Power amplifier protection by adaptive output power control

Cellular phone power amplifiers (PAs) operate in strongly varying environments and have to withstand extreme conditions. To avoid destructive breakdown a generic protection concept is proposed that is based on adaptive control of the output power. It provides over-voltage, over-temperature, and/or over-current protection by detection of the collector peak voltage, die temperature, and/or collector current to reduce the effective power control voltage once a threshold level is crossed. By applying protections, PAs can be implemented in low-cost silicon technology competitively to GaAs HBT implementations. In addition, requirements on package thermal resistance are relaxed. In this paper a theoretical analysis is given on the behavior of a class-AB amplifier under mismatch conditions. Measurement results on a silicon bipolar power transistor with integrated protection circuits are presented, proving the concept of adaptive protection. For a supply voltage of 5 V and nominal output power of 2 W no breakdown is observed for a VSWR of 10 over all phases when output power is adaptively reduced by 2.7 dB at most.