An important concept in electronic circuits is that of feedback. Feedback occurs when an output signal is fed around a device to contribute to the input of the device. Consequently, when positive feedback occurs, that is, when the output signal returns to reinforce itself upon being fed back, it can lead to the generation of signals which may or may not be wanted. Circuit designers need to be conscious of all possible feedback paths that are present in their circuits so that they can ensure that unwanted oscillations do not occur. In the case of negative feedback, that is, when the output signal returns to weaken itself, then a number of improvements in circuit performance often ensue; for example, the circuit can be made less sensitive to changes in the environment or element variations, and deleterious nonlinear effects can be minimized.
The devices in an electronic circuit are classified as being either passive or active. The passive devices change signal energy, as is done dynamically by capacitors and statically by transformers, or absorb signal energy, as occurs in resistors, which also act to convert voltages to currents and vice versa. The active devices, such as batteries, transistors, operational amplifiers, and vacuum tubes, can supply signal energy to the circuit and in many cases amplify signal energy by transforming power supply energy into signal energy.
Since active devices usually supply signal energy to an electronic circuit, and since energy can only be transformed and not created, a source of energy is needed when active devices are present. This energy is usually obtained from batteries or through rectification of sinusoidal voltages supplied by power companies. When inserted into an electronic circuit, such a source of energy fixes the quiescent operation of the circuit; that is, it allows the circuit to be biased to a given operating point with no signal applied, so that when a signal is present it will be processed properly. To be useful, an electronic circuit produces one or more outputs; often inputs are applied to produce the outputs. These inputs and outputs are called the signals and, consequently, generally differ from the bias quantities, though often it is hard to separate signal and bias variables. Biasing of electronic circuits is an important, non- trivial, and often overlooked aspect of their operation.
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.