With thermal phase lag measurements, current paths are tracked in a Class A radio frequency (RF) power amplifier at 2 GHz. The amplifier is heterodynally driven at 440 MHz and 2 GHz, and its resulting thermal field was inspected, respectively, at 1013 and 113 Hz with an infrared lock-in thermography system. The phase lag maps evidence with a higher sensitivity than thermal amplitude measurements an input-output loop due to a substrate capacitive coupling. This limits the amplifier’s performance, raising the power consumption in certain components. Other information relative to local power consumption and amplifier operation is also inferred. This approach allows the local non-invasive testing of integrated systems regardless of their operating frequency.