Design of Microwave Active Devices

Chapter 2

Power Amplification

2.1. Characteristics of power amplifiers

The analysis of amplifiers discussed in Chapter 1 assumes linear operation of devices, with amplitude excitation signals that are small enough such that the linearization of component characteristics is possible; the equations linking currents and voltages are approximated by linear relations. If the amplitude of excitation signals increases, these approximations are no longer possible and it is necessary to take the nonlinear nature of relations, between currents and voltages in the active devices, into account.

A power amplifier is designed to provide output power for which the value is as large as possible. The amplitudes of signals must be sufficiently large and the nonlinear nature of operation must be considered.

Let us suppose that a power amplifier is excited by a sinusoidal signal. The output signal will only be sinusoidal if the excitation amplitude is sufficiently low; if the amplitude of the input signal increases, nonlinearities of the device will generate harmonics such that the output signal is no longer sinusoidal.

The output signal thus causes nonlinear distortions to appear, which depend on the input signal amplitude.

Figure 2.1 shows the spectrum and temporal shape of the output signal for two values of the input signal power. We observe that, for a low value of input power, the output signal spectrum has no significant harmonics amplitude and the signal is sinusoidal, then the operation is ...