2-16
mentioned earlier and with the internal operation of the transistor, we will move on to the basic transistor
amplifier.
To understand the overall operation of the transistor amplifier, you must only consider the current in
and out of the transistor and through the various components in the circuit. Therefore, from this point on,
only the schematic symbol for the transistor will be used in the illustrations, and rather than thinking about
majority and minority carriers, we will now start thinking in terms of emitter, base, and collector current.
Before going into the basic transistor amplifier, there are two terms you should be familiar with:
AMPLIFICATION and AMPLIFIER. Amplification is the process of increasing the strength of a SIGNAL.
A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. An
amplifier is the device that provides amplification (the increase in current, voltage, or power of a signal)
without appreciably altering the original signal.
Transistors are frequently used as amplifiers. Some transistor circuits are CURRENT amplifiers, with a
small load resistance; other circuits are designed for VOLTAGE amplification and have a high load
resistance; others amplify POWER.
Now take a look at the NPN version of the basic transistor amplifier in figure 2-12 and let's see just how
it works.
So far in this discussion, a separate battery has been used to provide the necessary forward-bias voltage.
Although a separate battery has been used in the past for convenience, it is not practical to use a battery for
emitter-base bias. For instance, it would take a battery slightly over .2 volts to properly forward bias a
germanium transistor, while a similar silicon transistor would require a voltage slightly over .6 volts.
However, common batteries do not have such voltage values. Also, since bias voltages are quite critical and
must be held within a few tenths of one volt, it is easier to work with bias currents flowing through resistors
of high ohmic values than with batteries.
By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the
emitter-base battery eliminated. In addition to eliminating the battery, some of these biasing methods
compensate for slight variations in transistor characteristics and changes in transistor conduction resulting
from temperature irregularities. Notice in figure 2-12 that the emitter-base battery has been eliminated and
the bias resistor R
B
has been inserted between the collector and the base. Resistor RB provides the necessary
forward bias for the emitter-base junction. Current flows in the emitter-base bias circuit from ground to the
emitter, out the base lead, and through R
B
to V
CC. Since the current in the base circuit is very small (a few
hundred microamperes) and the forward resistance of the transistor is low, only a few tenths of a volt of
positive bias will be felt on the base of the transistor. However, this is enough voltage on the base, along
with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor.
