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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.

• is available in the other videos on this channel.

Computers and digital electronics are made from logic gates,

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# Logic Gates from Transistors: Transistors and Boolean Logic

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Henry 楊 posted on 2020/06/07
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