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[CS61C FA20] Lecture 14.2 - Intro to Synchronous Digital Systems: Transistors — Transcript

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0:01and welcome back
0:03now let's talk about the next big piece
0:05in our logic design puzzle
0:07the transistor so the transistor was
0:10born
0:11two days before christmas 1947 just a
0:13couple of years after the war ended
0:16boy fundamentally transformed everything
0:18we know about electronics
0:20and all the hardware devices you have
0:22give it
0:23if it's due to the fact that they
0:24invented this transistor back then it
0:26was huge it was this big it was
0:28it was the goal was to and the goal was
0:31to try to figure out
0:32what was happening they fully didn't
0:34understand what was happening in the
0:34early days if you look at the story
0:36there's a whole
0:36i linked on the top right a couple of
0:38videos and i encourage you to watch
0:40pbs.org transistor for the history
0:43they were like discovering this
0:45wonderful piece of science that
0:47like current can flow it's called a
0:49semiconductor because current flows
0:51um and is even amplified uh in this
0:54wonderful way if the distance between
0:57these two leads is just right so they
0:59had to take a razor blade and cut them
1:00watch that video it's a fascinating
1:01video but the idea is
1:03it's called a semiconductor for a reason
1:05sometimes it conducts sometimes it
1:07doesn't and
1:08you can control whether it conducts
1:09electricity or not with another source
1:12so it's like one source acts like the
1:14gate to
1:15tell you whether these two leads are
1:16connectable or connectable are either
1:18amplified
1:19uh the voltage here is either amplified
1:21or either connected or not connected so
1:23we can either use to be as an amplifier
1:25or as a switcher as well i want to give
1:27credit to bridene shockley and pretend
1:30as the adventures of this back in the
1:32day before that they had vacuum tubes
1:34which used to
1:35which were you know thousands of times
1:37the size and would
1:38break and fail at you know unexpected
1:42times and it was really hard
1:43just replacing the vacuum tubes and all
1:44the electronics before the transistor
1:46came in so it also had
1:47a better life a better longevity for
1:50them
1:52after that integrated circuits shrinking
1:55the size of these things every year we
1:57see
1:57um the the size of transistors shrink
2:00it's remarkable what they were able to
2:02do in today's
2:03computer engineering so it's incredible
2:05you build these things in networks um
2:08model digital modern digital systems use
2:10cmos
2:11cmos stands for complementary metal
2:13oxide semiconductor
2:14so this is the technology used and they
2:17use the vulture control switches so
2:18we're talking we're going to continue
2:19the idea of a switch we saw in the last
2:20lecture
2:21with the idea that could you make this
2:22transistor via switch i'm not going to
2:24try by amplifiers in the analog domain
2:25just a switch
2:27so here's a picture a transistor is
2:30usually a three lead device and you can
2:32tell whether it's
2:33going to be used as an amplifier or not
2:35whether it has a heat sink on it because
2:36these things get hot so if it's a big
2:38one that you're going to use
2:39if it has a big heat sink sink on it
2:40probably it's using is used as an
2:41amplifier and that heatsink is being
2:43used to dissipate that heat
2:45if it's not if it does if it's a smaller
2:46one you go to electronics store and you
2:48buy a smaller one that doesn't have a
2:49heatsink probably it's used as a
2:50switcher device or it's a low
2:51low level amplifier here's the idea
2:55you have three leads drain gate and
2:56source how do you remember those three
2:58names
3:00dgs well dan garcia says drain gate and
3:02source
3:04and here's the idea source has potential
3:07i have this water and i want to be able
3:08to flow to the to the drain
3:11but i can't there because there's a gate
3:12preventing it and that's the idea it's a
3:14voltage controlled gate that's the big
3:16idea here
3:17so as i get my pen let me just show you
3:20here
3:21here's my source i want to be able to
3:22send current to the drain i'm going to
3:24pour it into the drain it's too heavy
3:25i've got all this current
3:27got all this potential got to pour it
3:28down got to get some current flowing so
3:30i can pour this potential into the drain
3:32but the gate's saying uh hold on my gate
3:35controls it so that when the gates
3:37voltage
3:38is higher than the source voltage this
3:42thing closes
3:42and there's a path okay so only when
3:45this gate voltage goes above a certain
3:47threshold
3:47which is you know epsilon above the
3:49source voltage then that closes the
3:51switch and now there's a path and
3:52current flows and everybody's happy
3:54okay closed circuit this is called an
3:57n-channel transistor and i think of n as
4:00a normal that's the normal way of
4:01thinking about it okay
4:02p which has a circle is the opposite
4:05okay this is kind of like a
4:07complementary side of this
4:08so p is the opposite when you have high
4:11when you have high voltage when the gate
4:13voltage is bigger than the source
4:14voltage
4:15then it opens so normally it closes the
4:17gate
4:18you know the current goes up the voltage
4:19goes high it closes a circuit
4:22this is exactly the opposite current
4:23goes high it opens the circuit okay so
4:25let's actually see a little animation of
4:26this
4:26so here we go so g is low so my gate
4:30voltage is low therefore the n channel
4:33transistor is open
4:34the p channel transistor is closed and
4:38when the gate voltage is high the
4:40opposite happens
4:41i close the gate in the and channel and
4:43i open the gate in the p
4:44channel okay remember that because we're
4:45going to use it in one slide you got it
4:47ready
4:48and action okay
4:51i built a circuit this is pretty cool
4:54i've got a voltage source
4:56well three volts we'll call it one we
4:57call it a digital one
4:59okay and i've got ground like into the
5:01ground no no potential
5:03ground i can put a wire into the ground
5:04that's ground and that's zero volts
5:07so i've got a y out here and y needs a
5:10value
5:11now if nothing if i disconnect y y has
5:14no value it isn't either
5:150 or 3 volts it's just floating in there
5:18we can't have that
5:19so we always have to have our y have one
5:21connection either the 0 or three
5:24that's part one and part two is y can't
5:27also there can't ever be a short circuit
5:29i can't ever have a path
5:31from three volts from a one to a zero
5:33without going through a resistor
5:35that's going to be bad that would be
5:36like shorting out something you don't
5:37want to short that out
5:38so there has to also be uh never a
5:40connection in these circuits between
5:42three volts and zero volts
5:43or from one to zero remember those two
5:44things okay output has to have at least
5:46something driving it that's one
5:48and two you can never short circuit uh
5:51high to ground okay
5:52power to ground all right here we go
5:55what's the relationship though between x
5:57and y is a beautiful circuit
5:59okay so when it's three volts
6:03i'm drawing this picture here three
6:05volts remember this is the n-type down
6:06here
6:07there's the normal type and here's the
6:09p-type you remember it from the circle
6:11so let's think about this here we go
6:13well
6:14when i've got zero volts okay when x is
6:17zero x is low okay this is x is low okay
6:22x is low that means this closes
6:25the p type closes this is a connection
6:28now i've made a little wire like it goes
6:30up and down it goes across there is a
6:31connect
6:32the gate has been closed there but we
6:34saw that before on an end type
6:36it's an open it's an open switch okay
6:39well
6:39then does what that's that's it is there
6:41a connection from three volts to zero
6:43no okay because this is an open circuit
6:46that means there's no way that
6:47this three volts who's going to go three
6:49three volts three volts three volts
6:51y gets three volts and then here's a see
6:53this open circuit doesn't go to zero so
6:55that's good so far
6:56so when i put x is zero volts when x is
6:59low
7:00y is high they go low
7:04you go high huh see how it did that with
7:06the michelle obama okay
7:08now next what happens when x goes
7:12high you say michelle does that mean
7:14when when they go high we go low is what
7:15that means well let's see let's see what
7:17happens here
7:18so x goes high okay well that means this
7:22is going to open
7:23and then this is going to close and then
7:26when that closes what's y
7:27does y have a value yes it is it's
7:29connected directly to zero through that
7:31path
7:31and it is disconnected from three volts
7:33so we both y does have a value
7:35and it's not i'm not short circuiting
7:37one to zero
7:38and therefore it's when x is three
7:42y is zero can you see what gate that
7:45would be
7:46x is low we're high x is high we're low
7:50can you get it huh with the thing is it
7:52it's a not gate
7:53isn't that cool with your switches
7:55that's pretty cool
7:57we made a not gate so now
8:00we're going to see in these series of
8:01lectures we can one-to-one connect
8:03current flowing transistors to a boolean
8:06circuit and then a truth table on this
8:08beautiful page this is the this is the
8:10last slide before we end this lecture
8:12let's actually take a look at this so
8:13the idea is
8:15i've got a transition transistor circuit
8:16representation and i've got a block
8:18diagram and i'm going to say abstractly
8:20all i need to worry about is the block
8:21diagram from here out but if i
8:23if i lift the hood in that block diagram
8:25i'd say well this one transistor is
8:26doing the right thing wired a certain
8:28way
8:29let's see if this works let's actually
8:31take a look and
8:32and and discover the circuit more
8:34complicated circuit
8:35i've got a and b as input and c is
8:37output but also keep in mind
8:39again we never want to short circuit
8:40high to low and we always want to have c
8:42have a value so let's make sure this is
8:44true in all these cases
8:45and let's play with it okay so this is
8:48this is let me make sure okay this is
8:52a serial line two switches in cereal we
8:55said that
8:55and in fact c only connects when a and b
8:59if this is a so let's go back this is a
9:01these are both
9:02n type these are p types up here so
9:06when a and b go high here we go
9:09a and b go high so that's
9:12over here only when the a and b go high
9:16only then is c connected to ground
9:19now when a and b are high what happens
9:21well this opens up
9:22that opens up this opens up
9:26and b opens up remember when that goes
9:28high the p-type
9:30opens up so that means c does have a
9:33connection
9:34to one value there isn't a connection
9:37from c
9:38to the one up here to our three volts
9:41because both of these are open there's
9:42no way for that
9:43that ground to get to the three so we're
9:45okay we're in this row
9:48so now we're in one one zero and watch
9:50what happens the other three cases are
9:52identical
9:52watch out so that's like one set of
9:54cases okay we only get to that zero
9:56when a and b are both high the n channel
9:59closes
10:00and now c is ground and both of those
10:02guys are open
10:04now watch this here we go now all of
10:07these are the same category watch this
10:09this is really kind of fun
10:12how does this work well if either
10:15in those three cases we know either a or
10:17b or both are zero
10:19which means this is disconnected
10:22both of these guys are disconnected
10:25that's pretty cool
10:28so there's no connection from c to
10:30ground and
10:32this also means that one of these guys
10:36if one or both of them are zero one of
10:38these guys is closed remember when it's
10:40a zero
10:40the b type closes
10:44the end type opens with a zero so that
10:46means
10:47one there's gonna be a path either here
10:49or in here we saw that
10:50a parallel guy which means i don't care
10:52or both
10:53therefore c is gonna be one so in these
10:56three cases
10:57c is one when either one or both of a or
10:59zero c is one
11:01and only when both of these are one do
11:03both of these open up
11:04do both of these open up both of these
11:07open up oops i should draw this better
11:09both of these open up both of these open
11:12up
11:12no connection and c routes down there so
11:15this is my output
11:17we're going to see later this is called
11:19a nand gate
11:21it's like an and and says they're only
11:23one if both a and
11:24output is only one if both both a and b
11:27the inputs are
11:28both high nand is exactly the opposite
11:31so
11:31an an and gate would have been 0 0
11:350 1 and nand gate is the opposite of
11:37that so this is why this is a nand gate
11:40and here is a nand gate there that's
11:42pretty cool
11:44so we're going to see in the series of
11:45lectures how we're going to think about
11:47transistor networks
11:48and then very quickly move into the
11:50block diagram this is like a
11:52you know a building block call it an and
11:54move it on move forward from there
11:56once we live in the nan world we're now
11:58digital and we're not thinking about the
11:59transition anymore we're kind of
12:00abstract
12:00it's below our level level of
12:01abstraction for the rest of these
12:02lectures that's pretty cool
12:04also there's this really interesting
12:06fact that by wiring nand gates together
12:09if i just
12:09if you just went to fry's electronics
12:10and you bought a pool an arm full of
12:13nand gates
12:14could you build an and gate out of a
12:16nand gate out of wiring an and gate
12:18by you know maybe wiring one of the
12:20inputs to zero or to one
12:21or the two themselves or something like
12:23this could you make an and
12:25could you make an ore could you make a
12:27not gate and the answer is you can
12:29all you need to go to fry's is to buy
12:32series of nand gates that's not the
12:33optimal way to do it
12:34and from there you can build end or not
12:36and from and or not you can build
12:37everything after that it's pretty cool
12:39so nand gate is all you need it's really
12:41very very neat so
12:42that's kind of a cool thing so i started
12:44talking about your transistors i talked
12:46about transistors networks
12:47how to treat them with switches and how
12:49to move on to thinking of them as a
12:50block diagram as a
12:52you know digital circuit has some inputs
12:53has some outputs we got to make sure the
12:55transistor is never short circuit
12:57nothing in the transistor network short
12:58circuits and the output is always linked
13:00to at least one or one of one or
13:02zero never just kind of floating in the
13:03air so we got that we all
13:05that was true for this case and we're
13:06really happy so in the next series of
13:08lectures we're going to talk about
13:09signals and waveforms
13:11we'll see you there

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