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  • Computers and digital electronics are made from logic gates,

  • and logic gates are made from transistors.

  • A transistor is a device that can be made to behave like a switch.

  • When a switch closes, it causes the two different parts

  • of a circuit that it connects to be at the same voltage.

  • When a switch opens, it prevents any current from passing through.

  • When there is no current flowing through a light bulb,

  • both sides of the light bulb are at the same voltage.

  • The end of the light bulb that is connected to the battery

  • will always be at the voltage set by the battery.

  • The then other end of the light bulb will be

  • either at the voltage set be the battery, or at zero volts,

  • depending on whether the switch is open or closed.

  • The same thing is true of a device which we call a resistor.

  • If we replace the light bulb with a resistor, it will behave the same way.

  • Suppose we also replace the switch with a device called a field effect transistor.

  • This transistor has three terminals, which we will call Gate, Source, and Drain.

  • The terminals which we called Source and Drain

  • go where the terminals of the switch used to be.

  • So far, we have not yet connected the Gate terminal to the rest of the circuit.

  • If we apply a voltage to the gate terminal,

  • the voltage that will exist between the Gate and the Source

  • will determine whether the transistor behaves like

  • an open switch, a closed switch, or something in between.

  • If the transistor behaves as something in between

  • a closed switch and an open switch,

  • the transistor can be used as an amplifier.

  • Small changes in the voltage on the Gate Terminal,

  • will create much larger changes in voltage on the Drain terminal.

  • However, in our case, the voltage that we will place at the

  • Gate terminal of the transistor will always be just one of two values.

  • Either this voltage will be the same as the

  • voltage of the battery, or it will be at zero.

  • With this particular type of field effect transistor, if the

  • voltage between the Gate and the Source is at the voltage of the battery,

  • the transistor will behave like a closed switch.

  • If the voltage between the Gate and the Source is zero,

  • then the transistor will behave like an open switch.

  • Let us call this part of the circuit theinput”,

  • and let us call this other part of the circuit theoutput.”

  • When a part of the circuit is at zero volts,

  • we will say that it is atLogic Low.”

  • When a part of the circuit is at the voltage of the battery,

  • we will say that it is atLogic High.”

  • We have now made our first logic gate.

  • When the input of our circuit is logic low, the output of our circuit is logic high.

  • When the input of our circuit is logic high, the output of our circuit is logic low.

  • This is the definition of a “notlogic gate,

  • where the output is always the opposite of the input.

  • To make a more complicated logic gate, let us consider a circuit with two switches.

  • In this case, if either one of these switches closes,

  • the part of the circuit that we call the output will be at zero volts.

  • The part of the circuit that we call the output will be at the voltage of the battery only if both switches are open.

  • Suppose we implement this circuit with the

  • same type of field effect transistor that we used before.

  • In this case, we now have two inputs to our circuit,

  • which are the two gate terminals of the two transistors.

  • In this case, if either the first input or the second input is at logic high,

  • then the output will be at logic low.

  • This is what we refer to as a “norlogic gate.

  • Suppose that on the output of thisnorlogic gate,

  • we add thenotlogic gate that we created earlier.

  • Now, if either the first input or the second input is at logic high,

  • then the output will be at logic high.

  • This what we refer to as anorlogic gate.

  • We call this anorlogic gate because to generate a logichighoutput,

  • either the first input or the second input needs to be high.

  • Suppose that we instead take thenorgate that we created earlier,

  • and rather than adding a not gate to the output,

  • we instead add a not gate to each of the two inputs.

  • Now, if either the first input or the second input is at logic low,

  • then the output will be at logic low.

  • The output will be at logic high only if the

  • first input and the second input are both logic high.

  • We therefore call this anandlogic gate.

  • If we add a not gate to the output of theandlogic gate,

  • then it becomes what we call a “nandlogic gate.

  • We can also use logic gates to create a “xorlogic gate.

  • In the case of a “xorlogic gate,

  • the output will be high if one of the inputs is high, but not both.

  • We can also use logic gates to create memory.

  • As an extremely primitive example,

  • consider anorlogic gate with the output tied to one of the inputs.

  • Suppose we start out with both the inputs and the output at logic low.

  • If we send a logic high into the free input of theorgate,

  • then the output will go to logic high,

  • since the output is high if either one of the inputs is high.

  • The other input of theorgate will then also go tologic high”, since it is connected to the output.

  • If we now removed the logic high to the free input of the or gate,

  • and change it to a logic low, the output of theorgate will still stay high,

  • due to the fact that its other input is at logic high.

  • And the logic high output of theorgate will perpetuate the logic high of this other input.

  • Therefore, in this case,

  • once the free input to the logic gate goes tologic high”,

  • the circuit will remember this,

  • and then the output will stay atlogic highforever.

  • More information about electric circuits

  • is available in the other videos on this channel.

Computers and digital electronics are made from logic gates,

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B1 US gate output voltage input circuit transistor

Logic Gates from Transistors: Transistors and Boolean Logic

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    Henry 楊 posted on 2020/06/07
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