Various apparatus exist for amplifying signals.  In applications that involve high power amplification and transmission of modulated signals, a premium is placed on amplifier efficiency.  In addition, because many modulated signals have information in the amplitude envelope as well as in the phase of the signal, a premium is placed on the ability to faithfully reproduce the amplitude and phase of the signal.  In particular, the amplifier preferably exhibits very low distortion in both amplitude and phase. 
Communications devices which transmit digitally modulated signals having information in both amplitude and phase are an example of applications where these qualities are in demand.  In these applications, low distortion reduces undesired spectral components and ensures that occupied channel bandwidth is not degraded by the amplifier.  Having a high efficiency amplifier design can reduce transmit current consumption, which allows the devices to operate longer on a single battery. 
Some amplifiers are operated in their "linear" region.  This is a region of operation that produces low distortion, but it is inefficient in terms of power consumption.  One method of achieving increased efficiency is to allow the amplifiers to operate partially in "compression."  When operating in compression, amplifiers are more efficient, but they are also outside the linear region of operation, and distortion results.  Another method of achieving increased efficiency is to use saturating amplifiers.  Saturating amplifiers, such as class D, class E, and class F amplifiers, operate beyond compression, in a region of "saturation," and they achieve the goal of high efficiency at the expense of non-linearity.  Non-linear amplifiers, such as saturating amplifiers and amplifiers operating in compression, however, are generally not used in applications where information is included in the amplitude envelope, because that information is corrupted by the non-linear amplification. 
In addition to distortion and loss of information, non-linear operation of amplifiers causes an increase in intermodulation products.  Intermodulation products cause undesirable amounts of energy to be present in frequency bands other than the one intended for use.  This undesirable energy is commonly quantified and termed Adjacent Channel Power (ACP).  Excessively high levels of ACP cause undesired interference to users operating in nearby channels.  This degradation can cause an amplifier to be unsuitable for a particular application. 
Since efficiency has traditionally been gained by driving amplifiers into compression and saturation, and since driving amplifiers into compression and saturation causes intermodulation products to rise, high efficiency and low intermodulation products have historically been traded off against each other.  It would be very desirable to achieve low intermodulation products in a highly efficient amplifier, thereby obviating the need for a trade-off between the two. 
Accordingly, there is a need for high efficiency amplifiers operating in compression and saturation exhibiting low distortion and low intermodulation products.