so I've got two transistor circuits
drawn out here that should look fairly
similar these are these are similar to
two what we've kind of already been
exploring but I want to think of them in
a little bit different way in that so
each of these has kind of an input and
an output and I want to think of these
inputs and outputs in terms of voltage
and of course transistors are current
switches but if this input for example
is is a voltage then that'll if it's a
positive let's say it's +5 volts then
that's going to create a current and of
course the switch the transistor on it
if it's zero volts then there will be no
current and the transistor will will be
off so we can think of this in terms of
these inputs either being on or off five
volts or zero volts which which will
help us out when we get into digital
logic where we where we deal with ones
and zeros and so in fact we can actually
just start talking about ones and zeros
now as long as we keep in mind that a
zero means zero volts and a 1 means +5
volts and so I'll actually just talk in
terms of zeros and ones being our inputs
and our outputs because our input can
either be a zero zero volts or a one +5
volts and actually what we'll see is
that the output is also going to be
either one of these so let's just take a
look at this circuit on the left to
start with and see what happens when its
input is zero volts so if I have zero
volts here and next I'll make a little
table down here just to keep track of
what's going on so I have an input and
my input can be either a 0 or a 1 and I
have an output which we will see can
also be either a 0 or a 1 so if my input
is zero so I have zero volts coming in
then the transistor is going to be off
because there's no current remember we
have this this base current that has to
flow in order for the in order for the
collector current to flow
oops that should be high collector
current to flow so with no base current
flowing there's no collector current
flowing and so there our output is not
is not connected in any way to 5 volts
because
transistor is cut off and so the output
is connected through this resistor to
ground so if we measure the voltage
between ground and our output we get
zero volts and sour output in this case
is zero so our input is zero and our
output is zero now let's look at the
case where our input is 1 so if our
input is 1 which remember means +5 volts
so we're using we're using five we're
using one to symbolize +5 volts or or I
guess another way to think of it is
we're using +5 volts as as kind of our
our symbol for for the notion of 1 but
anyway if our input is 1 then we're
gonna have that 5 volts and so we're
gonna have a current flowing across from
our base to our emitter and so when that
current is flowing assuming there's
there's enough current and when we're
assuming that there's enough current to
get the transistor into saturation which
means that current will flow
unrestricted from +5 volts to ground so
when that's happening if current is
flowing unrestricted from plus 5 volts
to ground if we were to measure our
voltage from our output between output
and ground output is basically directly
connected to +5 volts because we have we
have current flowing unrestricted from
that +5 volts so in this case our output
is going to be 1 so this at first glance
doesn't seem to be terribly interesting
we put 5 volts in over here we get 5
volts out over there we put 0 volts in
over here we get 0 volts out over here
but of course we saw before that this
output can drive a whole lot more
current then then the input might be
able to so we might have our input have
a relatively low amount of current but
then our output is able to drive more
current and there are a couple different
names for for this circuit that I want
you to be familiar with one of them is
is is this is called an emitter follower
and let me let me rewrite this I
apologize my my handwriting is terrible
emitter follower emitter follower
and the reason this is called an emitter
follower is basically because the
emitter this is the emitter of our of
course this is the collector this is the
base of our transistor the emitter
follows the input it follows the base so
the emitter follows the base so this is
called an emitter follower another name
for this might be a buffer and basically
a buffer just is a is a logic device
that takes whatever its input is and
gives it to you as the output and
there's a symbol for the buffer that
we'll use when we get into digital logic
and it's basically just a triangle like
this and this is our input over here and
this is our output over here and so
whenever you see this symbol when we get
into digital logic it basically means a
circuit that does this and it may not be
implemented exactly like this with a
transistor and a resistor it might have
other components in it it might be a
little bit more complicated but
effectively it will do this it will take
whatever voltage is on the input and
it'll give and it'll give that same
voltage level on the output and actually
it may take a range of voltages that
signify one on the input and give out a
maybe a little bit more voltage on the
output it may boost the voltage but but
effectively whatever your definition of
zero is on the input if if that's what
you have there then you'll get whatever
your definition of zero is on the output
and whatever your definition of one is
if it's plus five volts if you see that
on the input then that's what you'll see
on the output that's what a buffer does
this this circuit is also called an
emitter follower so you might see both
terms now moving on to this circuit we
we actually just built one of these you
may recall but we can we can go through
the same process here and look at our
input and our output and if our input
can be either 0 or 1 if the input is 0
then the transistor is there's no
current flowing across the base our base
current is zero because our voltage is
zero so no current flows and so the
transistor will be cut off and our
output will be tied through this
resistor to +5 volts and so our output
will be 1
and another name for this resistor that
you might mic up might come across is
called a pull-up resistor pull-up
resistor and that's because it basically
pulls up this output so this output is
being pulled up to 5 volts unless the
transistor turns on and so well actually
let me just well back up a second so
when this is 0 the output is 1 when the
input is 1 let me just erase these when
our input is 1 then the transistor is we
now have current flowing so this is 5
volts coming in here and so we have
current flowing to ground and so the
current flowing across the base is going
to create a current flowing across the
collector to the emitter and with this
current flowing here our output is now
going to be at a lower voltage because
our output is effectively going to be
connected to ground and so we're gonna
have 0 coming out here this is this is
what we saw in the last video we built
one of these and so when our input is 1
our output is 0 and so like I said this
is kind of called pull-up resistor
because if if the transistor is is
conducting or if it's if it's in
saturation our output is kind of
connected to ground but if the
transistor goes away then the resistor
pulls it up to +5 volts and that's why
it's kind of called a pull-up resistor
so you'll see that you might also see
pulldown resistor you can think of this
as a pulldown resistor so when the
transistor is off our output is pulled
down to ground but then when the
transistor comes on the the transit the
transistor is going to give less
resistance than this resistor and so our
output voltage will will go up to +5
volts or very nearly +5 volts so with
that in mind there are also a couple
names for this circuit over here and
actually the most common name that
you'll see for this is called an
inverter inverter and there's also a
logic symbol for that and it looks very
similar to the buffer except that it has
a little a little bubble on its output
so this is our input over here
and this is our output over here and
likewise whenever you see this symbol in
a logic diagram it basically means that
we're talking about a circuit like this
and again it doesn't have to be exactly
this circuit with one transistor one
resistor it just has to be a circuit
that obeys this particular this
particular truth table so whenever
whatever the input is a zero the output
will be a 1 whatever the input is a 1
the output will be a 0