A resistor placed between the input and output of a first circuit has a low value when a control signal delivered by a threshold discriminator circuit has a first value and has a high value when the control signal has a second value. The input of an amplifier A.sub.3 of the first circuit is connected through a switch either to the output of a first amplifier A.sub.1 or to the output of a second amplifier A.sub.2. The first amplifier has a negative input connected to a coupling capacitor of the first circuit and to the output of the amplifier A.sub.3 and a positive input connected to ground. The amplifier A.sub.2 has a negative input connected to the output of the amplifier A.sub.3 and to ground and a positive input connected to the negative input of the amplifier A.sub.1.
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