I´m relatively new on circuits and have a few logical questions I´m pretty interesting in the functionality of an ALU and occupied myself with topics like the multiplier and full-adder. However, I want to know how these devices physically work so I looked at the internal structure of and, or and nand gates.....
(1) Let´s take for instance the nand gate, that you can see on the picture => The voltage at the output is always off, only when positive voltage is applied on the 2 bases of the 2 transistors, the output is low. However I`m not electronically educated enough to understand why this happens => If current flows through the 2 transistors via the resistor to Vcc, why is the Output low, I mean the electrons do not flow to the output, right?, since the are attracted by the strong positve voltage of Vcc, so there´s still positive voltage applied to the output and hence it´s high.
Thank you so much for clearance, haha
When current is not flowing, those transistors are effectively doing nothing, with respect to your sample point. So you are just connected to 6v (via the resistor).
When the two transistors are conducting, they behave like a switch. So effectively your resistor is a load in a circuit being powered by the voltage difference of 6v and ground. Your sample point is therefore on the ground side of this load.
But I don´t understand that if it´s on the ground side, why isn´t it on the ground side before? (the ground delivers electrons right, and if a voltage is applied then why aren´t these electroncs attracted before the transistors are closed?)
I think I got it, correct me if I´m wrong => Because the transistors are conducting the electrons can flow through them and therefore eliminate the electron-difference between the ground and the Vcc-source, hence there is no voltage anymore applied to the output! ?
yeah, but that´s the technical direction, right?. Physically electrons are negative charged and are therefore attracted by a positive pole and ground is the negative pole, so why should the flow to ground. Am I right, with my understanding in my previous post? Y/N?
Most of the world talks in terms of "positive current" rather than "electron flow". Technically you are correct but it is easier to talk about the non ground voltage being the one that "flows".
sinatra39:
The voltage at the output is always off, only when positive voltage is applied on the 2 bases of the 2 transistors, the output is low. However I`m not electronically educated enough to understand why this happens => If current flows through the 2 transistors via the resistor to Vcc, why is the Output low, I mean the electrons do not flow to the output, right?,
Originally (before they knew about electrons) it was believed that current flowed from positive to negative. Your circuit is probably easier to understand from that perspective.
When the transistors are OFF (i.e. the inputs to them are LOW) the output if HIGH because the 6v cannot flow through the transistors. In effect the transistors represent a very high resistance in a voltage divider of which the 4k7 resistor is the other part. The output is at the junction of this "voltage divider" and is HIGH.
When the 2 inputs are HIGH the transistors conduct power and, in effect, change to be a low resistance part of a voltage divider thus making the voltage at the output LOW.
You can mimic the same effect with a real voltage divider if you make the other resistor either (say) 1meg ohm or 1 ohms.
I also suspect you are a bit mixed up with what the flow of electrons means. It may help to think of an absence of electrons as HIGH and a flow of electrons as LOW. You must keep in mind that the notion of positive and negative is as arbitrary as using the name "elephant" for those large grey animals.
sinatra39:
yeah, but that´s the technical direction, right?. Physically electrons are negative charged and are therefore attracted by a positive pole and ground is the negative pole, so why should the flow to ground. Am I right, with my understanding in my previous post? Y/N?
The electrons come from spots in orbitals that the Fermi level of the conductor has raised them from. Even as electrons flow they still trade association to those spots which are known as 'holes'.
When current flows we are speaking of the holes, not the electrons. Fact is that you effectively have a 2-direction flow of opposite polarities every time that current flows.
You can keep all this and as much other trivia in mind all you want but it's not going to make understanding what works and how the least bit easier. --- Well unless you get to messing with vacuum tubes and the like. You won't find holes in a Klystron tube except in the electrodes.
Robin2:
When the 2 inputs are HIGH the transistors conduct power and, in effect, change to be a low resistance part of a voltage divider thus making the voltage at the output LOW.
Okay so, the part of the voltage divider after the 2 transistors is a low resistance area because electrons can flow there without problems right? Let´s make a scenario, let´s say there is no resistor before Vcc => then the voltage at output would be high, right because electrons would travel directly out of the transistor-logic to Vcc, without getting to the Output, since there´s no resistor?
If there was no resistor, the output would be tied directly to the power supply.
You could remove the transistors completely, the output would still measure high.
So what's that tell you about what the electrons are doing?
It's a lot more straight forward to think of current as flowing from High Voltage source to Gnd, with voltage drops along the way as the various parts in the path each have some voltage across them.
Once you start adding in the path of how electrons are flowing, things get a lot messier.
So think of a transistor as two diodes - one from C to E, the other from B to E.
If current flows from B to E, that allows current to flow from C to E. There is some voltage drop across that diode, say 0.5V to 0.7V.
So with your two-diode logic gate, the output will be 6V when either diode is off, and 1V to 1.4V when both are on.
I echo previous comment. You should simply adopt the convention
that has been given to you. You have come to us seeking advice.
That advice has been given. Accept it anx don't bring up electrons again.
Speak only of current flow direction as POS to NEG and leave it at that.
In answer to your original question, the transistors function as series
switches separating the output from ground when BOTH are OFF.
resulting in a HIGH. If yiu repaced the transistors wuth switches
would you still be asking us why the output is LOW when they are on ?
I realuze you are a beginner but can we move past the beginning?
If everyone learned quantum mechanics then there would be less confusion.
There is a positive charge that does flow in conductors exactly as much and opposite the negative charge. That is what was first understood because of compasses and early magnetic detection.
Electric potential is not like gravity that has one direction. Electric has two directions at the same time.
You can have your own private direction but it will only screw you up if you want to learn electronics as presented in the standard way, which WORKS. You can make your private way work too, if that's how you like to waste your time but don't expect others to play your silly game for long.
Voltage divider explained as voltage drops over individual resistors in serial with others.
And in the new tradition of doing things backwards, now you should go learn Ohm's Law.
I´m pretty interesting in the functionality of an ALU and occupied myself with topics like the multiplier and full-adder. However, I want to know how these devices physically work so I looked at the internal structure of and, or and nand gates.....
