Welcome back to Electronics, this is Dr. Robinson.
In this lesson, we are going to look at the Bipolar Junction Transistor.
In your previous lesson, we examined the common source amplifier based on a MOSFET.
Our objectives for today's lesson, are to introduce the bipolar junction transistor.
Let's take a look at the structure of an NPN BJT.
An NPN bipolar junction transistor.
And for reference, I have drawn the structure of a PN junction diode.
Remember, the diode consists of a single PN
junction with a circuit schematic symbol that looks like this.
P-type material, N-type material.
We have current into the diode, ID, and a voltage across the diode, VD.
Now let's compare this structure to the structure of an NPN bipolar
junction transistor.
The BJT is a three terminal device and you can see that it consists of N material,
P material, and N material configured in this way.
The three terminals are known as the collector, the base, and the emitter.
The collector is connected to N material, base is connected to P material, and
emitter, again to N material.
So it's known as NPN because of this, this configuration.
It's known as bipolar because, both positive and
negative charge carriers participate in the operation of the device.
And it's known as junction, because it consists of two PN injunctions.
Between the collector and the base, and the base and the emitter.
Now as I've drawn this transistor we can model in the same way as the diode,
remember the diode point is a P injunction,
that points between the P material and the N material.
So you can think of an NPN transistor as two back to back anode con,
anode connected diodes, like this.
Where this is the collector, this is the base, and this is the emitter.
Now as I've drawn this structure just to indicate the relative
Structure of the N and P type materials.
It appears to be completely symmetric.
In other words, we could swap the collector and the emitter and
have it behave in the same way, but in the actual manufacture of an NPN transistor,
the relative geometries and relative dopings of these materials.
Are varied to optimize the performance of the transistor.
So in an actual circuit, you could not interchange the terminals,
without changing the behavior of the circuit.
Now remember, in a PN junction diode,
we can vary the current through the diode by varying the voltage across the diode.
So we change VD which changes ID.
And remember, to forward bias this junction,
we need a voltage of approximately 0.7 volts.
Well in this three-terminal device, the BJT, this base-emitter PN
junction behaves in exactly the same was as it does for a diode.
If we apply a large enough voltage here,
approximately 0.7 volts, remember from our diode analysis.
Then we forward bias this junction.
But because it's a three terminal device, and
controlled by the physics of these PN junctions.
When we forward bias this junction,
we cause current to flow from collector to emitter.
So in the two terminal device the single PN junction diode, we can
control the current that flows through the device, by the voltage across the device.
But in our three terminal device,
our BJT we can control the current that flows through the device.
By a voltage between two other terminals, between the base and emitter terminals.
So, we can have, say, one portion of a circuit setting this voltage,
which controls the current through another portion of the circuit.
Now, we know that the behavior of this device,
this BJT, is governed by the PN junction physics.
But in the following analysis we are going to
analyze the behavior of the NPN based on its terminal characteristics.
How it behaves externally at it's terminals.
And we know that that behavior is due to the PN junction physics, but we
don't have to worry too much about exactly the relationship between the physics and
the external behavior.
Now, here I'm showing this schematic symbol for NPN BJT.
You can see its a three thermal device, I've labeled the terminal C for
collector, B for base, and E for emitter.
The emitter is defined by an arrow on the terminal.
And its pointing from the P type material of the base,
to the N type material of the emitter.
Just like for a PN junction diode.
And you can see that from the structure, the terminals aren't interchangeable.
Now, for an NPN BJT the current for
the collector, is typically defined as N to the device.
So the collector current I see is N to the collector.
The base current.
IB is into the base.
And just like for node in a circuit,
we know that the sum of currents into this device must be equal to 0.
So we know that IB plus IC must equal the current out of the device IE,
the emitter current.
So, I can write by Kirchhoff's Current Law, IC.
Plus IB is equal to IE.
Now, here I'm showing a picture of NPN BJT transitors in various packages.
This is a TO 92 package.
It's a small signal transistor low power transistors.
You can see three terminals.
Emitter, base, collector.
This plastic package is not able to dissipate heat very well.
So these transistors can't handle a lot of current from collector to emitter.
This is a TO 220 package.
It has a, a metal tab here which could be connected to a large metal heat sink.
So this is a higher power transistor.
It can handle more current, without damaging the device.
And here is an even higher power transistor.
I'm showing you both the higher power package.
I'm showing you both the front view and the back view.
You can see that there only two terminals on the transistor and
that's because the case of this transistor actually acts as the collector.
So the base and emitter are isolated from the collector by this insulating,
insulating material and this, this package, this TO 3 package.
Is designed to be mounted on a heat sink.
And I've shown you it mounted on one here.
So, it's a very high power, high current transistor.
So as lots of current flows through the transistor, and
a lot of heat is generated.
It's able to dissipate that heat through the case,
which is mounted to a heat sink, which keeps the transistor cooler.
Which allows it to Conduct more current.
So PN junction diode consists of one PN junction.
This two terminal device, can be either forward biased or reverse biased.
So there are essentially only two possible states.
The diode is on or
the diode is off, depending on the externally applied voltages.
Well in a three terminal device the BJT that consists of two PN junctions,
we now have four possibilities for the biasing of these two junctions.
And, depending on how those junctions are biased,
governs the operation of the transistor.
So here, I have a table that relates the region of operation to
the biasing of the two PN junctions, the junction between the collector and
the base, and the junction between the base and emitter.
And in each one of these particular regions the BJT
has particular characteristics.
So if we reverse bias the collector base PN junction, and we reverse bias
the base emitter junction, and remember reverse bias means that the voltage on
the P type material is lower than the voltage on the N type material.
If we add this configuration,
we're in what's known as the cutoff region of the transistor.
No current flows in the transistor, and
it looks like an open circuit between collector and emitter.
So in this region we can say that IC is equal to 0, and
you can consider this to be the transistor is off.
In a saturation region if we have voltages that bias the junctions in this way,
we have the transistor being fully on, so IC is greater than 0.
And the transistor for
the most part looks like a short circuit between collector and emitter.
If we have this configuration, we're in what's known as the active region, and
this is the region in which we use the transistor as an amplifier.
And because we have two junctions and four possibilities,
we have this final possibility known as the reverse active region, but for
the most part it is not a useful region to operate the transistor in.
If we alternate the region of operation of the transistor,
between the cutoff region and the saturation region.
Where the transistor is off, and the transistor is on.
We can use the BJT as an electronic switch.
I told you earlier that we were going to characterize the BJT based on
its external behavior, its behavior at its terminals.
And you can think of characterizing the BJT in that
way as performing an experiment, with this being the experimental setup.
I've connected the BJT.
In this way so that I have a voltage supply, a DC battery between emitter and
base, and I also have a battery connected between collector and emitter.
And what I want to do is fix this voltage at 12 volts, and
vary this voltage between 0 and 1 volts.
And as I do that I want to measure the collector current.
N to the transistor IC.
If I do that, remember fixing this at 12, and sweeping this from 0 volts to 1 volt,
we get a characteristic curve that looks like this,
known as the transfer characteristic curve.
A plot of IC versus VBE, for a constant VCE.
And you can see that this curve has exactly the same form,
as does the relationship between current voltage for a PN junction diode.
Except that remember the voltage on this axis is
the voltage between these two terminals,
while the current on this axis is the current between collector and emitter.
So again we're controlling a.
We're controlling a separate current with the BBE voltage, the base emitter voltage.
Lets relate this curve to the regions of operation that we talked about.
You can see that,
if we're less have VB of less than about .5 volts the transistor is off.
IC is equal to 0.
So this region, we can label as the cutoff region.
And this region, where VBE is greater than about .5 volts,
you can see the curve begins to leave the 0 volt isotope, and
the transistor turns on allowing current to flow from collector to emitter.
So in this region, this could be either the active, or the saturation region.
Because in both of these regions, current flows from collector to emitter.
Now remember when we analyzed the PN injunction diode.
We assume that there was a forward voltage drop across the diode of
approximately .7 volts and, for the same reason, we can assume that,
when the transistor is on, that we can approximate this very steep curve by
a straight line of approximately 0.7 volts.
So the intersection of this approximation.
With the x-axis is about .7 volts.
So when the transistor is on, we can assume that VBE, VBE is about .7 volts.
We can obtain a second set of characteristic curves for
the NPN, BJT using this setup.
So here I've replaced the voltage between base and emitter by a current supply.
And I still have a voltage connected between collector and emitter.
So to form this set of curves known as the output characteristic curves
I hold the base current as some constant value.
So, I set this to some constant value and
then I sweep VCE from 0 to 12 volts while measuring the collector current IC.
So just like the transfer characteristic curve the y-axis here is
the collective current.
So I, say I initially set IB equal to 0 and
sweep VCE from 0 volts to 12 volts, I would get this curve here.
And bec, because IB is equal to 0, IC is equal to 0 no matter what VCE is.
So this is the IB equals 0 microamps curve.
I then increase IB to 20 microamps, and make my sweet again, and
I get this green curve.
So this is the 20 microamp curve.
And I can continue that, 20, 40, 60, 80, 100 microamps.
To get this family of curves.
And again, the difference between curves, is the value of IB.
And the calc output characteristic curves is a plot of IC versus VCE for
constant IB.
Now just like we did for the transfer characteristic curves,
we can identify the regence of operation on this set of curves.
This red line where IC is equal to 0.
Would be the cut off region.
This region here, where the relationship between IC and
VCE is linear, see these straight lines.
But IC is greater than 0 is the active region.
And finally, this region here, this very narrow region.
Let's see if I can label that.
This region here is the saturation region.
Let me just put sat.
And we can see that a characteristic of the saturation region is that VCE is
approximately a constant of about .2 volts.
0.2 volts.
So when in saturation, we can assume that VCE is 0.2 volts.
And get a pretty accurate result.
If VCE is greater than 0.2 volts and IC is greater than 0 we're in the active region.
Here I've summarized the information we've obtained from the characteristic curves
for each of these three regions, cutoff, active, and saturation.
We know from the transfer characteristics curve that when we're in
the cutoff region,
the transistor when it's off, we have a VBE of less than, or equal to 0.5 volts.
And we also know in that region, that the base current, the collector currant, and
the emitter current are all equal to 0.
In the active region.
We know from the transfer characteristic curve,
that VBE can be approximated as 0.7 volts.
We know from the output characteristic curve, that VCE is greater than 0.2 volts.
We know the transistor is on, so the the base current is greater than 0.
And we didn't obtain this equation from the characteristic curve, but
I'm telling you that.
In the active region IC, IB and IE can related in this way.
Where beta is the base to collector current gain, with a typical value of 100
and alpha is the emitter to collector current gain with a typical value of .99.
You can consider these both to be parameters of the transistor.
In the saturation region, we that again from
the transfer characteristic curve that VBE is approximately 0.7.
We know from the output characteristic curve that VCE is equal to
approximately 0.2 volts.
The transistor is on, but in the saturation region the transistor is
saturated, which means that the maximum current into the collector is flowing.
So, the collector current is actually less than what we'd expect it to be,
from the beta IB product.
So if we were in the active region, IC would be equal to beta IB.
But because we've reached the maximum value of collector current,
as we further increase the base current, this number is less than this number.
Now it's possible to write equations that, define the behavior of the BJT and
all regions of operation.
But if we know were operating in the active region,
the amplifier region, we can make approximations to those equations.
And I've shown this approximations here.
Here we have the relationship between IC and VBE, you can see it's
an exponential relationship just as it was for the PN junction diode.
In this equation, IS is known as the saturation current, and
VT is the thermal voltage.
The thermal voltage is given by kT over q,
where k is Boltzmann's constant, q is the charge on an electron, and
T is typically assumed to be 300, degrees kelvin.
To give a typical value used in calculations of 0.0259 volts,
or 25.9 millivolts.
So, if we plot this curve,
this IC versus VBE curve, using this parameter of the transistor, and
this constant, we would get the transfer characteristic curve.
In the active region.
Now, this equation, I see is equal to beta IB.
If I make this substitution for the value of beta,
then we have a single equation that relates IC to both IB and VCE.
We have a linear relationship if IB is considered to be a constant.
And we get those straight lines that we saw in the active region of
the output characteristics curve.
Now, beta naught can be considered an intrinsic transistor
parameter along with ISO and this quantity VA, which is known as the early voltage.
So if you looked at the data sheet of a transistor.
These quantities, beta not, IS0 and VA, would be on there somewhere.
And those three parameters, give us the values to plug in to these equations.
And these equations determine the operation of the transistor in
the active region.
Now beta not is known as the 0 bias base to collector current gain.
It has a typical value of 100.
Which indicates that there's a current gain from base to collector.
So a small value of base current, because this is large,
can result in a large collector current.
Which is why this transistor can be operated as an amplifier.
Alpha is related to beta in this way, beta over beta plus 1.
So if beta is 100, or beta not is 100 alpha has a typical value of .99.
The early voltage VA has a typical value of 150 volts.
And the saturation current is a very small number,
typically about 1 times 10 to the minus 15 amps.
In summary, during this lesson I introduced the bipolar junction
transistor, a three terminal device that consists of two PN injunctions.
We also examined its terminal characteristics.
In the next lesson we're going to take a further look at the parameters of a BJT,
beta naught, IZ 0 and
VA, and figure out how to determine these parameters from measured data.
So thank you, and until next time.