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At higher frequencies, saturation voltage is significantly greater than the dc value. This is because the high-frequency active area is less than at dc. Typically, a high-breakdown-voltage device utilizing high-resistivity materials will have higher saturation voltage levels than low-breakdown-voltage device. The rf saturation voltage will thus have a great effect on the saturated power output of a transistor operated at too low a supply voltage. It is not advisable, therefore, to use a high-breakdown-voltage device for a low-voltage application. For a given power level, a much larger-area device is necessary of the saturation voltage is a significant percentage of the supply voltage.

Fig. 8. Ideal Class-B load linefor microwave transistors.

The ideal Class-B collector load line is shown in Fig. 8, where VSAT is the rf saturation voltage—higher than the dc value. The relation between power output, VSAT, and supply voltage is given by the equation:

From this equation, the effects of VSAT vs Vcc are shown in Fig. 9. It can be seen that VSAT has a significant effect on the maximum power output capability of a transistor. Thus, this is an important parameter from the standpoint of both circuit design and device design and must be taken into account when calculating the required load line to achieve a given power output.

Fig. 9. How saturated output power varies with rf saturation voltage with typical microwave transistors.

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