Wave-filtering device with an adjustable frequency bandwidth

A wave-filtering device with an adjustable frequency bandwidth installed in an output unit of an integrated system for receiving a first output signal, the first output signal comprising an alternating current signal and a direct current bias voltage, the wave-filtering device comprising:

• a direct current bias voltage detector for receiving the first output signal and obtaining a control voltage according to the direct current bias voltage of the first output signal;
• and
• a low-pass wave filter electrically connected to the direct current bias voltage detector for receiving the control voltage so as to adjust the frequency bandwidth of the low-pass wave filter, wherein the low-pass wave filter is used for receiving the first output signal and obtaining a second output signal from the first output signal to be an output of the wave-filtering device with an adjustable frequency bandwidth, and the low-pass wave filter comprises a variable resistance, and the resistance value of the variable resistance varies in accordance with the value of the control voltage, wherein the low-pass wave filter comprises:
• a photo-coupler comprising a light emitting diode (LED) and a transistor, the LED being used for receiving the control voltage so as to adjust the strength of the light radiated by the LED, and the on-resistance of the transistor changing in accordance with the value of the control voltage;
• and
• a capacitor electrically connected to the emitter of the transistor, wherein a voltage across the capacitor serves as the output of the wave-filtering device.