Achieving signal power amplification using energetically passive devices

This article discusses some fundamental properties of electronic amplifiers and the associated implications. When defining the passivity of a multiport circuit in the sense of net power consumption (rather than the ability to amplify signals), devices such as transistors, which are commonly described as “active”, have to be considered passive, which is referred to as “energetically passive” to avoid confusion with other definitions. It is shown that, when using such energetically passive devices to achieve signal power amplification, the amplifier circuit necessarily has to feature a non-linear transfer function. The article discusses how two common concepts (class A and B amplifiers) to achieve signal power amplification with such devices can be fundamentally related to Poynting’s theorem and that both of these concepts allow the realization of at least piecewise linear transfer functions in spite of the necessary non-linearity.