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 the “input”,
and let us call this other part of the circuit the “output.”
When a part of the circuit is at zero volts,
we will say that it is at “Logic Low.”
When a part of the circuit is at the voltage of the battery,
we will say that it is at “Logic 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 “not” logic 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 “nor” logic gate.
Suppose that on the output of this “nor” logic gate,
we add the “not” logic 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 an “or” logic gate.
We call this an “or” logic gate because to generate a logic “high” output,
either the first input or the second input needs to be high.
Suppose that we instead take the “nor” gate 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 an “and” logic gate.
If we add a not gate to the output of the “and” logic gate,
then it becomes what we call a “nand” logic gate.
We can also use logic gates to create a “xor” logic gate.
In the case of a “xor” logic 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 an “or” logic 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 the “or” gate,
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 the “or” gate will then also go to “logic 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 the “or” gate will still stay high,
due to the fact that its other input is at logic high.
And the logic high output of the “or” gate will perpetuate the logic high of this other input.
Therefore, in this case,
once the free input to the logic gate goes to “logic high”,
the circuit will remember this,
and then the output will stay at “logic high” forever.
More information about electric circuits
is available in the other videos on this channel.