LOW VOLTAGE PAs Techniques for Low Voltage Power Amplifier Matching

There is rapid expansion in the use of short range digital wireless systems [1], using the license-free allocations around 915 MHz in the USA and 869 MHz in Europe that permit up to 1 watt output power. These RF systems now use digital signal processing (DSP) for modulation and demodulation so to get better circuit packing density, and take advantage of finer IC process geometries that generally use low voltage supplies. Of course, lower operating voltages have the further benefit that they reduce the power consumption of the digital circuits, always an important parameter in battery powered products. However, reduced supply rails result in low impedances at the output of the power amplifier (PA) which can be difficult and inefficient to match. As a result, the power amplifier is often run from a separate, higher voltage, which has to be provided by an additional external regulator. With the constant pressure on commercial products for lower cost, the external parts count must be minimized. The approach we have taken is to run the PA off the 1.8 V supply hat is already used as a common interface level in many products. This tutorial shows how it is simple to use a balanced amplifier with a low cost balun incorporated into the circuit board, so power outputs of more than 1 W can obtained at 900 MHz, even on a 1.8 V supply, with a minimum of external matching components.