BASIC BJT AMPLIFIERS
In the last chapter, we described the operation of the bipolar junction transistor, and analyzed and designed the dc
response of circuits containing these devices. In this chapter, we emphasize the use of the bipolar transistor in linear
Linear amplifiers imply that we are dealing with analog signals.
A linear amplifier then means that the output signal
is equal to the input signal multiplied by a constant, where the magnitude of the constant of proportionality is in
general, greater than unity.
A linear amplifier is capable of producing signal power gain: that is the power in the
output signal is greater than the power in the input signal. We will investigate the source of this "extra" power.
We examine the properties of three basic single-stage, or single-transistor, amplifier circuits. These circuits are the
common-emitter, emitter-follower, and common-base configurations.
These configurations form the building
blocks for more complex amplifiers, so gaining a good understanding of these three amplifier circuits is an important
goal of this chapter.
We introduce a few of the many possible multistage configurations in which multiple amplifiers are connected in
series, or cascade, to increase the overall small-signal voltage gain or to provide a particular combination of voltage
gain and output resistance.
Our discussion includes the method of analysis required for these types of circuits and a
synopsis of their properties.
ANALOG SIGNALS AND LINEAR AMPLIFIERS
In this chapter, we will be considering signals, analog circuits, and amplifiers.
A signal contains some type of
For example, sound waves produced by a human contain the information the person is conveying to
Our physical senses, such as hearing, vision, and touch, are naturally analog.
Analog signals can
represent parameters such as temperature, pressure, and wind velocity.
Here, we are interested in electrical signals,
such as the output signal from a compact disc, a signal from a microphone, or a signal from a heart rate monitor. The
electrical signals are in the form of time-varying currents and voltages.
Time-varying signals from a particular source very often need to be amplified before the signal is capable of being
For example. Figure 4.1 shows a signal source that is the output of a compact disc system.
consists of a small time-varying voltage and current, which means the signal power is relatively small.
required to drive the speakers is larger than the output signal from the compact disc, so the compact disc signal must
be amplified before it is capable of driving the speakers in order that sound can be heard.
Other examples of signals
that must be amplified before they are capable of driving loads include the output of a microphone, voice signals
Introduction to Electronics