LiquidOrb24

I am currently taking a Digital Circuit Logic course at my school and I was getting really confused about AND, OR, and NOT Gates. I just have a few questions that I hope someone could clear things up for me.

1) What kind of signal gets put into the gates so that it knows weather to output a 0, or 1. (I'm talking about more spacific such as if it has to be a certain voltage, or current, or frequency, or what have you)?

2) If this signal has to be a certain type; If I wanted to make a digital circuit, where do resistors, capacitors, inductors, and other circuit elements come into play. Why are they there?

3) How can you simplify these circuits using algebra, and what would the methods be?

4) How many gates are there and what do some of the more common ones look like

5) How exactly do they work?

If anyone could answer even a few of these it would be most appreciated.
Thank you.

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bmcculla

1) The signal that is input to logic gates is Voltage. Voltages above a certain point are 1 and below are 0.

2) For pure digital circuits descrete resistors and capacitors are rarely used. Inductors arn't ever used as far as I know. However A digital gate has certain intrinsic Resistances and capacitances that get built into the device from the method used for their construction. These resistances and capacitances are what cause the gate to have a delay between input and output. However in theoretical devices you can ignore all this and assume a ideal device.

3) The method for simplifying digital circuits is called Boolean Algebra. Its really strait forward. Just like basic algebra with different operations. A google search should give you tons of pages about this technequie.

4) AND ,OR , NOT, NOR , NAND , XOR ... NAND and NOR are just AND and OR with a built in inverter. NAND and NOR are the easiest two input gates to make with transistors.

5) All of these devices are built with transistors. The simplest is NOT which is just two transistors an N channel and a P channel. The NMOS is connected to the output and ground and the PMOS is conected to output and V+. THe gates are connected together. So when the input is high the NOMS turns on and connects the output to ground. When the input is low the PMOS turns on and connects the output to V+. All gates work on this basic idea.

Hope this helps.

Brent

kinjalgp

Answers:
1) Logic Gates operate on 2 discrete voltage levels. In positive (active high) logic the voltage range from 0 to 1.5 volt is considered as logic 0 and from 3.5V to 5V is considered as logic zero. The logic in the range of 1.5V to 3.5V is not specified.

2) Since digital logic uses only two voltage states, inductors/capacitors are generally not used for signal modification. But capacitors and inductors are used for noise reduction in power rails of digital circuitry.

3) There are some basic laws and methods to reduce a boolean equation like De-morgans laws, K-map redutcion, Quine-Mccluskey reduction etc which you can use them to simplify your equations.

4) There are 5 basic gates
a) NOT gate
b) OR gate
c) AND gate
d) EX-OR gate
e) EX-NOR gate
There are 2 other gates derived from above gates
a) NOR gate
b) NAND gate
These 2 gates are called universal gates because all basic gates can be derived from these two gates.

5) You can find basic circuits of each gate in any digital electronics text book. Digital Electronics by Malvino is my favourite book for this subject.

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bmcculla

http://www.geocities.com/cmoslayoutd...k/gmask04.html

Here are some pictures to go with my 5) answer. The Inverter in at the bottom

plot

Well, since all the questions have already been answered, I'm just going to suggest searching http://electronics.howstuffworks.com/ ...site has alot of great stuff, as well as a good little section on logic gates.

LiquidOrb24

Thanks a lot everyone. That helps me out a bunch and I think I understand everything a little bit better.

Thanks.

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