- [Instructor] There are several varieties
of bipolar junction transistors
that are rated to handle different amounts of voltage,
current, and power for different types of applications.
For every different type of NPN transistor,
there's a complementary PNP counterpart
with similar characteristics to go along with it.
For example, the B547 is a common NPN transistor
that works well for low-powered applications
with moderately high voltages and current.
Its PNP counterpart is the BC557.
The BC337 and BC327
are another complementary pair of transistors
that can handle a bit more current than the BC547 and 557.
The TIP29 and TIP30 transistors are designed
for medium power switching applications
and can dissipate up to two or three watts,
and their high-powered cousins,
the TIP3055 and TIP2955 can dissipate up to 90 watts.
For this video, I'll dive into the datasheet
for a transistor known as the 2N3904 to discuss
several of the important characteristics to consider
when choosing the right transistor to use for a project.
The 2N3904 is a very common NPN transistor.
It's a good general-purpose transistor to use
for switching or amplifying small signals.
I always keep my parts kit stocked with a handful
of 2N3904s and their PNP counterpart, the 2N3906.
To find a datasheet for the transistor,
I'll go to my search engine and type in the part number,
2N3904, and the word datasheet.
I see that there are several datasheets available
for the 2N3904 transistor from different manufacturers.
For this video, I'll look at this .pdf link
posted on mouser.com, which is for the 2N3904 transistor
from Fairchild Semiconductor.
The first page of this datasheet shows
the three different types of device packages
that are available for this transistor.
The picture on the left, labeled as TO-92,
is a common package that's used
for a variety of different transistor types.
TO stands for transistor outline,
and the package consists of a plastic case
with three long metal leads on one side
for each of the three transistor terminals.
The other two device packages shown here
are SOT packages, which stands for small outline transistor.
SOTs are surface mount components with a small footprint
for compact printed circuit board designs.
For prototyping circuits on my breadboard,
I tend to use transistors with the TO-92 package type.
The transistor type is indicated
by a marking printed on the flat side of the plastic case.
Now, that marking can be really small, and it is often
hard to read, so you may need to get a magnifying glass
to help you identify the transistor.
According to this datasheet, the marking
on these 2N3904 transistors will be 2N3904,
and that's convenient, but don't expect all transistors
to simply have their part number printed on them.
You should always refer to the datasheet
for your specific component.
The pin configuration for the component can also vary
between manufacturers and different types of transistors.
Again, refer to the datasheet.
The pins on this particular transistor go in the order
of emitter, base, collector, going from left to right.
You can use the flat side of the transistor package
to orient yourself, so you know which side is which.
The amount of power that the transistor can dissipate
will be related to its physical size
and the materials used in the package.
The TO-92 package is relatively small,
and it's only able to dissipate a few hundred milliwatts.
Now, that works fine for the 2N3904,
which is intended for amplifying and switching
relatively small signals, but power transistors,
on the other hand,
usually need to dissipate several watts of power.
So they require a larger package to stay cool,
like the TO-220.
The TO-220 is a common package for power transistors
that has a metal plate on the back
to help transfer heat away from the semiconductor inside.
On its own, the TO-220 can safely dissipate
about one watt into the air around it,
but it's more commonly used with an external heat sink
that attaches to the back metal plate
with thermally conductive grease.
With an external heat sink in place,
the TO-220 package can dissipate up to about 50 watts.
Moving on down to the second page of the datasheet
brings us to the device's absolute maximum ratings.
Like all electronic components,
transistors have physical limits to the amount
of voltage and current they can safely handle.
Exceeding any one of these maximum rating values
can cause permanent damage to the component.
The collector emitter voltage rating listed
in the datasheet as V-CEO is the maximum voltage
that the transistor can withstand
between its collector and emitter terminals.
That comes into play when using the transistor
in the cutoff mode, where it acts like an open circuit.
It's an especially important rating to pay attention to
when using the transistor as a switch,
because when it's switched off,
it'll have the full collector to emitter voltage across it.
Since bipolar junction transistors contain a pair
of PN junctions, there's a rating for the maximum voltage
that can be safely placed across each of those
diode-like junctions in their reverse bias directions.
V-CBO is the maximum voltage from the collector to the base,
which would reverse bias that top PN junction,
and V-EBO is the maximum voltage
from the emitter to the base,
which would reverse bias the bottom junction.
According to the datasheet, this transistor can handle
a maximum collector current of 200 milliamps.
Notice that it's labeled as the maximum continuous current,
meaning this transistor can handle a constant flow
of 200 milliamps through its collector.
When looking at other transistor datasheets,
you'll often encounter specifications
for the maximum pulsed collector current
as shown here on the datasheet for a TIP29 power transistor.
The pulse current is higher than the continuous DC current
because the transistor can dissipate more power
in short bursts than in a steady stream.
It's important to note that the maximum current rating
listed here is based on the amount of power
the transistor can dissipate
when it's operating in the saturation mode,
which means it will have the smallest possible voltage drop
from the collector to the emitter.
The power dissipated by a transistor
is equal to its collector current
times the collector emitter voltage.
When this transistor is saturated,
it'll have a collector emitter drop of about .2 volts.
So, if it has 200 milliamps of collector current,
it'll only need to dissipate 40 milliwatts of power.
However, if the transistor is not saturated,
then the emitter collector voltage could be much larger.
And in that case, the transistor will likely exceed
the maximum power dissipation rating
or the collector emitter voltage rating
before it reaches the 200 milliamp collector current limit.
The thermal characteristics section
describes the amount of power this transistor can dissipate.
These power ratings are always referenced to the ambient
temperature of the air surrounding the transistor.
On this datasheet, the transistor is characterized
for operating around standard room temperature
of 25 degrees Celsius,
which is around 77 degrees Fahrenheit.
At that ambient room temperature, the 2N3904 can safely
dissipate up to 625 milliwatts of power.
However, if the transistor is going to be used
in a hotter environment, then its maximum power rating
needs to be lowered or derated.
For every degree above 25 degree Celsius,
you should subtract five milliwatts
from that 625-milliwatt maximum.
These temperature considerations are important because heat
is the mortal enemy of semiconductor components.
The silicon in a transistor starts to break down
and lose its doping when the temperature rises
above around 150 degrees Celsius.
That's why the operating and storage temperature range
listed in the maximum rating section only goes up to 150.
150 degrees Celsius might seem like a lot of heat,
but you can quickly reach that fatal temperature range
if you try to dissipate
too much power through your transistor.
The third page of this datasheet lists a variety
of electrical characteristics for the transistor.
The first subsection lists characteristics
that are relevant
when the transistor is operating in the cutoff mode.
The three breakdown voltage parameters are the same values
as the breakdown voltages listed in the absolute
maximum rating section on the previous page,
describing the voltage that each of the three junctions
can handle in the reverse bias direction.
When those junctions are reverse bias,
I expect the transistor to be in the cutoff mode,
and it'll act like an open switch.
However, these are not perfect components.
So even in the the cutoff mode,
a very small amount of current
will still leak through.
According to the conditions of the base cutoff current
and collector cutoff current parameters,
if there are 30 volts between the collector and emitter
and the emitter to base junction
is reverse biased by three volts, then a maximum
of 50 nanoamps will leak through the base and collector.
The next subsection on this page lists characteristics
related to the transistor when it's turned on.
The DC current gain is the ratio of the collector current
to its base current
when the transistor is in the active mode.
And it's usually abbreviated as h-FE,
or with the capital letter beta.
It's the key parameter that characterizes
the BJT's ability to amplify a signal.
To simplify calculations, it's common to treat
this amplification factor as a constant value,
which is usually estimated as 100.
Unfortunately, that beta figure
is not actually a constant value in practice
and can vary significantly
depending on the operating conditions.
Because of that variability,
this datasheet lists the DC current gain
over a range of five operating conditions
where the collector-emitter voltage is held constant
at one volt, but the collector-current ranges
from .1 milliamps up to 100 milliamps.
The table lists a minimum current gain factor
at each condition, and for the middle entry,
it lists a maximum value.
Generally, the gain factor will be the highest
for the middle range of collector currents
and decreases for very low and very high collector currents.
At that middle collector current,
the gain factor can be anywhere from 100 to 300,
which is a pretty wide range.
And it really does show how much the gain factor can vary
from one component to another.
A bag of transistors is like a box of chocolates.
You never know what you're going to get.
Below the DC current gain, this table lists
the saturation voltages for the collector-emitter
and base-emitter junctions.
Just like the DC current gain, these parameters
can vary depending on the operating conditions.
So they're each listed for two combinations
of different base and collector currents.
When the transistor is saturated,
we want the collector-emitter voltage to be low,
so the transistor acts as close
to a short circuit as possible.
That's why the table lists maximum values
for the collector-emitter saturation voltage.
We can always count on it being below those values.
The base-emitter saturation voltage is listed in a range
from .65 to .85 volts, which encompasses
that rule of thumb value I used that the base
emitter voltage is usually around .7 volts.
The remaining two sections of the electrical
characteristics table describe the transistor's
small signal characteristics and switching characteristics.
They're related to how quickly the transistor can change
its output in response to input signal changes,
and they're beyond the scope of this course.