Abstract: A switched capacitor pilot cancellation circuit included in an FM stereo decoder signal path which substantially eliminates the 19 kHz pilot from the FM composite by subtracting a reconstructed pilot frequency signal from the FM composite. The reconstructed pilot signal is obtained by applying the DC output voltage of a pilot magnitude detector to a switched capacitor network which is sampled by clock signals which convert the DC to 19 kHz.

Abstract: A closed-loop attenuator for an FM stereo receiver provides attenuation to the L+R audio signal in a controlled and predictable reaction to an ultrasonic noise (USN) voltage presented to an input amplifier. This is accomplished through a closed-loop circuit configuration including a multiplier which modulates the current output of the input amplifier in accordance with the output of a master differential amplifier and supplies the modulated current to a gain stage connected to the input of the master differential amplifier. The output of the master amplifier is mirrored by a slave differential amplifier which applies the actual audio signal attenuation. The closed-loop configuration confines this linear control action to a very narrow range. Outside of this range, the circuit provides either full attenuation or no attenuation.

Abstract: A current mirror implemented preferably in MOS technology provides low total harmonic distortion and high output impedance and maintains the input signal within one Vgs of the supply voltage source. For an input signal having DC and AC components I and i, a constant curent of 2I is fed to the input terminal. A transistor couples the input terminal to the output terminal. The difference of the constant current and the input current is the output current I-i which passes through the transistor to the output terminal. An amplifier having its input connected to the input terminal and its output fed back to the gate or base of the transistor helps maintain the input terminal at a constant voltage.