Method and apparatus for sensing the envelope of high level multi frequency band RF signals in power amplifiers. For each frequency band, an RF transistor, such as a FET or a bipolar transistor is operated essentially at a non-linear operating point (e.g., in Class B, AB or C) at the frequency band. The RF transistor is fed by a DC power supply trough an RF filter and terminated by a dummy load that is tuned to the frequency band so as to terminate the RF components in the output signal of the RF transistor. An RF signal of the frequency band is fed into the input of the RF transistors and an output signal representing the envelope is obtained from the fluctuating current drawn from the DC power supply by the RF transistor, during the time period when the RF signal is applied to the input. The output signals obtained from all RF transistors that operate within their corresponding frequency band are combined to a common output, such that the output signal at this common point is essentially equal to the output signal that corresponds to one of the frequency bands.