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When the input to a NOT gate is 1, the output is 0. When the input is 0, the output is 1.

Now, does the input being 0 mean that no current is flowing into the gate and then out of the gate towards the load? And does the input being 1 mean that current is flowing into the gate and then out towards the load?

If it does mean this then i have a question. When the input of NOT gate is 0 i.e current is not flowing into the NOT gate then how can the output be 1 i.e current is flowing out of the NOT gate.

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  • \$\begingroup\$ What type of logic gate is that? Current operation is rather unusual. \$\endgroup\$ Apr 26 '13 at 19:15
  • \$\begingroup\$ I have used the word "NOT gate" 5 times.... \$\endgroup\$ Apr 26 '13 at 19:18
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    \$\begingroup\$ He means is it CMOS, TTL, ECL, PECL etc. it matters HOW the logic is implemented. He is giving you the benefit of the doubt in case you are asking about a (relatively rare) current mode type of logic. \$\endgroup\$ Apr 26 '13 at 19:20
  • \$\begingroup\$ Oh. I am just doing the basics. And there is NOT gate in my course and I have asked a question about that....Don't know what CMOS, TTL or ECL are...:) \$\endgroup\$ Apr 26 '13 at 19:25
  • \$\begingroup\$ Does "doing the basics" give anyone the excuse for believing the first commentor may have over-looked you mentioning NOT-gates several times? When you refer to current flowing in, and current flowing out, this gives the impression you are not working with standard gates. Do you actually mean voltage in and voltage out? \$\endgroup\$
    – Andy aka
    Apr 26 '13 at 20:53
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Usually such gates are voltage-controlled devices, and not current controlled devices. They generally have very high input impedance, which means that very little current is drawn from the input. How much, depends on the device, so see the datasheet for that.

The output side is usually meant to be likewise voltage indicator of the logic state. Whether the is any current flowing out depends a) the state of the logic gate (current can be drawn only from non-zero voltage) and b) on the input impedance of whatever is hooked up to the gate: if the device connected to the gate output has low impedance then much current will flow from the output of the gate. How much current such a gate can source also depends on the specific gate.

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A NOT gate is made up of more than than just one input and one output. You must also connect it to a power source (Vcc on schematics) and ground. When the input is 0 it is connecting the output to Vcc, when the input is 1 it is connecting the output to ground.

(see pin 14 and pin 7)
enter image description here

Here is what a NOT gate looks like internally

schematic

simulate this circuit – Schematic created using CircuitLab


P.S. Logic gates normally operates on measuring voltage, not current.

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  • \$\begingroup\$ Please, if you could rephrase the first line of your answer.. \$\endgroup\$ Apr 26 '13 at 19:19
  • \$\begingroup\$ @MuhammadRafique Done \$\endgroup\$ Apr 26 '13 at 19:24
  • \$\begingroup\$ I'm going to have to be nitpicky here. But I'd venture that there never has been a JFET based NOT gate, they being depletion mode devices and all. \$\endgroup\$ Apr 26 '13 at 19:35
  • \$\begingroup\$ Any user can edit any post on this site, if you can find a better part on the schematic editor built in to the website feel free to edit the answer. \$\endgroup\$ Apr 26 '13 at 19:49
  • \$\begingroup\$ A NOT gate has a single input and a single output. They are, however, often sold in packages of 6 - that is, a 7404 NOT gate IC contains 6 individual NOT gates. (NOT gates are more often called "inverters".) \$\endgroup\$ Apr 26 '13 at 20:12
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I will address your last paragraph: " If it does mean this then i have a question. When the input of NOT gate is 0 i.e current is not flowing into the NOT gate then how can the output be 1 i.e current is flowing out of the NOT gate."

All logic gates require a power supply, in addition to the logic inputs and outputs. We generally think of logic 1s and 0s as being voltage levels, but, depending on what is connected to the logic inputs and outputs and the technology used to make the gate, current may flow into or out of both input and output of the gate - this current will come from the power supply, and is not "passed through" from input to output.

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As others, e.g. @angelatlarge, have mentioned, logic gates are typically voltage controlled. The mapping between voltage and logic varies quite a bit between families. If you look at, e.g., a 74HC04 data sheet:

http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdf

you see in section 9 (Static characteristics) a lot of details on this. The key parameters there are V_IH, V_IL, V_OH, and V_OL. For the data sheet above, what this works out at V_CC=4.5V and 25C is:

  • If the input voltage is smaller than 1.35V (Max of V_IL), the input is considered logical 0, so the output is logical 1 and the output voltage is guaranteed to be at least 3.98V (Min of V_OH) (though that depends on the current you draw).
  • If the input voltage is larger than 3.15V (Min of V_IH), the input is considered logical 1, so the output is logical 0 and the output voltage is guaranteed to be at most 0.26V (Max of V_OL).

Note that these values are designed to provide a reasonable safety margin (Min of V_OH is larger than min of V_IH, and max of V_OL is smaller than max of V_IL), so you can depend on being able to hook up the output of one gate of this family to the input of another gate in the same family. These guarantees don’t necessarily hold between different families, though.

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This is true, no current when 0, current when 1, but logic gates have power supply's that make the logic work. So a NOT gate has 4 pins: power supply, input, output, ground (negative).

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  • \$\begingroup\$ No, this isn't true. Whilst it might be that some current is flowing when a 1 is provided, the key point here is voltage, not current. See here. \$\endgroup\$
    – user17592
    May 15 '13 at 20:25

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