CMOS NAND Circuits Draw Current? Lab Notes Explained

In summary, the conversation discusses the issue of non-negligible current in CMOS NAND circuits. The person is confused because they thought CMOS circuits were known for using little current, but their lab notes state otherwise. They mention a data sheet for a CMOS NAND gate that has a maximum current of 1 uA, but it is still unclear what is considered non-negligible current. The explanation given is that CMOS circuits only use significant current during input transitions, but the circuit in question is designed to keep the inputs in a no man's land, resulting in continuous current flow. This is not a typical scenario and does not reflect the usual low power consumption of CMOS circuits.
  • #1
mmmboh
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In my lab notes, it says that the connection of CMOS NAND circuits will draw non-negligible current...but I thought the point of the CMOS was because it uses such little current, at least that's what I wrote in my notes..I don't get it, what's going on?
 
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  • #2
Yes, the CMOS implementation draws little current. Your lab notes are wrong ?!
 
  • #3
I don't think they are wrong as it says so explicitly, and it's done in a way that acknowledges that this is unusual..this is the circuit if it makes a difference [PLAIN]http://img10.imageshack.us/img10/8317/lab7w.jpg.

I just don't get this "contradiction".
 
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  • #5
Hm, I'm not sure what they consider non-negligible, it just says non-negligible...I guess any more current than it theoretically should. I don't know the explanation for this non-negligible current though, it shows us how the CMOS NAND is built and tells us the reason has to do with that, but I don't know the explanation.
 
  • #6
Hey mmmboh,

The idea is that CMOS circuits use virtually no current when they're in steady-state. If you have some network of NANDs and all of the inputs are constant, not changing in time, then the CMOS circuit uses very little current. The only current it uses is called "leakage," and it's negligible in most situations. When the inputs change, though, the gate capacitance of the transistors has to be charged or discharged, and that does consume current.

The circuit they gave you is kind of a trick. They're driving the input of the NAND gates to mid-scale, halfway between the two supplies. This is a no man's land, somewhere between logic-0 and logic-1. This voltage is adequate to partially turn on both the NMOS and PMOS devices in the gates, so a continuous (and possibly large) current will flow.

Normally, when a CMOS gate changes state, its output changes so rapidly that the voltages are only in the no man's land for a short time. Except during these brief transitions, the voltage is stable, very close to either supply. Because the transitions are so rapid, the total power loss is very small.

- Warren
 

Related to CMOS NAND Circuits Draw Current? Lab Notes Explained

1. What is a CMOS NAND circuit?

A CMOS NAND circuit is a type of logic gate that uses complementary metal-oxide-semiconductor (CMOS) technology to implement Boolean logic functions. It is composed of two or more inputs and one output, and it produces a logic high (1) output only when all of its inputs are logic low (0).

2. How does a CMOS NAND circuit work?

A CMOS NAND circuit is made up of a series of transistors that are connected in a specific way to create the desired logic function. When the inputs are all set to logic low (0), the transistors are turned on, allowing current to flow from the power supply to the output. When any of the inputs are set to logic high (1), the corresponding transistor is turned off, breaking the connection to the output and preventing current from flowing.

3. What is the purpose of CMOS NAND circuits?

CMOS NAND circuits are commonly used in digital electronics to perform logical operations such as AND, OR, and NOT. They can also be used as building blocks for more complex circuits, such as adders, multiplexers, and flip-flops.

4. Why do CMOS NAND circuits draw current?

CMOS NAND circuits draw current because they use transistors, which require a flow of current to function. When the inputs are set to logic low (0), the transistors are turned on and current is allowed to flow from the power supply to the output. This current is necessary to power the circuit and produce the desired output.

5. How can I interpret lab notes about CMOS NAND circuits?

Interpreting lab notes about CMOS NAND circuits may require some background knowledge of digital electronics and Boolean logic. The notes may include information about the circuit's design, function, and performance, as well as any troubleshooting or experimental results. It is important to carefully read and understand the notes, and to refer to other resources if necessary, in order to fully comprehend the information being presented.

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