Abstract This invention describes a circuit having a variable impedance which varies in accordance with the input signal level. The inventive circuit is used in conjunction with the deemphasis circuit ordinarily employed in FM and other types of radio receivers. By varying the impedance between the output of the deemphasis circuit and the ground, the sensitivity of the audio frequency stage of the receiver is automatically varied. The variation in sensitivity increases the receiver''s ability to reject interference and weak signals. The inventive circuit includes two parallel impedance paths connected between the output of deemphasis circuit and the ground reference. The first path is a fixed impedance path and the second contains a voltage sensitive switch. The impedance of the second path is a function of the current through the voltage sensitive switch and therefore the receiver sensitivity varies directly with the variation in current through the switch when the switch is in a conductive condition.
A variable impedance circuit comprising: a fixed impedance network; a variable impedance network connected electrically in parallel with said fixed impedance network; and means for connecting input signals to said impedance networks; said variable impedance network including a transistor having a base and an emitter, said base providing the output of said variable impedance circuit; said fixed impedance path including a first resistor connected between said means for connecting and ground; a second resistor connected between said first resistor and one end of a third resistor; the other end of said third resistor being connected to said base; the junction of said second resistor and said third resistor being connected to an RC high pass network; said variable impedance path including said transistor and a fourth resistor connected between said emitter and ground.
Claim 2
Dependent Claim
133 words
A variable impedance circuit comprising: a fixed impedance network; a variable impedance network connected electrically in parallel with said fixed impedance network; and means for connecting input signals to said impedance networks; said variable impedance network including a transistor having a base and an emitter, said base providing the output of said variable impedance circuit; said fixed impedance path including a first resistor connected between said means for connecting and ground; a second resistor connected between said first resistor and one end of a third resistor; the other end of said third resistor being connected to said base; the junction of said second resistor and said third resistor being connected to an RC high pass network; said variable impedance path including said transistor and a fourth resistor connected between said emitter and ground.
Claim 3
Dependent Claim
53 words
The circuit of claim 1 wherein said means for connecting includes a diode, a capacitor, and a resistor; said resistor being connected between a first terminal of said diode and a first terminal of said capacitor, a second terminal of said diode and a second terminal of said capacitor being connected to ground.