A wireless power transmitting device transmits wireless power signals to a wireless power receiving device using an output circuit that includes a wireless power transmitting coil. Measurement circuitry is coupled to the output circuit to help determine whether the wireless power receiving device is present and ready to accept transmission of wireless power. Oscillator circuitry for supplying signals to the output circuitry while making measurements with the measurement circuitry is coupled to the output circuit using an impedance injection network. The impedance injection network includes an inductor and a resistor coupled in series. Control circuitry opens a transistor in the output circuit when making measurements with the measurement circuitry and closes the transistor when transmitting the wireless power signals.