Welcome back to electronics, this is Dr. Robinson.
In this lesson, we're going to look at the BJT terminal characteristics.
In our previous lesson, we introduced the Bipolar Junction Transistor or BJT.
And our objectives for this lesson are to examine the BJT terminal characteristics.
And the curves and equations that represent these characteristics.
I told you earlier that we were going to characterize the BJT,
based on its external behavior, it's behavior at its terminals.
And you can think of characterizing the BJT in that way,
as performing an experiment with this being the experimental set up.
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 zero and one volts and
as I do that, I want to measure the (uuu) current Into the transistor Ic.
If I do that, remember fixing this at 12 and
sweeping this from zero volts to one 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 and
voltage for a p-n 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 separate current with the VBE voltage,
the base emitter voltage.
Let's relate this curve to the regions of operation that we talked about.
You can see that if we're less,
have a Vb of less than about .5 volts the transistor is off, Ic is equal to zero.
So this region, we can label as the cutoff region.
And this region where VBE is greater than about 0.5 volts,
you can see the curve begins to leave the zero volt asymptote.
And the transistor turns on, allowing current to flow from collector to emitter,
ao 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 analyze the p and
junction diode, we assume that there was a forward voltage drop.
Across the diode of approximately 0.7 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 point seven volts.
So when the transistor is on,
we can assume that the DBE is about point seven 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 a meter by A current supply.
And I still had 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 at some consent values, so
I set this to some consent value.
And then, I sweep VCE from zero to 12 volts,
while measuring the collector current I see.
So just like the transfer characteristic code,
the y axis here is the collector current.
So I say, initially said, IB equal to zero and
sweep DCE from zero volts to twelve volts.
I will get this curve here and
because IB is equal to zero, IC is equal to zero no matter what DCE is.
So this is the IB equals zero,
microamps curve.
I then increased Ib to 20 microamps and make my sweep 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 output characteristic of curves.
Is the plot of IC versus VCE for constant IB.
Now, just like we did for the transfer characteristic curves,
we can identify the regions of operation.
On this set of curves, this red line where IC is equal to
zero 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 zero, 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 it the SAT.
And we can see that a characteristic of the saturation region
is that VCE is approximately a constant of about 0.2 volts.
Zero point two volts, so when in saturation,
we can assume that VCE is zero point two volts and get a pretty accurate result.
If VCE is greater than zero point two volts and
IC is greater than zero, we're in the active region.
Here I've summarized the information we 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 0.5V and
we also know in that region that the base current.
The collector current and the emitter current are all equal to zero.
In the active region, we know from the transfer characteristic curve that
VBE can be approximated at 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 base current is greater than zero and
we didn't obtain this equation from the characteristic curve.
But I am telling you that in the active region, IC, IB, and
IE can be 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 know 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 maximum current into the collector is flowing.
So, the collector current is actually less than what we'd expected to be
from the beta IB product.
So if we're 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 we're operating in the active reason,
the amplifier region, we can make approximations to those equations.
And I've shown these approximation 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 dot iode.
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 use in calculation of 0.0259 Volts or 25.9 Millivolts.
So if we plot this curve, this IC versus DVE curve,
using this parameter of the transistor and this constant.
We would get the transfer characteristic curve In the active region.
Now this equation, Ic 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 Bce.
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 naught, 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 naught is known as a zero bias base to collect current gain.
There's 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 one, so
if beta is 100, our beta null is 100.
Alpha has a typical value of 0.99, the early voltage Va,
has a typical value of 150 volts.
And the saturation current is a very small number,
typically about one times 10 to the minus 15 Amps.
So in summary,
during this lesson, we examined the terminal characteristics of the BJT.
In our next lesson,
we'll continue to look more at the parameters of the BJT, Beta, IS0, and VA.
And we'll look at how those parameters affect the characteristic curves.
So thank you and until next time.