electrons in a closed circuit flow opposite the arrow symbol on the transistor.

So wiith NAND 2 NPN transistor circuit with LED and 2 normally open switches, cathode is to ground with anode connected to collector/voltage in (via resistor). So the circuit is already closed, and the LED is on. The open switches have no bearing, nor does 1 closed switch.

However, when both switches are closed, a flood of electrons goes to the LED anode, essentially creating 2 grounds on the LED and the light goes off.

My previous confusion about why the output LED was "above" or "below" the transistors had to do with not understanding the difference of connecting the output to the collector vs. the emitter: unlike the NAND circuit where voltage is already being supplied to the output/LED, when the output is connected to emitter, no voltage is flowing through the transistor when both bases are open.

Grumpy_Mike

Just a few points to get your thinking a bit better:-
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no voltage is flowing
Voltage doesn't flow. Current flows and voltage pushes it.

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A key point to my understanding has come about with learning that electrons in a closed circuit flow opposite the arrow symbol on the transistor.
If you consider the direction of current flow matters you are misunderstanding something. We consider that current flows from the positive to the negative, this is called "conventional current". There is no need to consider electron flow. In a particular materiel the conduction mechanism will be dominated either by electron flow or hole flow, the two flow in opposite directions.   

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However, when both switches are closed, a flood of electrons goes to the LED anode, essentially creating 2 grounds on the LED and the light goes off.
No that is not right, there are not two grounds in any respect.

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My previous confusion about why the output LED was "above" or "below" the transistors .....
Yes that is right.

Jiggy-Ninja

Just a few points to get your thinking a bit better:-Voltage doesn't flow. Current flows and voltage pushes it.
If you consider the direction of current flow matters you are misunderstanding something. We consider that current flows from the positive to the negative, this is called "conventional current". There is no need to consider electron flow. In a particular materiel the conduction mechanism will be dominated either by electron flow or hole flow, the two flow in opposite directions.   
No that is not right, there are not two grounds in any respect.
Yes that is right.
Actually, it's electrons moving in both cases. The distinction you're trying to make is between free electrons in N regions, and holes in P regions, but both of those are caused by electrons. Holes can't actually move since they aren't an actual thing, they are an absence of a thing.

But your larger point is correct. With respect to electrical and magnetic effects, there is no difference at all between positive charges flowing in one direction, and negative charges flowing in the other. The distinction is completely arbitrary.

Grumpy_Mike

#22
Jan 23, 2017, 07:05 pm Last Edit: Jan 23, 2017, 07:06 pm by Grumpy_Mike
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Holes can't actually move since they aren't an actual thing, they are an absence of a thing.
Of of course holes can move. They even have a positive mass.

So how can the absence of something have a positive mass?
Because the things that are absent have a negative mass. That means that they are repelled by a gravitational filed. But that is quantum mechanics for you.

Jiggy-Ninja

So I go to the electron hole page on Wikipedia and see this subheading: Detailed picture: A hole is the absence of a negative-mass electron.

I will say nothing more about the subject. Mother Nature is certifiably insane.

Grumpy_Mike

"Sometimes the appropriate response to reality is to go insane." (Philip K. Dick)

MarkT

Of of course holes can move. They even have a positive mass.

So how can the absence of something have a positive mass?
Because the things that are absent have a negative mass. That means that they are repelled by a gravitational filed. But that is quantum mechanics for you.
Actually the effective mass of a hole depends on the actual semiconductor material involved
and can be either sign even IIRC.

For analysing circuits (as opposed to understanding semiconductor device operation), forget electrons
and holes, treat positive charge as the thing that flows, then all your signs come out right and you
won't confuse people - conventional current is a flow of positive charge, that's the abstraction level
you want at the circuit level.

Whether the actual charge carriers are +ve or -ve doesn't matter, until you are designing actual devices!
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

Jiggy-Ninja

Whether the actual charge carriers are +ve or -ve doesn't matter, until you are designing actual devices!
Or doing chemistry, like electrolysis, electroplating, or anodizing.

MarkT

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Or doing chemistry, like electrolysis, electroplating, or anodizing.
Most circuits don't involve that, unless you spill acid on a PCB/breadboard by mistake of course! :)
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

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