The term given to describe the amount of current consumed by a circuit when it is not performing any work (sometimes referred to as standby current). This is a particularly important concept in designing battery-operated systems such as wireless beltpack transmitters. Battery life is determined by the total current drain composed of quiescent current and load current. Usually battery-operated devices are in standby mode more than in operation mode so the quiescent current consumption is the more dominant consideration. Quiescent current consumption should be as low as possible in order to prolong the battery’s life.
Class A amplifiers have the general property that the output device(s) always carry a significant current level, and hence have a large quiescent current. The quiescent current is defined as the current level in the amplifier when it is producing an output of zero. Class A amplifiers vary the large quiescent current in order to generate a varying current in the load, hence they are always inefficient in power terms.