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A CMOS RF power amplifier that can change the output transformer ratio is presented. The CMOS power amplifier is fully integrated in a 0.13 µm process and has a power added efficiency (PAE) of 38% at 2.1 GHz and an output power of 30.7 dBm with 3.0 V supply voltage. The PAE at an output power of 16 dBm was increased by 40% by altering the transformer ratio.
Introduction: Variable load impedance can increase the average efficiency of a power amplifier (PA), if the load impedance is changed to a higher value at lower output power [1-4], PAs with variable load impedance have been implemented using a varactor [2], switching transistors [3], or multipass structures such as a Doherty PA [4], The large voltage swing at the output saturates the varactor, thereby generating distortions. The on-state resistance of the switching transistor degrades the efficiency of the PA. Moreover the area of the switching transistors is typically very large. A Doherty PA requires more passive components which increase the chip area. In this Letter, a simple method of modifying the load impedance is presented without requiring any additional components, or chip area.
Principles: Fig. 1 shows the concept of transformer ratio switching. The transformer ratio at the high-power mode is 4:1, as in Fig. la. The Cl and C2 capacitors were designed for zero voltage switching (ZVS) in the switching PA. To switch to the low-power mode, the inner differential input signals that are connected to C2 must be off, and the Cl and C2 capacitances changed to C3 and C4, as in Fig. lb. Cl should be decreased by half to satisfy the ZVS condition, and C2 increased to infinity for an AC short. In this case, the transformer ratio can be changed from 4:1 to 2:1.
Circuit design: Fig. 2 shows...