Introduction

In Chapter Modeling and specification of amplifiers, we discussed the characterization of amplifiers. We concluded that an amplifier should provide its load with an accurate copy of its source signal, while the power available to the load should exceed the available power of the source. In this chapter, we will discuss the principle of amplification, as it is based upon the application of nonlinear, passive devices and power sources. We will discuss under which conditions the available small-signal power gain of a network comprising nonlinear, passive devices and bias sources can exceed unity. To this end, a formal introduction of the concepts operating point, biasing and available power gain will be presented.

Active devices

Electronic devices with amplifying capabilities, such as, MOSFETs, Junction FETs, Bipolar Junction Transistors and Vacuum Tubes are often referred to as active devices. However, the adjective active is somewhat misleading. These devices themselves are passive because they do not provide any electrical power. The term active means that when these devices are properly combined with power sources it appears as if they can provide electrical power themselves.

According to network theory, a network delivers power if the sum of the products of its branch currents and the corresponding branch voltages is negative.

From now on, we will also use the term active devices, while bearing in mind the fact that we actually refer to passive devices that require power sources to behave as such.

This chapter

In this chapter, we will show the way in which passive devices have to be combined with power sources, such as to obtain amplifying capabilities. In section Two-terminal resistive elements we will do this for two-terminal, passive, resistive elements. We will also give a formal definition of complementary elements and of the concepts operating point, biasing and available power gain.

In section Multi-terminal resistive elements we will discuss the deployment of the amplifying capabilities of biased, passive, resistive, multi-terminal elements and especially of biased, passive, resistive, two-ports and biased, passive, resistive three-terminal elements.

In section Introduction to biasing we will give a formal approach to the biasing of electronic devices. We will discuss the selection of independent and dependent bias sources and briefly introduce methods to derive those sources from the power supply sources. We will also discuss the application of a biased, passive, three-terminal element as basic amplifier stage and elucidate the amplification mechanism.