DE2920792A1 - Constant current supply bridge for telephone exchange - has transistor in each current generator, whose collectors are connected cross-wise to their bases - Google Patents

Abstract

The supply bridge has two current generators connected in series with each other and in parallel to the wires of a subscriber line loop. Each generator has a transistor. The transistors (T1), T2) in the generators (G1, G2) are complementary to each other. To achieve mutual voltage control a resistor (R3) connects a first transistor (T1) collector to the base of a second transistor (T2) and a second resistor (R4) connects the second transistor (T2) collector to the first transistor (T1) base. There is an optoelectronic coupler (AO) between the supply unit (CT) and the generator (G1).

Description

Constant current feed bridge for a telephone

exchange.

The invention relates to a constant current feeder bridge according to the preamble of claim 1, i.e. a feed bridge in which the the Telephone sets supply the subscriber with a specified current of length the line loop connecting the telephone sets to the central office has independent value.

Compared to conventional feed bridges, however, also have electricity feed bridges the advantage that they ensure constant effectiveness of the carbon microphones, which are used in most telephone sets and are known to be in theirs Performance are dependent on the supply current.

Known constant current feed bridges have at least one current generator, the subscriber telephone set with a constant, from the fluctuations the impedance of the telephone set and the subscriber line loop in the case under consideration existing load feeds independent electricity.

For reasons of symmetry, one with the a-wire of the line loop is generally used connected first current generator and a second current generator connected to the b-wire intended.

These power generators are therefore in series and in parallel with each other switched to the wires of the line loop. The series connection of two power generators but causes difficulties because the transistors with which they usually are equipped, influence each other, which has the consequence that an operating point shift in the case of one transistor, an operating point shift in the case of the other transistor evokes. In particular, the voltage of one transistor increases when the collector voltage drops at constant output current the collector voltage of the other Transi-Lrrs on, resulting in considerable heat dissipation from one of the two transistors and thus can damage or destroy the transistor. In addition, the Transistors with different collector voltages also have a different one have dynamic resistance i.e. the speech currents find two wires on the two wires Paths of different impedance to ground or

against the voltage pole of the battery, which is why the known type represent a crosstalk source.

The invention is based on the Aufgage, one in particular from good specify existing feed bridge for components suitable for an integrated circuit, which are based on the different collector voltages of the transistors Difficulties are avoided.

This task is characterized by that in claim 1 Feed bridge released.

With such a feed bridge, the mutual regulation Displacements of the intended operating points of the transistors avoided.

The invention is based on a non-limiting exemplary embodiment explained in more detail. In the drawing: FIG. 1 shows the current profile in a constant current feed bridge and FIG. 2 shows a more detailed illustration of the feed bridge described here.

In Fig. 1, the transformer for galvanic isolation is with TR Line loop of a subscriber, whose one winding with the telephone exchange CT is connected, while the other winding is connected to the via the line loop Telephone set TL is connected and a capacitor C for blocking the direct current supply of the telephone set TL contains. The direct current feed is supplied by a first and a second current generator G1 and G2 respectively, which are supplied in series with one another and are connected in parallel to the line loop. When the telephone is occupied TL, the current delivered by the generator G2 cannot pass the winding of the transformer TR flow through as it is blocked by the capacitor C; therefore it flows through the telephone TL and the generator G1, the circuit is closed. The voice current signals generated in the telephone set pass through the transmitter TR to the exchange CT.

Since with a feed bridge constructed in this way, the current generators G1 and G2 are connected in series and therefore influence each other, was previously able to reduce the collector voltage when the collector voltage of a transistor fell of the other transistor rise, so that with a constant output current one of the two transistors has to consume more energy than the other transistor. This is undesirable because heat is generated in one of the two transistors, which leads to the Damage or destruction of the transistor. There are also the two transistors have different collector voltages, they also have a different one dynamic resistance, which gives the speech currents a different resistance to ground or to the voltage pole of the battery, what crosstalk phenomena has the consequence.

The details of a circuit arrangement of a feed bridge according to of the invention are shown in FIG. Accordingly, the current generators G1 exist and G2 from two complementary transistors T1 and T2, whose emitters are through the Resistors RE1 and RE2 are biased. The bias of the base-emitter junctions of the transistors T1 and T2 takes place through a voltage divider which is connected to the supply voltage source (-V) is connected and from the series circuit of a resistor R1, one between the bases of the two transistors inserted Zener diode DZ and another resistor R2 is built up.

The generators G1 and G2 contain means for mutual regulation, which keep the two transistors in the linear operating range and also any To prevent deviations from the predetermined conditions. This means consist of a first resistor R3, which is the collector of transistor T1 with the base of the transistor T2 connects, and a second resistor R4, the connects the collector of transistor T2 to the base of transistor T1.

The following is the principle of mutual regulation under the Assumption will be explained that for some reason the collector voltage of the transistor T1 drops by an amount βV, which would result in the disadvantages mentioned above.

Since the transistor T1 is an npn transistor, so with respect to the Eitter operating positive voltages and currents causes a decrease the collector voltage with a constant collector current an increase in the base current, which causes an increase in the voltage drop across resistor R4.

Since the transistor @ 2 is a pnp transistor, it has negative voltages and currents; therefore causes an increase in the voltage drop across resistor R4 a drop in the collector voltage this transistor T2 and as a result also a drop in the base current. So if the base current of transistor T2 falls , the voltage drop across the resistor R3 decreases, resulting in an increase in the collector voltage of the transistor T1 causes, whereby the said deviation AV is canceled. The resistors R3 and R4 thus effect a mutual regulation of the bias voltages for the transistors T1 and T2 what possible deviations of the operating point of the transistors removed from the designated area.

In series with that provided for biasing transistors T1 and T2 Voltage dividers are a means of detecting the occupancy criterion for the considered line loop lying telephone set switched. You insist from an optoelectronic coupler AO, the one in series with the voltage divider lying photodiode and a phototransistor connected to the exchange CT contains.

When the handset of the telephone set TL is hung up, the diode from a weak one flowing through the elements of the voltage divider Current flowed through, which is considerably smaller than to illuminate the phototransistor is required. After removing the handset, the power generators generate G1 and G2 a much higher current through the photodiode, which gives a strong glow causes the photodiode and the saturation of the phototransistor. This is the case with the Exchange recorded as an occupancy criterion.

As can be seen from FIG. 2, that implemented according to the invention is Due to the components used, the feed bridge can be used as an integrated circuit without any problems realizable.

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Claims (5)

Claims constant current feeder bridge for a telephone exchange with two in series and each to the wires of a subscriber line loop current generators connected in parallel, each containing a transistor, d a d u r c h g e k e n n n z e i c h n e t that the two transistors (T1, T2) of the current generators (G1 and G2) are complementary to each other, and that for mutual voltage regulation a first resistor (R3) connects the collector of the first transistor (T1) to the base of the second transistor (T2) and a second resistor (R4) the collector of the second Transistor (T2) connects to the base of the first transistor (T1). 2.) Feed bridge according to claim 1, d a d u r c h g e k e n nz e i c h a e t that a voltage divider is provided for the voltage before the transistors (T1, T2) which is the series connection of a third resistor (R1), a Zener diode (DZ) and a fourth resistor (R2) and the voltage for the first transistor (T1) or for the second transistor (T2) at the ends of the third resistor (R1) or the fourth resistor (R2) generated. 3.) Feed bridge according to claim 2, d a d u r c h g e k e n nz e i c h n e t that the bases of the two transistors (T1, T2) to the connection points connected between the Zener diode (DZ) and the third or fourth resistor (R1, R2) are. 4.) Feed bridge according to claim 2 or 3, d a d u r c h g e -k e n n z e i c h n e t that in series with the voltage divider an arrangement for detection the occupancy criterion for the connected to the relevant line loop Telephone set (TL) is switched. 5.) Feed bridge according to claim 4, d a d u r c h g e k e n nz e i c h n e t that the arrangement for detecting the occupancy criterion from an optoelectronic Coupler (AO) consists of a photodiode in series with the voltage divider and a phototransistor connected to the central office (CT).