- [Instructor] A PNP Bioplar Junction Transistor
is the counterpart to an NPN Transistor,
working in basically the same, but opposite fashion.
So, the easiest way to understand how a PNP works
is to compare it to the NPN.
Structurally, where the NPN Transistor consists
of a section of P-Type material sandwiched
between two N-Type sections, the PNP Transistor,
has two P-Type sections, with a region of N-Type material
in the middle.
In terms of usage, both transistors use the same amount
of current at their Base terminals
to regulate the current flowing
between the other two terminals.
However, the direction of those currents is different
between the two types of transistors.
The control current for an NPN flows into the Base terminal
and the larger regulated current flows from
the Collector to the Emitter.
In a PNP, on the other hand, current flows out
of the Base terminal and the regulated current
though the transistor flows in the opposite direction
from the Emitter to the Collector.
One key difference between the two types of transistors,
is that an NPN Transistor, electrons are treated
as the important carriers, but in PNP Transistors,
the important carriers are the holes,
which are the absence of electrons
and represent positive charge.
So, unlike the Emitter in a NPN Transistor,
which emits electrons, the Emitter
in a PNP Transistor emits holes.
Those holes travel from the PNP's Emitter
to its Collector terminal, and since by convention
we say that electric current flows in the direction
of the holes, instead of the direction of electrons,
the conventional current through a PNP,
flows from its Emitter to the Collector
along with the holes.
The schematic symbol for a PNP looks similar
to the symbol for an NPN, except the arrow is pointing
in the opposite direction to indicate that the current
through a PNP flows into the Emitter.
Since the current through a PNP flows from the Emitter
to the Collector, the voltage at it's Emitter terminal
will always need to be higher or more positive
than the Collector terminal.
That's the opposite of an NPN Transistor
who's Collector voltage is always higher than the Emitter.
Since it's common practice to organize schematic drawings
such that the circuit nodes near the top of the page
are at a higher voltage than nodes near the bottom
of the page, you'll usually see PNP Transistors drawn
with their Emitter terminal at the top
because it's at the higher potential,
where as NPN Transistors will have their Emitter terminal
at the bottom because it's lower.
In both cases, the arrow on the symbol
is always connected to the Emitter.
Now, I have a hard time remembering which symbol
is the PNP and which is NPN, so to keep things straight,
I like to use the mnemonic Not Pointing iN
to remember that the arrow on an NPN Transistor
is pointing away from the bar.
It's a slightly odd sounding negated statement
but it works.
I don't have a similar mnemonic to describe the PNP,
so if the Not Pointing iN statement doesn't apply,
you're looking at a PNP.
A PNP Transistor has the same three operating modes
as an NPN.
In Cutoff mode, it acts like an open circuit
to block current between the Emitter and Collector.
In the Active mode, it let's some amount of current through,
proportional to the Base current,
and in the Saturation mode, it acts like a short circuit,
allowing current to freely flow between the Emitter
and Collector.
The conditions that cause PNP Transistors
to behave in these three operating modes
are basically the opposite of an NPN Transistor.
For that top diode-like junction between the Base
and Emitter to become forward biased,
the voltage at the transistor's Base
needs to drop below the Emitter voltage
by a certain threshold, which is usually around -0.7 volts.
I've chosen to describe that value in terms
of the Base voltage minus the Emitter voltage
so the value is negative, indicating that the Base voltage
is 0.7 volts lower than the Emitter.
For compactness, I'll use the standard label of Vbe
to indicate that difference between the Base and Emitter.
If Vbe is greater than the -0.7 volt threshold,
the PNP Transistor will be in the Cutoff mode.
The diode-like junction is off, so the transistor blocks
any current from passing through it
and acts like an open circuit.
When the Base voltage drops low enough below the Emitter
to reach that threshold, the PN junction
between the Emitter and Base,
acts like a forward biased diode
and turns on, allowing the transistor to begin operating
in the Active mode.
In the Active mode, the amount of current passing
through the transistor will vary based on the amount
of current flowing out of the Base.
The transistor will allow a proportional amount of current
to flow out through the Collector,
based on the transistor's individual current gain
or beta factor.
The Active mode PNP Transistor
will produce a Collector current
that is equal to beta times the Base current.
All of that current flowing out of the Base
and Emitter terminals, needs to come from somewhere,
so, in accordance with Kirchhoff's Current Law,
the current flowing into the Emitter terminal
will be equal to the sum of the Base and Collector currents
exiting the transistor.
As the amount of Base current increases or decreases,
the Emitter and Collector currents
will follow these equations to rise and fall too,
as long as the transistor remains in the Active mode.
If the Base current increases too much,
the PNP will become Saturated.
The Emitter terminal has maxed out the amount of current
it can draw from whatever source it's connected to,
so this Saturated transistor no longer follows
the linear relationship between the Base
and Emitter currents.
Just as with an NPN Transistor,
when a PNP becomes Saturated,
it acts like a short circuit for current
between the Emitter and Collector,
but there will still be a small voltage drop
between those two terminals of around -0.2 volts or less.
Again, I've made this value negative
to indicate that the Collector is at a lower potential
than the Emitter.
Since there is a -0.7 volt drop from the Base to the Emitter
and only a -0.2 volt drop from the Collector to the Emitter,
the voltage at the Base terminal of a Saturated PNP
will be lower than the voltage at either
of the other two terminals.
The Emitter voltage will be the highest
and the Collector will be slightly below that.
If I look at that alongside the voltage conditions
required for a NPN Transistor to be in the Saturation mode,
I can see that the greater than and less than symbols
are just oriented in opposite directions.
In fact, for each of the three transistor operating modes,
the voltage relationship between the Emitter, Base,
and Collector terminals, will have the opposite polarities
for NPN versus PNP Transistors.
Just flip the greater than and less than signs around
to convert the expression for one type of transistor
to the other.