The bipolar transistor is a three terminal component made from three
layers of alternating semiconductor material. The layers form two PN
junctions & in some ways work like two diodes connected in series with
pointing away from the point where they connect, this point being the
third terminal of the transistor.
The terminals in the transistor are called Collector, Base & Emitter.
Although similar in construction, the particular way in which the layers
in a transistor are arranged give it some interesting properties.
The bipolar transistor functions as what is called a current controlled
current regulator. When a small current flows through the forward biased
base-emitter junction, a large current is also allowed to flow from
collector to emitter.
This seems counter-intuitive with the way you learned about diodes; a
reverse biased diode should not allow current through it. This emergent
property of the transistor is what gives it most of its uses, since a
little input current at the base generates a large output current
through the transistor, in essence it amplifies the current using an
external power source.
There are two types of bipolar transistors, PNP & NPN, named after the
combination of material types that make them up, they differ in
polarity of voltages applied. The explanations given are for NPN
transistors, use the reverse polarities for PNP transistors.
Transistor Operation
With collector connected to a more positive voltage than emitter & no
current flowing into the base of the transistor, no current flows from
collector to emitter, & the transistor is said to be in cutoff.
When the voltage applied to the base is slightly higher than the
junction voltage of the base emitter junction, some current starts to
flow from base to emitter, as well as from collector to emitter. The
current that flows through the collector is roughly the same as the
current going into the base times the current gain of the transistor
(Typically written as hfe or B [beta]).
Consider a transistor's collector connected directly to the voltage
source & the emitter connected to ground, by kirchoff's second law you
can see that the voltage the transistor gets will always be equal to
the supplied voltage.
In cutoff, no current flows through the transistor, so the voltage
source "sees" an infinite resistance, that is equivalent to an open
switch.
With only a voltage of a little over the junction voltage of base
emitter, let's say enough for 1mA to flow & a beta of 100, we get a
collector current of roughly 100mA. So in theory, if we supply 100mA we
should get a collector current of roughly 10A right?
In theory, yes, that should be possible. In practice however, current
flowing through any conductor generates heat, & with small transistors
even a current of less than 500mA could be enough to create enough heat
to burn & destroy the transistor. There's also the fact that any
voltage source has a limit on the amount of current it can supply.
Let's now consider another similar circuit, now instead of being
connected directly to voltage ground, we use a resistor of 90 ohms as a
load the collector. Let's also use the base current again from the
previous example, 1mA & a voltage source of 10v.
The theoretical collector current should be Ic = B Ib = 100 1mA = 100mA.
With 100mA flowing through it, the resistor gets an induced voltage of
9v, close to our voltage supply, with 1v across the transistor, we
account for all 10v of supply. But what happens if we increase the base
voltage to 2mA?
In theory, collector current should be Ic = 100 2mA = 200mA.
With 200mA flowing through it, the resistor should get an induced
voltage of 18v, which is clearly higher than our supply voltage. To
compensate, the transistor should have to be 8v lower than ground
potential, which it simply cannot do.
What happens in this situation is that the transistor will try to keep
the voltage across it as close to ground as it can to accommodate the
current that should be flowing through its collector. The base current
at which the transistor cannot lower the voltage across it , in other
words the transistor is fully on, is called the saturation current, &
the state itself called saturation.
