Active Region or Linear Region: Sometimes called the linear region. It refers to that part of the collector curve that is approximately horizontal. A transistor operates in the active region when it is used as an amplifier. In the active region, the emitter diode is forward-biased, the collector diode is reverse-biased, the collector current almost equals the emitter current, and the base current is much smaller than either the emitter or collector current.

Active-load resistor: A FET with its gate connected to the drain. The resulting two terminal device is equivalent to a resistor.

Active Loading: This refers to using a bipolar or MOS transistor as a resistor. It's done to save space or to get resistances that are difficult with passive resistors.

Active Filter: In the good old days, filters were made out of passive components like inductors and capacitors. Some filters are still made this way. The problem is that at low frequencies, inductors become very large in passive filter designs. Op amps give another way to build filters and eliminate the problem of bulky inductors at low frequencies. Any filter using an op amp is called an active filter.

Active Current Gain: The current gain in the active region of a transistor. That is what you usually find on a data sheet and what most people mean when they talk about current gain.

AC Saturation: The upper end of the ac load line. At this point, the transistor goes into saturation and clips the ac signal.

AC Resistance: The resistance of a device to a small ac signal. The ratio of a voltage change to a current change. The key idea here is changes about an operating point.

AC Load Line: The locus of instantaneous operating points when an ac signal is driving the transistor. This load line is different from the dc load line whenever the ac load resistance is different from the dc load resistance

AC Ground: A node that is bypassed to ground through a capacitor. Such a node will show no ac voltage when it is probed by an oscilloscope, but it will indicate a dc voltage when it is measured with a voltmeter.

AC Equivalent circuit: All that remains when you reduce the dc sources to zero and short all capacitors.

AC cutoff: The lower end of the ac load line. At this point, the transistor goes into cutoff and clips the ac signal.

AC current gain: AC Current gain with a transistor, the ratio of ac collector current to ac base current.

Acceptor: A trivalent atom, one that has three valence electrons. Each trivalent atom produces one hole in a silicon crystal.

Absolute value: The value of an expression without regard for its sign. Sometimes called the magnitude. Given +5 and -5, the absolute value is 5.