BJT: Bipolar Junction Transistors

BJT: Bipolar Junction Transistors

BJT.1 Basic Operation

A bipolar junction transistor is a three-terminal device that, in most logic
circuits, acts like a current-controlled switch. If we put a small current into one
of the terminals, called the base, then the switch is "on"—current may flow
between the other two terminals, called the emitter and the collector. If no
current is put into the base, then the switch is "off"—no current flows between
the emitter and the collector.
To study the operation of a transistor, we first consider the operation of a
pair of diodes connected as shown in Figure BJT-1(a). In this circuit, current can
flow from node B to node C or node E, when the appropriate diode is forward
biased. However, no current can flow from C to E, or vice versa, since for any
choice of voltages on nodes B, C, and E, one or both diodes will be reverse
biased. The pn junctions of the two diodes in this circuit are shown in (b).
Now suppose that we fabricate the back-to-back diodes so that they share a
common p-type region, as shown in Figure BJT-1(c). The resulting structure is
called an npn transistor and has an amazing property. (At least, the physicists
working on transistors back in the 1950s thought it was amazing!) If we put
current across the base-to-emitter pn junction, then current is also enabled to
flow across the collector-to-base np junction (which is normally impossible) and
from there to the emitter.
The circuit symbol for the npn transistor is shown in Figure BJT-1(d).
Notice that the symbol contains a subtle arrow in the direction of positive current
flow. This also reminds us that the base-to-emitter junction is a pn junction, the
same as a diode whose symbol has an arrow pointing in the same direction.

It is also possible to fabricate a pnp transistor, as shown in Figure BJT-2.
However, pnp transistors are seldom used in digital circuits, so we won't discuss
them any further.
The current Ie flowing out of the emitter of an npn transistor is the sum of
the currents Ib and Ic flowing into the base and the collector. A transistor is often
used as a signal amplifier, because over a certain operating range (the active
region) the collector current is equal to a fixed constant times the base current
(Ic = β ⋅ Ib). However, in digital circuits, we normally use a transistor as a simple
switch that's always fully "on" or fully "off," as explained next.
Figure BJT-3 shows the common-emitter configuration of an npn transistor,
which is most often used in digital switching applications. This
configuration uses two discrete resistors, R1 and R2, in addition to a single npn
transistor. In this circuit, if VIN is 0 or negative, then the base-to-emitter diode
junction is reverse biased, and no base current (Ib) can flow. If no base current
flows, then no collector current (Ic) can flow, and the transistor is said to be cut
off ( OFF).

Since the base-to-emitter junction is a real diode, as opposed to an ideal
one, VIN must reach at least +0.6 V (one diode-drop) before any base current can
flow. Once this happens, Ohm's law tells us that

(We ignore the forward resistance Rf of the forward-biased base-to-emitter
junction, which is usually small compared to the base resistor R1.) When base
current flows, then collector current can flow in an amount proportional to Ib,
that is,

The constant of proportionality, β, is called the gain of the transistor, and is in
the range of 10 to 100 for typical transistors.
Although the base current Ib controls the collector current flow Ic, it also
indirectly controls the voltage VCE across the collector-to-emitter junction, since
VCE is just the supply voltage VCC minus the voltage drop across resistor R2:

However, in an ideal transistor VCE can never be less than zero (the transistor
cannot just create a negative potential), and in a real transistor VCE can never
be less than VCE(sat), a transistor parameter that is typically about 0.2 V.
If the values of VIN, β, R1, and R2 are such that the above equation predicts
a value of VCE that is less than VCE(sat), then the transistor cannot be operating in
the active region and the equation does not apply. Instead, the transistor is
operating in the saturation region, and is said to be saturated ( ON). No matter
how much current Ib we put into the base, VCE cannot drop below VCE(sat), and
the collector current Ic is determined mainly by the load resistor R2:

Here, RCE(sat) is the saturation resistance of the transistor. Typically, RCE(sat) is
50 Ω or less and is insignificant compared with R2.

BJT.2 Transistor Logic Inverter

Figure BJT-4 shows that we can make a logic inverter from an npn transistor in
the common-emitter configuration. When the input voltage is LOW, the output
voltage is HIGH, and vice versa.
In digital switching applications, bipolar transistors are often operated so
they are always either cut off or saturated. That is, digital circuits such as the
inverter in Figure BJT-4 are designed so that their transistors are always (well,
almost always) in one of the states depicted in Figure BJT-5. When the input
voltage VIN is LOW, it is low enough that Ib is zero and the transistor is cut
off; the collector-emitter junction looks like an open circuit. When VIN is HIGH,

it is high enough (and R1 is low enough and β is high enough) that the transistor
will be saturated for any reasonable value of R2; the collector-emitter junction
looks almost like a short circuit. Input voltages in the undefined region between
LOW and HIGH are not normally encountered, except during transitions. This
undefined region corresponds to the noise margin that we discussed with
Figure 1-2 on page 8.
Another way to visualize the operation of a transistor inverter is shown in
Figure BJT-6. When VIN is HIGH, the transistor switch is closed, and the output
terminal is connected to ground, definitely a LOW voltage. When VIN is LOW,
the transistor switch is open and the output terminal is pulled to +5 V through a
resistor; the output voltage is HIGH unless the output terminal is too heavily
loaded (i.e., improperly connected through a low impedance to ground).

Fuente: http://esminfo.prenhall.com/engineering/wakerlyinfo/samples/BJT.pdf