Two transistors (T1, T2) have their bases connected to respective inputs (A, B). The collector of one transistor is tied to a supply voltage (Va) through a resonator (L1, C1) and also to the base of the other transistor via a capacitor (C2), while the other transistor has its collector directly tied to the supply voltage. The emitters of both transistors are connected to each other and to ground through a common resistor (R3). For comparison of multiple input signals, these may be sampled and converted in parallel/serial converters (PSA, PSB), each having a respective parallel input (A1 to AN, B1 to BN) for homologously receiving the signals under monitoring, and each having a single output line (UA, UB) leading to one of the two main inputs (A, B). The two converters are controlled by respective conversion enabling signals (ACQA, ACQB) which are synchronized with each other and which have a period (TC) shorter than the shortest period of the signals under monitoring (TP) divided by the number of signals under monitoring plus 2.
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