A PROTECTIVE CIRCUIT

Abstract: A protective circuit in a power amplifier (1) is adapted, by limiting the maximum output current of the power amplifier (1) and by quickly recharging compensating capacitors (C1, C2, C3) in the power amplifier (1), to limit the amount and duration of the power drain in the power amplifier (1) upon over-excitation of the power amplifier (1).

Description: A PROTECTIVE CIRCUIT TECHNICAL FIELD The invention relates to a protective circuit and more specifically to a protective circuit for protecting a power amplifier in a subscriber line interface circuit against sudden voltage changes, e.g. caused by lightning strikes, on the associated telephone line. BACKGROUND OF THE INVENTION When sudden voltage changes appear on the output terminal of a power amplifier, the power amplifier will be over-excited and give rise to a maximum output current and power loss until all compensating capacitors in the power amplifier have been recharged to the new voltage appearing on the output teraiinal. This can lead to the destruction of the power amplifier. Current limitation in power ampUfiers is effected in a manner known per se in that a resistor is interconnected in series with the power transistors in the amplifiers. When the current in such a resistor becomes high enough, a transistor is caused to conduct so that the driving current is shunted past the power transistor. The problem with these series resistors is that they, also during normal conditions, require extra overhead voltage which is not always readily available. SUMMARY OF THE INVENTION The object of the invention is to limit the output current in a power amplifier to its value as well as duration in case of over-excitation. This is generally attained in that compensating capacitors in the power amplifier upon over-excitation are recharged quicker in accordance with the invention than in known power amplifiers. Hereby, the duration of a damaging, high output current will be restricted. BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing, on which the single figure shows a power amplifier known per se, having an embodiment of a protective circuit in accordance with the invention. PREFERRED EMBODIMENT The figure shows a power amplifier known per se, having a protective circuit in accordance with the invention. The power amplifier is generally denoted 1 and has two input terminals 2 and 3, respectively, and one output terminal 4. In a manner known per se, the power amplifier 1 comprises a transconductance amplifier Al having two input terminals and one output terminal. The input terminals of the transconductance amplifier Al are connected to the input terminals 2, 3 of the power amplifier 1 and, thus, constitute, at the same time, the input terminals of the power amplifier shown. The output terminal of the transconductance amplifier Al is connected to the input terminal of an integrator- coupled amplifier FI between whose input teixninal and output terminal, a compensating capacitor C 1 is connected. The output terminal of the amplifier FI is connected, via a node N, to a first input terminal 5 of a transconductance amplifier A2 whose other input terminal 6 is connected to the output terminal 4 of the power amplifier 1. The transconductance amplifier A2 has two output terminals 7 and 8; the output terrninal 7 being connected to the input terminal of an integrator-coupled amplifier F2 between whose input terrninal and output terrninal a compensating capacitor C2 is connected and whose output teπninal is connected to the output terrninal 4 of the power amplifier 1. The supply voltage terminal of the amplifier F2 is connected to ground. In a corresponding manner, the output terrninal 8 of the transconductance amplifier A2 is connected to the input terminal of an integrator-coupled amplifier F3 between whose input terminal and output terminal a compensating capacitor C3 is connected and whose output terrninal is also connected to the output terminal 4 of the power amplifier 1. The supply voltage terminal of the amplifier F3 is connected to a supply voltage VBAT. The components hitherto described as being included in the power amplifier 1, are components that are normally included in such an amplifier. The embodiment of the protective circuit in accordance with the invention, illustrated on the drawing, comprises a first resistor Rl which is interconnected between the output terminal 4 of the power amplifier 1 and a wire connection terminal 9 via which the line interface circuit, not shown in any detail, is intended to feed one wire of a two-wire telephone line (not shown). To anti-series connected zener diodes Zl and Z2 are interconnected between the wire connection terrninal 9 and the node N, i.e. the interconnection point between the output terrninal of the amplifier FI and the input terminal 5 of the trans- conductance amplifier A2. A resistor R2 is interconnected between the supply voltage terrninal of the amplifier FI and the supply voltage VBAT. The supply voltage teπriinal of the amplifier FI is, moreover, connected to the base of two transistors Tl and T2, whose emitters are connected to the supply voltage VBAT. The collector of the transistor Tl is connected, on the one hand, to the input terminal of the amplifier FI and, on the other hand, to the emitter of a transistor T3. The base of the transistor T3 is connected to the supply voltage VBAT and its collector is connected, on the one hand, to the input terminal of the amplifier F2 and, on the other hand, to the anode of a diode Dl whose cathode is connected to ground. The collector of the transistor T2 is connected, on the one hand, to the input terminal of the amplifier F3 and, on the other hand, to the cathode of a diode D2 whose anode is connected to the supply voltage VBAT. In the embodiment shown on the drawing, the input terminals 2 and 3 of the power amplifier 1 are connected, on the one hand, to a respective current generator II and 12, respectively, and, on the other hand, via a respective resistor R3 and R4, respectively, to the wire connection terminal 9 and the output terminal 4, respectively, of the power amplifier 1. In normal operation, the output current from the power amplifier 1 is an amplified copy of the difference between the input currents II and 12. In this condition, the voltage on the wire connection terrninal 9 is equal to the voltage in the node N, i.e. the interconnection point between the output terminal of the amplifier FI, the input terrninal 5 of the transconductance amplifier A2 and the anode of the zener diode Z2. At sudden positive or negative changes of the voltage on the wire connection terminal 9, the voltage in the node N will not change quite as sudden since the voltage in the node N is limited by the slew rate of the amplifier FI. The voltage difference between the wire connection terminal 9 and the node N corresponds to the zener voltage of the zener diode Zl or Z2. When this voltage difference amounts to the zener voltage for the zener diode Zl or Z2, also the voltage in the node N will be forced to change by voltage changes on the wire connection terrninal 9. Thus, the voltage difference between the wire connection terminal 9 and the node N will be kept constant at the zener voltage for Zl or Z2, whereby the maximum output current of the power amplifier 1 will be limited to the zener voltage divided by the resistance of the resistor Rl. If the change of the voltage on the wire connection terrninal 9 is larger than the zener voltage of the zener diode Zl or Z2, thus, also the voltage in the node N will be changed by force. At a quick, positive change of the voltage in the node N, the current through the resistor R2 will be so large that the transistor Tl starts to conduct. Hereby, the capacitor Cl across the amplifier FI will quickly be recharged to the voltage appearing in the node N. In the same manner, the capacitor C3 across the amplifier F3 will be recharged through the transistor T2 which also becomes conductive by the voltage drop across the resistor R2. The capacitor C2 across the amplifier F2 will also be recharged through the diode Dl, but this is not critical since the amplifier F2 is not active under the circumstances in view of the fact that its supply voltage terrninal is connected to ground. At a quick, negative change of the voltage in the node N, the capacitor Cl will attempt to pull the emitter of the transistor T3 to a voltage below the supply voltage VBAT. Hereby, the transistor T3 becomes conductive and the capacitor Cl is quickly recharged to the voltage appearing in the node N. The charging current for the capacitor C 1 through the transistor T3 is also used to quickly recharge the capacitor C2. The capacitor C3 is recharged via the diode D2, but this recharging is not critical since the amplifier F3 is not active under the circumstances. By quickly recharging the compensating capacitors included in the power amplifier by means of the protective circuit according to the invention, thus, upon over- excitation, the maximum output current that can be outputted by the power amplifier, on the one hand, and the time during which the power amplifier is overexcited and outputs the maximum output current, on the other hand, are limited. The protective circuit described above for protecting the power amplifier 1 against sudden positive and negative voltage changes appearing on its output terminal 4, is according to the invention adapted to quickly recharge the compensating capacitors Cl, C2, C3 included in the power amplifier 1. By quickly recharging the capacitors Cl, C2 and C3, each individual amplifier FI, F2 and F3, respectively, in the power amplifier 1 is, thus, protected. Therefore, the protection of the power amplifier 1 is based on the protection of the respective amplifier FI, F2 and F3. Thus, the amplifier FI is protected in accordance with what has been stated above against sudden, positive voltage changes appearing on its output terrninal, i.e. in the node N, in that the transistor Tl becomes conductive and establishes a current path to the supply voltage VGAT so that the capacitor Cl is quickly recharged. Against sudden, negative voltage changes appearing on its output terminal, i.e. in the node N, the amplifier FI is protected in accordance with what has been stated above in that the transistor T3 becomes conductive and establishes a current path via the diode Dl to ground so that the capacitor Cl is recharged. It should be pointed out that the transistors Tl and T3, in the general case, can be replaced by other switches known per se. In the embodiment of the power amplifier 1 and the protection circuit illustrated on the drawing, both amplifiers F2 and F3 are protected in accordance with what has been stated above by recharging the associated capacitor C2 and C3, respectively, via a current path established by the diode Dl to ground and via a current path established by the transistor T2 to the supply voltage VBAT, respectively. In view of the fact that the output terminals of the amplifiers F2 and F3 are interconnected, the capacitor C2 will thus be recharged at sudden, negative voltage changes, while the capacitor C3 will be recharged at sudden, positive voltage changes.

Claims: CLAIMS 1. In an amplifier (FI) having a capacitor (Cl) interconnected between its input terminal and its output terminal, a protective circuit for protecting the amplifier (FI) against sudden voltage changes appearing on its output terminal, characterized in - that a first switch (Tl) is adapted, at sudden, positive voltage changes, to establish a first current path from the input terminal of the amplifier (FI) to a first supply voltage source (VBAT), and - that a second switch (T3) is adapted, at sudden, negative voltage changes, to establish a second current path from the input terminal of the amplifier (FI) to a second supply voltage source. 2. A protective circuit for a power amplifier (1) having two input terminals (2, 3) and one output terminal (4) in a line interface circuit for feeding one wire of a telephone line via a wire connection terminal (9), the power amplifier (1) comprising - a first transconductance amplifier (Al) having two input terminals and one output terrninal, the input terminals of the transconductance amplifier (Al) constituting the input terminals of the power amplifier (1), and the output teiminal of the trans- conductance amplifier (Al) being connected to the input terminal of a first amplifier (FI) between whose input terrninal and output terminal a first capacitor (C l) is connected, - a second transconductance amplifier (A2) having two input teπninals (5, 6) and two output terminals (7, 8), the first input terminal (5) being connected to the output terminal of the first amplifier (FI) and the second input terminal (6) being connected to the output terminal (4) of the power amplifier (1), - a second amplifier (F2) between whose input terminal and output terrninal a second capacitor (C2) is connected and whose input terrninal is connected to the first output teπninal (7) of the second transconductance amplifier (A2), and whose output terminal is connected to the output terminal (4) of the power amplifier (1), while its supply voltage terrninal is connected to ground, - a third amplifier (F3) between whose input terrninal and output terminal a third capacitor (C3) is connected and whose input terrninal is connected to the second output terminal (8) of the second transconductance amplifier (A2), and whose output terminal is connected to the output terminal (4) of the power amplifier (1), while its supply voltage terminal is connected to a supply voltage (VBAT), the protective circuit being characterized in that it comprises - a first resistor (Rl) which is interconnected between the output terrninal (4) of the power amplifier (1) and the wire connection terrninal (9), - two anti-series connected zener diodes (Zl, Z2) which are interconnected between the wire connection terminal (9) and the output terrninal of the first amplifier (FI), - a second resistor (R2) which is interconnected between the supply voltage terminal of the first amplifier (FI) and the supply voltage (VBAT), the supply voltage terrninal being connected to the base of a first and a second transistor (Tl, T2) whose emitters are connected to the supply voltage (VBAT), the collector of the first transistor (Tl) being connected, on the one hand, to the input terminal of the first amplifier (FI) and, on the other hand, to the emitter of a third transistor (T3) whose base is connected to the supply voltage (VBAT) and whose collector is connected, on the one hand, to the input terrninal of the second amplifier (F2) and, on the other hand, to the anode of a first diode (Dl) whose cathode is connected to ground, while the collector of the second transistor (T2) is connected, on the one hand, to the input terrninal of the third amplifier (F3) and, on the other hand, to the cathode of a second diode (D2) whose anode is connected to the supply voltage (VBAT).

Inventors: ERIKSSON HANS, LARSSON ELISABETH, ERIKSSON, HANS, LARSSON, ELISABETH
Applicants: ERICSSON TELEFON AB L M, ERIKSSON HANS, LARSSON ELISABETH, ERIKSSON, HANS, LARSSON, ELISABETH, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1997
Main classification: H03K1
IPC codes: H03F1