A circuit for blocking undesired input direct current of AC-coupled broadband circuits. The circuit includes a capacitor coupled to an input port and a common node. The input port receives a RF input signal. Additionally, the circuit includes a current source supplying a DC current to the common node leading a bias current to an output port. Further, the circuit includes a variable voltage source through an internal load and a close loop with an application circuit having an input load coupled to the output port to determine various bias voltages to control the bias current at the output port in association with a RF output signal that is substantially free of any input direct current originated from the RF input signal and is associated with an inherent low cut-off frequency independent of the various bias voltages.