4 - Other Factors Affecting Amplifier Design

Abstract

The performance of an amplifier depends on how it is biased. Low power transistors are characterized as Class A, and many high power amplifiers are characterized as Class C. Each class is limited to a specified portion of the input signal during which current flows in the amplifying device. The definitions of classes apply regardless of whether the amplifier is a vacuum tube or a transistor, or whether it is a bipolar transistor or a field effect transistor (FET). The significance of the class of operation has to do with the amplitude linearity of the amplification process. It is important to note that only Class A amplifiers are linear in that the output signal (in the ideal case) is a faithful reproduction of the input signal. Radio frequencies propagate in space and permit information to be transferred from a transmitter to a receiver when information is added to the RF signal by a process called modulation. There are two basic kinds of modulation used to put information on an RF carrier. They are amplitude modulation and frequency modulation. Depending on the specification of a specific application, various degrees of requirements are set for the base bias voltage source. The RF energy in the form of pulses is utilized in many applications, including medical electronics, laser excitation, and various types of radar. Additional considerations in the design of a pulsed amplifier are energy storage near the device, and minimizing the inductance in the emitter leads. These two items affect the rise time of the pulse and prevent droop resulting from voltage decay during the duration of the pulse.