Content area
Full Text
This technique of controlling PA module output power has advantages in dynamic range and accuracy compared to traditional current-sensing and voltage-sensing power-control methods.
Power-control methods for integrated Global System for Mobile Communications (GSM) power-amplifier (PA) modules are many. New methods include approaches based on sensing current and sensing voltage. But the best performance can be achieved with a linear-in-dB technique that provides an accurate and predictable method of controlling PA output power. In comparing these different approaches, the usual measures include output power and power-added efficiency (PAE). But other areas to consider include the PA's output-power stability as functions of temperature, frequency, load VSWR, and battery power; the dynamic range of the power-ramping function; the ease of calibration; trade-offs between PAE/battery current and output power; and the impact of the power-control circuitry on efficiency.
The voltage-sensing (Fig. 1a) and current-sensing (Fig. 1b) methods are often used for PA output-power con-trol. In the voltage-sensing method, a high-speed control loop is incorporated to regulate the collector voltage of the amplifier while the PA stages are held at a constant bias. By regulating the power, the stages are held in saturation across all power levels. As the required output power is decreased from full power down to 0 dBm, the collector voltage is also decreased. The current-sensing method senses the cur-rent supplied to the PA through the power supply. A complementary-metal-oxide-semiconductor (CMOS) controller controls the base voltage of the field-effect transistor (FET) with an error voltage generated by applying the ramp voltage, from the DAC, and the current measured to an error amplifier. These are both indirect closed loop methods as there is no direct measurement of the PA's output power.
Figure 2 shows a simplified block diagram of a more accurate way to control power by directly detecting the RF power from the amplifier. The module comprises a PA, a silicon power controller, and a number of passive components. The PA output power is controlled by adjusting the bias on the bases of the power transistors. The output power is regulated by a classic automatic-gain-control (AGC) loop. This measures the actual output power of the PA by coupling off a small proportion of the output power using a directional coupler. The sensed power level is...