A signal strength indicator circuit, configured to detect a power of an output signal outputted by a power amplifier, includes a voltage gain circuit, a current gain circuit, a multiplier, and a buffer stage. The voltage gain circuit provides a first gain to the output signal to generate a first value of an indicating voltage when a voltage of the output signal is not greater than a threshold, and provides a second gain to generate a second value of the indicating voltage when the voltage of the output signal is greater than the threshold. The first gain is greater than the second gain. The current gain circuit generates an indicating current according to an input signal corresponding to the output signal. The multiplier multiplies the indicating voltage and the indicating current to generate an indicating power. The buffer stage converts the indicating power to the indicating signal.