# How to make Logic Gates with Transistors

Logic gates normally come packaged in integrated circuits, but to study the internal works of them you need to be able to build them using discrete components. This is a great exercise for those interested in digital electronic circuits, specially if you like to learn exactly how things work internally.

## Basic gate Diagrams

AND gate - Here you need to connect two transistors in a way that the output is high only when both inputs are high.

OR gate - This is simpler, you connect them in a way that when either input is high the output will be high.

NOT gate - This is the simplest one to build, as mentioned in the introduction, you only need one transistor for this one.

AND gate
OR gate
Inverter - NOT gate

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## Comments 14 comments

Allen Hundley 5 years ago

Hey. Im trying to make an ALU so i need to make adders which i need to make logic gates for. So im confused on where the out put comes out. Also if im correct i can put any resistor in as long as i apply enough voltage to the imputs. On my schematics of adders i drew there is no fourth lead needing to be grounded. I assume i can just have all my grounds connect to a metal chasis?

Gergy008 5 years ago

The NOT gate is incorrect, The wrong type of transistor was used in the circuit? You need to use a PNP transistor not an NPN as displayed

wtfman 5 years ago

"The NOT gate is incorrect, The wrong type of transistor was used in the circuit? You need to use a PNP transistor not an NPN as displayed"

wtf r u saiyn'

MrFish 5 years ago

He is saying that you need a different type of transistor for the NOT gate. There are two types of bipolar transistors, NPN, and PNP. An NPN transistor lets current flow from the collector to the emitter when current flows into the base. So with this NOT gate, when it gets a HIGH through the base, it produces the output HIGH, which is the opposite that you want it to do. PNP transistors would work though.

mac 5 years ago

you must realize that the and gate connot be used as displayed if you leave the collector or emitter of a pnp or npn transistor disconected it can function like a diode letting power flow through b to the output if a is low

sakibmoon 5 years ago

Thanks for your hub. There may be some mistakes, but still it is great. I searched in hubpage and there is not many hubs on this topic. Maybe 4-5 hubs. I think, we who are interested in this, should write more. I except more hubs on this topic from you.

I believe you will enjoy my hub on logic gate. Check out when you are free. Don't forget to share your thoughts.

http://hubpages.com/technology/Standard-Logic-Gate...

deepika 4 years ago

thats amazing

deepika 4 years ago

its ausm

Aaron 3 years ago

The NOT gate is just fine as an NPN. Although there should be a resistor to limit the flow of current through the transistor. It simply inverts the logic by shorting the output to ground (0) when current is applied to the gate (1).

aruna 3 years ago

why transistors are series in circuit?

Cameron 3 years ago

Why would a PNP work? When I look at it, I see that if I ground the input because it needs to be at a lower potential than the collector to be correctly biased. So now the transistor is ON, The output isn't at 5V, its at ground roughly because most of the current is going to flow through the transistor and not through the output. It has no direct connection to the positive rail.

Cameron 3 years ago

I realized that I put collector instead of emitter.

fghfgh 2 years ago

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Johne592 2 years ago

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