Crown Audio Class-I manual Transistor Operation, Class-A, CLASS-B and Time Alternation

The Class-I Amplifier

The Class-I Amplifier

Class-I, also known as BCA (Balanced Current Ampli- fier) is Crown’s patented, cutting-edge technology that gets more power out of an amplifier with less waste than was ever before possible.

Class-I technology offers several key advantages. It provides unprecedented efficiency, requiring less power from the AC supply than other designs and that can add up to significant cost savings over the life of the ampli- fier. Class-I handles reactive loudspeaker loads easily and gracefully, by reusing energy returned from the loudspeaker rather than dissipating it as heat or forcing the amp into premature current limiting. This character- istic means class-I amplifiers run better and longer— especially at lower impedances. It also makes them more reliable, since they are not constantly stressed to their limits or subjected to excessive heat. Best of all, as proud owners can attest, amplifiers with class-I technol- ogy sound great, with a powerful, accurate sound that stands out from the competition.

Crown’s class-I “switching” technology is a completely new adaptation of switching (PWM) amplifier design. This paper provides a simplified overview of Class-I, but before we explore its inner-workings, we need to look at the foundation of all previous high-power amplifier de- signs in order to fully appreciate how class-I stands apart.

TRANSISTOR OPERATION

To understand the different amplifier classes, it helps to understand a little about how transistors operate. Bias is a technical term referring to the static operating condi- tion of an electronic device, such as a transistor. In other words, bias defines how much conduction takes place in the transistor with no dynamic signal input. Transistors may operate in three possible states: cutoff, saturation, or somewhere in between. The cutoff state is when there is not enough signal present to cause the device to conduct. Saturation refers to when the device has reached maximum conductivity. Amplifiers referred to as “dissi- pative” control their output by operating in the region between cutoff and saturation.

CLASS-A

Class-A amplifiers are the simplest in design, and can be the most distortion-free of all amplifier classes. In class-A, the output devices are biased on all the time with a current large enough to produce the largest output signal.

Some class-A amplifiers may employ both a positive and negative device in a push-pull arrangement to increase output power, but both devices still are biased

Class-A Operation

on and conduct all the time. Class-A amplifiers are generally considered to be the most accurate of all classes in low to moderate power ranges and are useful for applications such as preamp stages; however, they create tremendous amounts of heat due to their very low efficiency, making them impractical for high-power am- plification. Other amplifier classes have been devel- oped over time to overcome the class-A efficiency problem.

CLASS-B AND TIME ALTERNATION

Class-B was invented as a solution to the efficiency problem with class-A. The invention of class-B is signifi- cant in that with it came the concept of time alternation, which has been the foundation of virtually all power amplifier designs used for audio reproduction and in- dustrial power since about 1931 to the present. While many incremental improvements have been brought to market since that time, none have varied from the basic time-alternation paradigm.

The basic class-B amplifier implements two devices in the output stage in a “push-pull” arrangement, with each amplifying half of the waveform, and the devices oper- ate in strict time alternation. When the signal goes positive, the positive device conducts while the negative

Class-B Operation