DYNAMIC RANGE ENHANCEMENT FOR LINEAR CLASS AB AMPLIFIER
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2002-Feb-22
Dynamic range requirements in terms of output power verses intermodulation distortion (IMD) per- formance of linear high power amplifiers in applica- tions such as the MIRS Base Repeater have been specified in the 20 to 30 dB range with a 350 Watt PEP capability. Such a design requires multiple cas- caded and paralleled stages of Class AB and Class AB push-pull stages. The IM characteristic of this class of amplifiers is described in Figure 1. At higher power levels IM performance is limited by the inter- cept point of the device, assuming any peripheral combining circuitry is optimized for optimum phase, amplitude balance, and impedance match. At lower power levels, cross-over distortion is observed and contributes significantly to IM distortion. Figure 1 illustrates that the AB bias point is a compromise between IM at high power levels verses IM perform- ance at lower power levels. As we bias towards Class B, IM at higher power levels improves, while it suf- fers at lower levels. As the bias is moved towards Class A, the curve flattens out. From the curves it is shown that while the IiVlR is improved at higher power levels the absolute level of the IM products will be nearly the same !at lower power levels for a given bias. Thus, there iis no improvement .to IM splatter when such an amplifier is operated at lower power levels. A second observation is that the gain of the device suffers significantly at lower levels for the near Class B bias. This is expected, as a mini- mum amount of power must be available to turn on the device for such a lightly biased junction. Depending on the application, the gain variation may or may not be desirable. Finally, the amplifier biased for optimum IM performance at higher power lev- els will also be most efficient at that bias given the IM requirement, For optimum IM and gain per- formance, it is apparent, that the bias point on the device must be modified appropriately for a given RF level.