Abstract: A device switches a coil (2) between a first state (REC) where it is connected between a supply source (V.sub.REC) and ground through a variable current source (I.sub.REC), and a second state (PB) where it is connected between ground and the input of an amplifier (3) through a coupling capacitor (CC). The device comprises a first controlled switch (T.sub.PB) connected in parallel to the current source (I.sub.REC); a second controlled switch (T.sub.REC) placed between the supply source (V.sub.REC) and the coil (2); a threshold comparator (4, V.sub.T), the inputs of which are connected across the capacitor (CC); a logic control circuit (5) receiving a state control signal (REC/PB) and the output signal (V.sub.C) of the threshold comparator (4, V.sub.T) and, as a function of these signals, alternatively controlling switches (T.sub.PB, T.sub.REC) with no overlapping of their closed states.

Abstract: A comparison circuit associated with a video tape recorder having two identical legs (41, 42), one of which (41) is connected to a head of a first type and the other (42) to a head of a second type, the outputs of both being connected to a respective input of a comparison means (56) for selecting in "search" mode the head which presents the highest signal amplitude. Each branch comprises, in series, a high-pass filter (45), a variable gain amplifier (46), a multiplier (47), a low-pass filter (48) and a capacitance amplifier (49).

Abstract: A voltage-current amplification circuit comprises two sub-circuits (B1, B2), each of which comprises a differential amplifier (D), a resistor (R4), a transistor (T), and a first switch (K1) connected between the transistor base and ground. Each sub-circuit (B1, B2) also comprises an additional amplifier (A1, A2), and an additional resistor (R2a, R2b), respectively. The inputs of the additional amplifiers being connected to ground through a common resistor (R) and to the emitters of the two transistors through a common resistor (R1). Each sub-circuit further comprises a second switch (K2) formed by a single transistor.

Abstract: A broadband amplifier comprises a bipolar transistor (T), having a base connected to the input terminal (2), a collector connected to the output terminal (3) which is in turn connected to a supply source (11) through a first resistor (R1), and an emitter grounded. A second resistor (R2), the value of which varies simultaneously with the current gain (.beta.) of the transistor is connected between the output terminal and the transistor base. This broadband amplifier further comprises a capacitor (C) connected in parallel across the terminals of the second resistor and a third resistor (R3) connected between the second resistor and the transistor base.

Abstract: A voltage-current amplification circuit comprises two sub-circuits (B1, B2), each of which comprises a differential amplifier (D), a resistor (R4), a transistor (T), and a first switch (K1) connected between the transistor base and ground. Each sub-circuit (B1, B2) also comprises an additional amplifier (A1, A2), and an additional resistor (R2a, R2b), respectively. The inputs of the additional amplifiers being connected to ground through a common resistor (R) and to the emitters of the two transistors through a common resistor (R1). Each sub-circuit further comprises a second switch (K2) formed by a single transistor.