device is biased off. In turn, when the signal goes negative, the positive device biases off while the nega- tive device turns on and conducts the negative portion of the signal.
This configuration provides much greater efficiency than class-A. The problem with class-B is that it can create distortion at the zero-crossing point of the wave- form, making it unsuitable for precision amplifier appli- cations. Improvements to and variations of class-B have been developed to solve the crossover-distortion prob- lem, and many are in common use today; however, all operate under the push-pull and time-alternation para- digms.
All amplifier classes have theoretical limits to their effi- ciency, meaning they all waste a portion of the energy they draw from the AC mains supply. Dissipative de- signs such as class-A, class-B and other variations, have theoretical limits well below those of “switching” designs since they operate in that region between cutoff and saturation. This lower efficiency may not be a particular problem with lower power applications; how- ever, it can be a major factor in large amplifiers, and when several amplifiers are being used in an applica- tion. AC mains power may be limited and/or expensive to supply, and excessive heat created by dissipative amplifiers can also be inconvenient and costly to deal with.
PWM
PWM (Pulse Width Modulation) has been used for years in non-precision amplifiers to achieve very high effi- ciency. A PWM converter modulates the analog signal onto a fixed-frequency carrier wave, creating pulses
PWM Sampling of an Analog Waveform
that vary in width depending upon the amplitude of the signal's waveform. This creates a “sampling” of the signal, which is then converted back to analog to drive the load. The output devices “switch” between fully on (saturation) and fully off (cutoff) states, so they waste very little energy.
THE TIME-ALTERNATION PROBLEM IN PWM
The primary drawback to using previously existing (class-D) PWM technology in a precision, high-power audio amplifier output stage has been that in order to keep distortion sufficiently low, accurate timing circuitry is absolutely critical. Even the slightest variation in timing can cause both positive and negative switching devices to be on at the same time, allowing high “shoot-through” current to destroy the output circuitry. In essence, the time-alternation paradigm works well for dissipative amplifiers, but not so well for switching ones.
Crown invented a highly reliable PWM amplifier output stage that produces very little audible distortion, and solves the reliability challenge. This was made possible by adapting, for the first time in amplifier design, the very opposite of time alternation.
CLASS-I OPERATION
The push-pull paradigm is part of class-I but the time- alternation paradigm is not. In class-I, two sets of switching output devices are arranged in a “parallel” fashion and operating balanced in time, with both sets sampling the same input waveform. One set is dedi- cated to the positive current portion of the waveform, and the other to the negative current portion. When there is no signal applied, or when a signal varying in ampli- tude reaches the "zero crossing" between positive and negative, the switching devices are being turned on and off simultaneously with a 50% duty cycle. The result is the formation of two balanced and canceling high- frequency output currents with no net output at the no- signal condition. The two output currents are said to be “interleaved,” and class-I is named from this interleaved characteristic.
To produce a positive output signal, the output of the positive switching device is increased in duty while the negative switching device is decreased by the same
Class-I Switches, Signal at Zero Amplitude
