Muting circuit with operational amplifier

Abstract

An FM muting circuit has a common input terminal and uses an operational amplifier which has a pair of input terminals. The two amplifier input terminals are coupled to the common muting circuit input terminal. An electronic switch is connected between ground and one of the operational amplifier input terminals. When the muting circuit receives an input signal which is free of noise, a 'high-level' control signal is developed to switch the electronic switch into a saturated condition, thereby grounding one of the operational amplifier input terminals. Thereafter, the operational amplifier amplifies only the signal applied from the common input terminal of the muting circuit to the other amplifier input terminal, with a resulting amplified output signal. However, when noise appears in the input signal received by the muting circuit, a 'low-level' control signal is applied to the electronic switch and it turns off. The signal appearing at the common input terminal of the muting circuit is thereafter supplied, in phase, to both amplifier input terminals, which causes a cancellation therein. The operational amplifier output signal is then about 100dB below the conventional level, thereby effectively rejecting the signal containing noise. A resistor network at the two inputs of the operational amplifier prevents any substantial change of bias potential responsive to the electronic switch operations. The elimination of such a bias potential change also eliminates the popping noise and causes the switching operation of the muting circuit to occur very fast.