Drain Efficiency Vs Power Added Efficiency in Power Amplifier (PA)

Introduction

Power amplifiers (PAs) are critical components in RF and microwave communication systems, converting DC power into amplified RF signals. Efficiency is a key performance metric, determining power consumption, heat dissipation and overall system performance. Two commonly used efficiency metrics for power amplifiers are drain Efficiency (η) and Power Added Efficiency (PAE). While both measure how effectively a PA converts DC power into RF output, PAE provides a more realistic efficiency measure by accounting for the RF input power. Understanding the difference between these metrics is essential for optimizing amplifier performance in wireless and radar applications.

Each measurement involves signal generator, signal analyzer and power supply or Source Measure Unit (SMU). SMU is a critical instrument when measuring PA efficiency due to its ability to measure DC current consumption of PA. Engineers can measure efficiency over range of supply voltages and use SMU to measure current consumption at each supply voltage.

Drain Efficiency

It is a metric of PA efficiency. It describes the percentage of DC power that is converted to RF energy. The term drain is derived from use of FET (Field Effect Transistor) in PA design, in which DC power is applied to drain terminal of FET device.

Drain Efficiency = P_{RF Output} / P_{DC Supply}

As mentioned in the formula, it is calculated by dividing the output power of PA by applied DC power.

Drain efficiency is useful metric of PA performance but it is less useful in PAs with low gain where input power is often significant. Hence PAE is measured.

Power Added Efficiency

The calculation of PAE is similar to drain efficiency except that it characterizes power added to PA instead of power at its output.

PAE = (P_{RF output} - P_{RF Input}) / P_{DC Supply}

Practically, PAE depends on range of operating conditions, including bias voltage and output level of amplified signal.

Conclusion

Both Drain Efficiency and Power Added Efficiency (PAE) are crucial for evaluating a power amplifier’s performance. Drain Efficiency focuses on how much DC power is converted into RF output, whereas PAE accounts for the impact of RF input power, providing a more accurate efficiency assessment. In practical RF amplifier designs, PAE is preferred, as it reflects the actual gain and efficiency of the amplifier. Engineers must carefully consider both metrics to optimize amplifier design for power consumption, thermal management and overall system efficiency.