Power amplifiers (PA) in wireless communications devices are typically designed to drive antennas at a higher power than necessary. An impedance mismatch between the antenna and the power amplifier causes standing waves at the output of the power amplifier. This reduces the total radiated power of a wireless communications device.
Load impedance variations at the antenna of the wireless communications device are caused by reflections of the radiated electromagnetic (EM) waves from objects in the vicinity of the antenna. The resulting standing waves at the output of the power amplifier can damage and reduce the total radiated power of the wireless communications device. The reflected power is expressed as a voltage standing wave ratio (VSWR), which is a ratio of the maximum to minimum amplitude (voltage or current) of the corresponding field components on the line that feeds the antenna.
To provide enough total radiated power, the power amplifier is typically over-designed to be able to provide sufficient output power to the antenna such that the total radiated power specification can be met even under poor impedance matching conditions. As a result, the power amplifier is designed to produce more output power than what it would need if the impedance match was to be perfect. This causes the efficiency of the power amplifier to be lowered at the nominal output power at the antenna as compared to another power amplifier that was designed to operate under perfect impedance matching conditions with the antenna.
One approach to address this problem is to use a programmable impedance matching network to adjust the impedance presented by the antenna. Impedance matching elements such as Barium Strontium Titanate (BST) capacitors or micro-electromechanical (MEM) capacitors may be used to provide a programmable capacitance under an applied control voltage. The applied control voltage may typically be within a range of 0-30V, for example. The DC point sets the capacitance value which can be used to create the programmable impedance matching network. The complete matching network is formed by making a pi to T network with two programmable capacitors and an inductor, or one programmable capacitor and two inductors.
Directional couplers are typically used with programmable impedance matching networks, and are placed on the path between the power amplifier and the antenna to measure forward and reflected power. Detection of the forward and reflected power is performed using log-amplifiers and non-linear detectors associated with the directional couplers. The impedance matching elements can then be adjusted to increase the forward power or reduce the reflected power. A disadvantage of directional couplers is that they are limited in dynamic range and directivity over a wide frequency band of operation. To support a wide frequency band of operation, multiple directional couplers are required. However, this increases the cost and complexity of a wireless communications device.