Class AB amplifier

A class AB amplifier is defined as an amplifier using a power stage that has output current flow for more than half, but less than all, of the input cycle (Gilbilisco 1994). 

TABLE 6.1 Characteristic Measurements Made on Class AB Amplifier Circuit... 

In order to measure the frequency response of the class AB amplifier, the input voltage source was changed to a small-signal AC stimulus source (AC 1). The breadboard results are shown in Fig. 6.31. The IsSpice, PSpice, and Micro-Cap results are shown in Figs. 6.32, 6.33, and 6.34, respectively. 

The rf transmitter amplifies an rf pulse signal of about 1 mW up to several W or up to several kW. The amplifier should work in a linear mode (class AB) because excitation pulse shape for slice selection must be reproduced. Class AB rf transmitters such as these with blanking gates are widely available commercially. 

A class AB power amplifier is capable of generating more power—using the same tube than the class A amplifier, but more intermodulation distortion will also be generated. A class B RF linear ampMer will generate still more intermodulation distortion, but is acceptable in certain applications. The plate efficiency is typically 66%, and stage gain is about 20-25 dB. 

In Fig. 7.52 the loacUine passes through the zero collector current point of each transistor where one device is turning off and the other is turning on. At this precise point both amplifier devices could be ojf or on. To avoid this glitch or discontinuity, strict Class B is never really used, but instead, each transistor is biased on to a small value of current to avoid this crossover distortion. The choice of bias current has an affect on 1M3 so that it maybe adjusted for best IM performance under a multitone test condition. The quiescent bias current is usually set for about 1-10% of the highest DC collector current under drive. Biased under this condition, the active devices are not operating in strict Class B, but rather, in a direction toward Class A, and is usually referred to as Class AB. 

Third-order intermodulation performance for Class AB or B is generally worse than Class A, and harmonic distortion is better in the sense that even harmonics are canceled to a large degree in the primary of the output transformer. These currents flow in opposite directions at the same time and net to zero, whereas odd order harmonic currents do not. An amplifier model for a Class AB or B amphfier is shown in Fig. 7.54. Its topology is the same as that for the Class A in Fig. 7.51 except that listandardLoad(sL) inside the amplifier is replaced by liintemai or Ri t- For a maximum power efficiency, //niax = 78.5%,... 

FIGURE 7.54 The Class AB or B amplifier may be modeled as having a measurable input resistance when driven and an internal output resistance that maintains the power efficiency at or below the theoretical maximum of 78.5%. For... 

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