Bipolar Junction Transistor

The bipolar junction transistor (BJT) is a three-element (emitter, base, and collector) device made up of alternating layers of n- and p-type semiconductor materials joined metallurgically. The transistor can be of pnp type (principal conduction by positive holes) or of npn type (principal conduction by negative electrons), as shown in Fig. 3-1 (where schematic symbols and positive current directions are also shown). The double subscript notation is utilized in labeling terminal voltages, so that, for example, Vbe symbolizes the increase in potential from emitter terminal E to base terminal B. For reasons that will become apparent, terminal currents and voltages commonly consist of superimposed dc and ac components (usually sinusoidal signals).

Diode Applications: Clamping Circuits

A clamper is a circuit that is designed to shift a waveform above or below a dc reference voltage without altering the shape of the waveform. This results in a change in the dc average of the waveform.

Both of these statements are illustrated in the figure below. (The clamper has changed the dc average of the input waveform from 0 V to +5 V without altering its shape.)

A clamper with its input and (ideal) output waveforms.

Diode Applications: Clipping Circuits

Clippers or Diode Limiters are networks that employ diodes to "clip" away a portion of an input signal without distorting the remaining part of the applied waveform. For a clipping circuit at least two components—an ideal diode and resistor are required and sometimes a dc battery is also employed for fixing the clipping level. The diode acts as a closed switch when forward biased and an open switch when reverse biased. The input waveform can be clipped at different levels by simply changing the voltage of the battery and by interchanging the positions of the various elements.

Introduction to Electronics: Terminology

This article contains the terms that we will often encounter in the succeeding lessons in Electronics.

AC and DC

Electricity flows in two ways, either in alternating current (AC) or in direct current (DC). The difference between AC and DC has to do with the direction in which the electrons flow.

In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards." DC is the kind of electricity generated by a battery. On the other hand, the power that comes from wall outlets is AC.

AC can even be changed to DC by an adapter, such as that used to power the battery on a laptop.
Source: Wise Geek