- [Instructor] A bipolar junction transistor
or BJT
consists of three alternating layers
of doped N-Type and P-Type semiconductor material
joined together.
And there are two ways that those three layers of material
can be stacked.
The BJT in the left
is called an NPN Transistor
because it contains a section of P-type material,
sandwiched between two N-Type sections.
And its counterpart on the right
is a PNP Transistor.
Because it has two P-Type sections
with a region of N-Type material in the middle.
For now,
I'll just focus on how an NPN Transistor works.
From a physical standpoint,
the three alternating layers of material
create two PN junctions.
At first glance,
those back to back PN Junctions
might look somewhat like a pair of diodes.
With their cathodes connected together
pointing away from each other.
This is a common simple vocation
that many people like to use
when explaining BJTs.
But it's important to recognize
that this is a major simplification.
If you try to understand how a transistor can behave
like two diodes,
you'll just end up confusing yourself
because it doesn't.
You cannot replicate a transistor on your breadboard
by connect two diodes together,
they are physically different things.
There are several important aspects
of the N and the P layers
that give the BJT its unique behavior.
For one, the middle layer of a BJT
between the two junctions
is designed to be as thin as possible
which affects how the holes in electrons
shift around amongst the BJT layers
as it operates in different modes.
Additionally,
the top and bottom layers
will have different amounts of doping concentrations.
So each of the two PN junctions
will behave slightly differently.
A bipolar junction transistor is not symmetrical.
Each of the N and P type regions
is connected to one of the three BJT terminals.
Called the collector, the base
and the emitter.
You can think of the base terminal
like the handle of a valve.
I'll be applying a signal to the base terminal
to control the transistor.
Turning it on or off.
The amount of voltage and current
that I provide at the base
will control the amount of current
that can pass between the other two terminals.
The emitter and collector were given their respective names
because in an NPN transistor when it's turned on,
electrons begin their journey through the transistor
at the emitter.
They flow up thought the base
and then pass through the collector on their way out.
However,
since by convention, we say that electrical current
is the flow of the holes,
which represent the absence of electrons
and those holes move in the opposite direction
of the electrons,
the conventional current through an NPN transistor
flows from the collector terminal
to the emitter terminal.
The schematic symbol for an NPN
has an arrow that indicates the direction
of conventional current flow through the transistor.
On the symbol shown here,
the top terminal is the collector,
the bottom terminal with the arrow is the emitter
and the base terminal is connected to the flat bar
on the left.
Before diving into how the BJT operates,
it's important to go over some common notation
that's used to describe the various voltages involved.
The voltage at each of the transistor terminals
relative to ground
is labeled using a single subscript.
VC is the collector voltage,
VB is the base voltage
and VE is the emitter voltage.
A double subscript is used to indicate the voltage
between two transistor terminals.
So V BE
is the voltage drop between the base and the emitter
and V CE
is the voltage drop from the collector to the emitter.
When using an NPN transistor,
the voltage at the collector has to be higher
than the emitter.
So VCE will always be positive.
That's a requirement because the current through an NPN
always flows from the collector to the emitter.
I'll use a similar set of subscripts
to refer to the current flowing in and out
of each of the terminals.
IB indicates the current entering the base terminal,
IC is the current flowing into the collector
and IE is the current exiting the emitter.
When the same letter is repeated in the subscript
for a voltage,
that indicates a power supply voltage.
VCC is the positive power supply voltage
associated with the collector
and VEE
is the negative supply voltage
associated with the emitter.
It's the relationship between all of those voltages
and currents at the transistor's terminals
that determines its operating mode.
And it's the responsibility of the circuit designer
to build up an appropriate network of other components
surrounding the transistor
to control those voltages and currents
to meet their needs.
Now that's a task easier said than done.
In designing transistor circuits really is its own art form.
For simplicity,
as I dive into the transistor's operating modes,
I'll focus on just the transistor
and the voltages and currents directly at its terminals.
But keep in mind that those input voltages and currents
are coming from a network of other components
surrounding the transistor
that I'm just not showing.
When a transistor is operating in the cutoff mode,
it's considered to be fully off.
It acts like a closed valve on a faucet
and prevents any current from passing through.
My NPN will be in the cutoff mode
when the voltage drop from the base to the emitter
is less than a certain threshold voltage.
Referred to here as VTH.
To understand where that threshold voltage comes from,
remember, that inside of an NPN transistor,
there are two PN junctions
which kind of looks like diodes.
Although as a whole,
the BJT doesn't behave like two connected diodes.
If I focus on just the bottom PN junction
between the base and the emitter terminals,
I'll see that it does behave similar to a diode
which means it will require
a certain amount of forward voltage to turn on.
For most common BJT transistors,
that threshold to turn on the base emitter junction
is usually somewhere around 0.7 volts.
Which corresponds to the forward voltage
that's necessary to turn on the bottom diode light junction.
When the base to emitter voltage is less than 0.7 volts,
the bottom diode will be in the cutoff region.
Which means no current flows into the base
and therefore no current flows between the collector
and the emitter.
The transistor is basically acting like an open circuit
between its three terminals.
If I raise that base voltage up
to the 0.7 volt threshold,
the base to emitter junction will become forward biased
and allow current to flow into the base.
The voltage between the base and emitter
will not rise above that threshold
for the same reason that the voltage
across a forward biased diode
doesn't increase above its forward voltage threshold.
At this point,
it's acting like a short circuit.
Under this condition,
the transistor is said to be biased, or turned on
and it will not limit the amount of current
that flows into its base.
So, it's very important that whatever circuit
is connected to the base terminal,
is designed to control and limit the current into the base.
That usually involves having a resistor
in series with the base terminal.
Now that the transistor is biased,
the amount of current that flows into the base
will control whether the transistor is operating
in the active mode
or the saturation mode.
When the amount of current into the base is relatively small
the transistor will be in the active mode
where it acts like a partially opened valve
to control the current flowing into the collector terminal.
The active mode is often called the linear region
because when the transistor is operating
in the active mode,
the current into the base and collector terminals
have a linear relationship.
With the collector current being equal
to the base current
times a factor called the current gain.
The transistor's current gain
or amplification factor
is a unitless value that describes the ratio
between the collector and the base currents.
When the transistor is operating in the active mode.
Current gain is usually represented
with a capital letter beta,
but you'll also see it represented as a lower case H
with a subscript FE.
This beta value is dependent
on the transistor's physical characteristics
and it will differ from transistor to transistor
due to variations in the manufacturing process.
It's rare to encounter two transistors
that have the exact same beta characteristics.
To make things even more confusing,
the beta ratio doesn't remain stable
for all operating conditions.
It can change based on the amount of collector current,
the transistor's temperature,
the frequency of the input signal
and other factors.
Because of all that variability,
good transistor circuits will be designed
so that their behavior is independent
of that transistor's unique beta value.
For simplicity in my examples,
I'll assume that my transistors have a beta of 100.
This is a common rule of thumb to use
when designing circuits,
because most basic small signal transistors
have beta values around 100.
If the transistor is operating in the active mode
with one milliamp of current flowing into its base,
the current flowing into the collector terminal
will be 100 times that.
Which is 100 milliamps.
Now according to Kirchhoff's Current Law,
the amount of current entering any circuit node
has to be equal to the amount of circuit exiting that node
because electrons are neither created nor destroyed.
Therefore, all of the current that's entering the transistor
through the base and collector terminals
must be exiting through the emitter terminal.
The emitter current will be equal
to the sum of the base and collector currents.
So, if the base current is one milliamp
and the collector current is 100 milliamps,
the emitter current will be 101 milliamps
out of this NPN transistor.
As the current into the base terminal
is increased or decreased,
the current through the collector and emitter
will change accordingly.
If the base current increases to two milliamps,
the collector current will be 100 times that
or 200 milliamps
and the emitter current will be 202 milliamps.
In the other direction,
if the base current was lowered
down to half a milliamp,
the collector current will decrease to 50 milliamps
so the emitter current would only be 50.5 milliamps.
One of my favorite ways to understand
the transistor's behavior
is using a simple model known as the transistor man
which was made famous by the book
The Art of Electronics.
Imagine that inside of every transistor,
there's a tiny little man.
His job is to look at the amount of current
flowing into the base terminal
which behaves like a forward biased diode
connected to the emitter.
And then, based on what he sees,
he adjusts the resistance of a rheostat on the other side
between the collector and emitter
to control the amount of current
flowing into the collector.
The transistor man's entire goal in life
is to maintain that beta relationship
between the base and collector current.
When he sees the base current decrease,
he'll raise the rheostat's resistance
to reduce the collector current.
And when he sees the base current increase,
he'll lower the rheostat's resistance
to let in more collector current.
If the base current increases too much,
the transistor man will lower the rheostat
as much as he can
until it basically acts like a short circuit
between the collector and the emitter.
At this point,
the transistor man runs into a problem.
Because he can no longer maintain the relationship
between the base and the collector.
He's letting as much current as possible into the collector.
If the base current continues to rise,
there's nothing he can do
to increase the collector current more.
When that happens,
the transistor is operating in the saturation mode.
The transistor is considered to be fully on
and acts like an open valve on a faucet
or a short circuit.
Letting current flow freely from the emitter
to the collector.
Since the transistor is acting like a short circuit,
ideally, the voltage drop between the collector and emitter
should be zero.
In reality,
there will be a small voltage drop
between the saturated transistor's collector and emitter.
That drop is usually around 0.2 volts or less
and it's special in the value
that it gets its own subscript.
Indicating that it's the saturated collector
emitter voltage.
It's important to note that in this condition,
the base voltage VB
will actually be higher than the collector voltage.
And both the base and collector will be higher
than the emitter.
Since the saturated transistor lets current flow freely
between the collector and emitter,
it'll be up to whatever circuits are connected
to those two terminals
to limit the current through the transistor.
For example,
if the circuit connected to the collector terminal
only provides 200 milliamps of current
in the saturated state,
if the base current was five milliamps,
then the output from the emitter will be 205 milliamps.
Even though the transistor is saturated,
it can never violate Kirchhoff's Current Law.
The relationship that the emitter current
equals the base plus collector current
will always hold true.
If the base current increased from five to 10 milliamps,
the collector current would remain the same
at 200 milliamps
because the circuit's saturated.
But the emitter current would increase to 210 milliamps
in accordance to Kirchhoff's Law.