Functional diagrams showing the processes that occur within an article and within individual components are used in studying the functional capabilities of the article, as well as in installation, adjustment, control, and repair work.
Fortunately, most of these transistors occur in pairs, such as the npn and the pnp bipolar junction transistors, or the n-channel and the p-channel MOSFETs, allowing designers to work symmetrically with positive and negative signals and sources. This statement may be clarified by noting that transistors can be characterized by graphs of output current i versus output voltage v that are parametrized by an input current (in the case of the bipolar junction transistor) or input voltage (in the MOSFET and JFET cases). Typically, the curves for an npn bipolar junction transistor or an n-channel field-effect transistor are used in the first quadrant of the output i-v plane, while for a pnp bipolar junction transistor or a p-channel field-effect transistor the same curves show up in the third quadrant. Mathematically, if i = f(v) for an npn bipolar junction transistor or n-channel field-effect device, then i = -f(-v) for a pnp bipolar junction transistor or p-channel field-effect device when the controlling parameters are also changed in sign.
Schematic diagrams define the components of the article that constitute a complete set and the interconnections between components; they usually give a detailed representation of the working principle of the article and may serve as a basis for developing other design documents, such as electrical installation blueprints and specification sheets.
Devices with an individual physical identity, such as amplifiers, transistors, loudspeakers, and generators, are often represented by equivalent circuits for purposes of analysis. These equivalent circuits are made up of the basic passive and active elements listed above.