12.1. General Considerations

As the first step in our study, we consider a transmitter delivering 1 W (+30 dBm) of power to a 50-Ω antenna. The peak-to-peak voltage swing, V_pp, at the antenna reaches 20 V and the peak current through the load, 200 mA. For a common-source (or common-emitter) stage to drive the load directly, the configurations shown in Figs. 12.1(a) and (b) require a supply voltage greater than V_pp. However, if the load is realized as an inductor [Fig. 12.1(c)], the drain ac voltage exceeds V_DD, even reaching 2V_DD (or higher). While allowing a lower supply voltage, the inductive load does not relax the “stress” on the transistor; the maximum drain-source voltage experienced by M₁ is still at least 20 V (10 V above V_DD = 10 V) if the stage must deliver 1 W to a 50-Ω load.

Figure 12.1. CS stages with (a) resistive, (b) current source, and (c) inductive load.