8 Transistor Amplifiers
8.1 Biasing the BJT
8.2 Biasing the FET
8.3 BJT Amplifiers
8.4 FET Amplifiers
8.5 Frequency Response of Amplifiers
8.6 Learning Objectives
8.7 Practical Application: A Case Study—Mechatronics: Electronics Integrated with
Mechanical Systems
Problems
Amplifiers are circuits that produce an output signal which is larger than, but proportional to,
an input signal. The input and output signals can be both voltages or currents, or one or the
other, as in voltage-in current-out and current-in voltage-out amplifiers. The amplifier gain is
just the network’s transfer function, which is the ratio of output-to-input complex signals in the
frequency domain as found by complex analysis. Amplifiers find extensive use in instrumentation
applications. Sometimes, amplifiers are used for reasons other than gain alone. An amplifier may
be designed to have high input impedance so that it does not affect the output of a sensor while at
the same time giving a low output impedance so that it can drive large currents into its load, such
as a lamp or heating element. In some other applications, an amplifier with a low input impedance
might be desirable.
The first step in designing or analyzing any amplifier is to consider the biasing. The biasing
network consists of the power supply and the passive circuit elements surrounding the transistor
that provide the correct dc levels at the terminals. This is known as setting theQpoint (quiescent
or operating point) with no signal applied. A good bias circuit must not only establish the correct
dc levels, but must maintain them in spite of changes in temperature, variations in transistor
characteristics, or any other sources of variation.
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