A protection circuit for HBT RF Power Amplifier under load mismatch conditions

This paper investigates the radiofrequency failure mechanisms of power amplifier (PA) integrated in an HBT technology, and proposes a circuit solution for PA protection against impedance mismatches. It exposes the failure mechanisms that occur when a PA under extreme conditions (high battery voltage and high input power) is exposed to impedance mismatches at its output. Protection against high voltage operation is addressed by integrating a parallel base resistor which increases significantly emitter collector breakdown. A current sensor is then associated to a feedback loop on the PA biasing circuit that operates when the PA is in high dissipated power conditions by limiting the collector current. These protections are easily implemented on the PA die without any extra area. Experiments confirm the effectiveness of these principles: the protection is indexed on the collector supply voltage and acts for all output loads leading to VSWR up to 1 0:1, whereas output power and power efficiency on a 50 Omega load are not affected.