Underdamped audio phase shift oscillator connected across the audio voltage amplifier section of a radio receiver that sharply increases the selectivity or the response to signal of predetermined frequency and simultaneously relatively attenuates all other frequencies transmitted to output of receiver. The feedback circuit included in the phase shift oscillator is particularly useful in improving the selectivity of a radio receiver working in the continuous wave (C.W.) mode.
An improved filtering circuit connected to the inverting terminal of an operational amplifier having connected in parallel a first resistor and three series capacitors. Two series resistors are connected in parallel with the center capacitor and the junction between the two series resistor is connected to a fixed potential.
