Patent Publication Number: US-6911866-B2

Title: Method and device for switch-on current limiting in push-pull amplifying power stages

Description:
This application claims priority under 35 U.S.C. § 119 to PCT Application No. PCT/EP02/06285 filed on Jun. 7, 2002 which claimed priority to German Application No. 10128772.0 filed on Jun. 13, 2001. 
    
    
     The present invention relates to a method for amplification of analog push-pull signals by means of a push-pull amplifier output stage, and relates in particular to a method for inrush current limiting in push-pull output stages. 
     In wire-based transmission systems such as ISDN and xDSL, line drivers are used in order to feed analog signals with the required transmission power to a cable. The analog signals are frequently in the form of analog push-pull signals, with the operational power amplifiers which are used having a power output stage in order to provide the necessary output currents. In general, for technological reasons, low supply voltages are used for a push-pull amplifier stage which is intended to amplify analog push-pull signals, so that an output stage must be provided with appropriately greater output currents for a constant output power level. 
       FIG. 2  shows an outline circuit arrangement of a conventional push-pull amplifier output stage which is formed from two output stage transistors. The complementary output stage transistors in the example shown in  FIG. 2  are a p-channel (pMOS) output stage transistor and an n-channel (nMOS) output stage transistor (MOS=metal oxide silicon). For reasons of clarity, the electronic circuits of the input stage devices and of the amplifier output stages (output connection) have been omitted in  FIG. 2 , in order to illustrate the fundamental disadvantages and problems of conventional push-pull amplifier output stages. 
     The input stage devices (arrows in  FIG. 2 ) drive the gate connections of the two output stage transistors. A supply voltage is applied via a measurement sensor MF to the source connection of the pMOS output stage transistor, while the source connection of the nMOS output stage transistor MN is connected to ground via a second measurement sensor MF. In a corresponding manner to the gate connections in the push-pull amplifier output stage being driven correctly, the currents I MP  and I MN  flow alternately to the output connection. The currents I MP  and I MN  are increased in a corresponding manner owing to the supply voltage which, as mentioned, is kept low for technological reasons. 
     During normal amplified operation, the push-pull amplifier output stage alternately supplies currents to the load and draws currents from the load. At the moment when the circuit arrangement is switched on, the electrical relationships are undefined. Until a predetermined operating point is set, the circuit may assume different states which are dependent on internal and external factors. One particular disadvantage of conventional circuit arrangements is that both output stage transistors may be switched on simultaneously at the moment of switching on, as a result of which very large inrush currents occur. The high inrush currents may disadvantageously exceed the current carrying capacity of the transistors or, very high inrush currents may lead to destruction of electronic components in the push-pull amplifier output stage. 
     A further disadvantage is that electronic components of a device for providing the supply voltage and the supply current can also be destroyed by an excessively high inrush current. Electromigration may disadvantageously occur, and this considerably reduces the life of electronic components. 
     As is shown in  FIG. 2 , the drain connections of the two output stage transistors are connected to one another and to an output connection. In order to ensure that excessively high inrush currents are detected, the conventional circuit arrangement has two measurement sensors MF which may be in the form of non-reactive resistors. Detection of a voltage drop which occurs across the measurement sensors MF and is caused by the currents I MP  and I MN  makes it possible to use controlled electronics (not shown) to switch off the push-pull amplifier output stage (or some other suitable measure if the inrush currents are excessive). 
     One disadvantage of the use of measurement sensors MF for current monitoring is that considerable power levels can be produced in the measurement sensors as well when the current levels are high, reducing the efficiency of the push-pull amplifier output stage. 
     In conventional circuit arrangements, the control voltages for the output stage transistors are often limited to a maximum value, which disadvantageously means that the maximum output current during normal operation is also limited. 
     One object of the present invention is thus to provide a method for amplification of analog push-pull signals by means of a push-pull amplifier output stage, in which any inrush current is safely limited. This ensures that only one of the two output stage transistors is in each case switched on. 
     According to the invention, this object is achieved by the method as specified in patent claim  1  and by a circuit arrangement having the features of claim  20 . 
     Further refinements of the invention can be found in the dependent claims. 
     One major idea of the invention is to provide a fault signal when the two output stage transistors are switched on at the same time, with this fault signal being fed back to a limiter transistor so as to limit any inrush current. 
     One advantage of the present invention is that there is no need for any resistive measurement sensors formed by non-reactive resistors, which reduce the efficiency of the push-pull amplifier output stage. 
     A further advantage of the circuit arrangement according to the present invention is that very low supply voltages and, associated with them, very high currents, can be used in the push-pull amplifier output stage since the invention prevents the two output stage transistors from being switched on at the same time. 
     The method according to the invention for amplification of analog push-pull signals by means of a push-pull amplifier output stage which has a first output stage transistor and a second output stage transistor essentially comprises the following steps:
     a) application of a first analog push-pull signal to the first output stage transistor and to a first control transistor by means of which a first control current is controlled;   b) application of a second analog push-pull signal to the second output stage transistor and to a second control transistor by means of which a second control current is controlled;   c) comparison of the first control current with a first reference current which is produced by a first reference current device;   d) comparison of the second control current with a second reference current which is produced by a second reference current device;   e) outputting of a fault signal as a function of the comparison, when not only is the first reference current exceeded by the first control current but the second reference current is also exceeded by the second control current;   f) feeding back the fault signal such that an inrush current is limited by limiting at least one control voltage of at least one output stage transistor; and   g) outputting of the amplified analog push-pull signal as output currents via an output connection.   

     The dependent claims contain advantageous developments and improvements of the respective subject matter of the invention. 
     According to one preferred development of the present invention, the first analog push-pull signal is input in complementary form to the second analog push-pull signal. 
     According to a further preferred development of the present invention, the second control current is mirrored by means of a current mirror device into a mirrored second control current. 
     According to yet another further preferred development of the present invention, the first control current is mirrored, rather than the second control current, in the current mirror device. 
     According to yet another further preferred development of the present invention, the fault signal which is output from an output connection of the AND gate is used for diagnosis and test purposes in which it is supplied to a fault output connection. 
     According to yet another further preferred development of the present invention, the ratio of the first control current to the mirrored second control current corresponds to the ratio of the first output current to the second output current. 
     According to yet another further preferred development of the present invention, the ratio of the first control current to the first output current, and the ratio of the mirrored second control current to the second output current are selected such that the energy consumption caused by the two control currents is negligible and the efficiency of the push-pull amplifier output stage is not reduced, so that the fault signal is provided with negligible energy consumption. 
     According to yet another further preferred development of the present invention, the fault signal which is supplied to the gate connection of the limiter transistor is used to limit a control voltage for the second output stage transistor. 
     According to yet another further preferred development of the present invention, the fault signal which is supplied to the gate connection of the limiter transistor is used to limit a control voltage for the first output stage transistor. 
     According to yet another further preferred development of the present invention, the fault signal, which is output from an output connection of the AND gate is supplied to an external processor thus making it possible to process the fault signal further, for example for test and analysis purposes. 
     According to yet another further preferred development of the present invention, a parallel current via the push-pull amplifier output stage during switching on is prevented, that is to say this prevents both output stage transistors from being switched on during switch-on. 
     According to yet another further preferred development of the present invention, digital circuit elements and logic circuit devices, such as the AND gate, and analog circuit elements are combined. 
     According to yet another further preferred development of the present invention, a first reference voltage level at a first reference voltage connection changes from a low level (logic L level) to a high level (logic H level) when the first control current is greater than the first reference current. 
     According to yet another further preferred development of the present invention, a second reference voltage level at a second reference voltage connection changes from a low level (logic L level) to a high level (logic H level) when the second control current is greater than the second reference current. 
     According to yet another further preferred development of the present invention, the first reference voltage level and the second reference voltage level are linked in a logic circuit unit in order to produce a fault signal with the logic circuit unit outputting a fault signal when both the first reference voltage level and the second reference voltage level are at a high level (logic H level) at the same time. 
     According to yet another further preferred development of the present invention, a gate connection of the limiter transistor has the fault signal applied to it, order to limit the inrush current through at least one output stage transistor. 
     According to yet another further preferred development of the present invention, the fault signal is fed back to electronic circuits of input stage devices such that any inrush current is limited in that at least one control voltage of at least one output stage transistor is limited. 
     The circuit arrangement according to the invention for amplification of analog push-pull signals also has:
     a) a first output stage transistor;   b) a second output stage transistor;   c) a first control transistor which is used to control a first control current;   d) a second control transistor which is used to control a second control current;   e) a first reference current device for producing a first reference current which is compared with the first control current;   f) a second reference current device for producing a second reference current which is compared with the second control current;   g) a limiter device for driving the at least one output stage transistor such that any inrush current is limited as a function of a comparison of the control current with the respective reference currents; and   h) an output connection for outputting the amplified analog push-pull signals as output currents as a function of limiting by the limiter device.   

    
    
     
       DRAWINGS 
       Exemplary embodiments of the invention will be explained in more detail in the following description and are illustrated in the drawings, in which: 
         FIG. 1  shows a circuit arrangement relating to the amplification of analog push-pull signals by means of a push-pull amplifier output stage, with any inrush current being limited, according to one exemplary embodiment of the present invention; and 
         FIG. 2  shows an outline circuit diagram of a conventional circuit arrangement for amplification of analog push-pull signals. 
     
    
    
     Identical reference symbols denote identical or functionally identical components or steps in the figures. 
     Exemplary Embodiment 
     In the circuit arrangement as illustrated in  FIG. 1  according to one exemplary embodiment of the present invention, a push-pull amplifier output stage  100  is supplied with a first analog push-pull signal  101   a  and with a second analog push-pull signal  101   b , with the gate connections of a first output stage transistor  102   a  and of a second output stage transistor  102   b , the two which together form the push-pull amplifier output stage  100 , having first and second respective analog push-pull signals applied to them. 
     During normal operation, only one of the two output stage transistors  102   a  or  102   b  is switched on, alternately, so that either a first output current  113   a  is supplied to a load which is connected to an output connection  114 , or a second output current  113   b  is drawn from the load (not shown) which is connected to the output connection  114 . The first output stage transistor  102   a  is in the form of a p-channel MOS field-effect transistor, while the second output stage transistor  102   b  is in the form of an n-channel MOS field-effect transistor. 
     The source connection of the first output stage transistor  102   a  is connected to a supply voltage  116 , while the source connection of the second output stage transistor  102   b  is connected to ground  115 . 
     The two drain connections of the first output stage transistor  102   a  and of the second output stage transistor  102   b  are connected to one another and to the output connection  114 . In order to prevent the two output stage transistors  102   a ,  102   b  from changing to a switched-on state at the same time, for example on switch on, the output currents  113   a ,  113   b  are mapped into control currents  104   a ,  104   c , as will be described in more detail further below. 
     The first analog push-pull signal  101   a  is also supplied to the gate connection of the first control transistor  103   a , while the second analog push-pull signal  101   b  is supplied to the gate connection of a second control transistor  103   b . The source connection of the first control transistor  103   a  (p-channel field-effect transistor) is connected to the supply voltage, while the source connection of the second control transistor  103   b  (n-channel field-effect transistor) is connected to ground. The control currents  104   a  and  104   b , respectively, which are produced by the drives for the control transistors  103   a  and  103   b , respectively, have a fixed ratio to the two output currents  113   a  and  113   b , respectively, such that the control currents are negligible in comparison to the output currents, so that any energy consumption resulting from the control currents can be ignored. 
     In order to make it possible to logically link the two control currents which reflect occurrence of the output currents, one of the two control currents must be mirrored in a current mirror device. 
     In the exemplary embodiment of the present invention as described here, the second control current  104   b  is mirrored in a current mirror device  105  which comprises a first current mirror transistor  105   a  and a second current mirror transistor  105   b , in order in this way to produce a mirrored second control current  104   c . The first control current  104   a  is then compared with a first reference current, which is produced by a first reference current device  109   a , such that a first reference voltage level  107   a  changes from a low level (L level) to a high level (H level) when the first control current  104   a  is greater than the first reference current. 
     In the same way, the mirrored second control current  104   c  is compared in a second reference current device  109   b  with a second reference current. If the mirrored second control current  104   c  is greater than the second reference current, then a second reference voltage level  107   b  changes from a low level to a high level. 
     The first reference voltage level  107   a  is tapped off at a first reference voltage connection  108   a , and is supplied to a first input connection of a logic circuit unit  110  (in this exemplary embodiment of the invention, the logic circuit unit  110  is in the form of an AND gate, but it is not restricted to this), while the second reference voltage level  107   b  is supplied from a second reference voltage connection  108   b  to a second input connection of the AND gate  110 . The AND gate  110  is used to logically link the first reference voltage level  107   a  and the second reference voltage level  107   b.    
     The following text describes the production of a fault signal  111 . The linking of the two reference voltage levels  107   a ,  107   b  in the AND gate  110  results in a high level (logical H level) when both the first reference voltage level  107   a  and the second reference voltage level  107   b  are at an H level. This corresponds precisely to the fault situation in which an excessively high inrush current is flowing, when both output stage transistors  102   a  and  102   b  are in a switched-on state, since the two control currents  104   a ,  104   c  exactly reflect the ratios of the output currents  113   a ,  113   b . If, on the other hand, only one of the two reference voltage levels  107   a ,  107   b  is at an H level, or if both are at an L level, then the output stage is not overloaded by any excessively high inrush current, so that the fault signal  111  is a logic L level. In the fault situation where the fault signal  111  is a logic H level, a fault condition is provided via a fault signal output connection  117 . 
     This fault condition can be used for diagnosis and test purposes, in which case, according to the circuit arrangement of the exemplary embodiment of the present invention, the fault signal  111  is fed back in order to actively prevent a fault situation. The fault signal  111  is supplied to the gate connection of the limiter transistor  112 , whose drain connection is connected to the gate connection of the first output stage transistor  102   a , and whose source connection is connected to ground. 
     When a fault signal occurs, the limiter transistor  112  is switched on, and a control voltage for the second output stage transistor  102   b  is reduced or is connected to ground potential. This means that only one of the two output stage transistors, in this case the first output stage transistor  102   a , is in a switched-on state, thus limiting any inrush current. 
     In the circuit arrangement according to the invention for amplification of analog push-pull signals  101   a ,  101   b  by means of a push-pull amplifier output stage  100 , the first and second output stage transistors  102   a ,  102   b  as well as the first and second control transistors  103   a ,  103   b  may be in the form of field-effect transistors (FET) or bipolar transistors. 
     The current mirror device is formed from at least one first current mirror transistor  105   a  and at least one second current mirror transistor  105   b.    
     Furthermore, the first current mirror transistor  105   a  and the second current mirror transistor  105   b  in the current mirror device  105  are in the form of field-effect transistors (FET) or bipolar transistors. 
     The first and second reference current devices  109   a ,  109   b  are likewise in the form of field-effect transistors (FET) or bipolar transistors. 
     It should be mentioned that the fault signal  111  may be used in the same manner to limit the control voltage of the first output stage transistor  102   a , thus resulting in a defined state in which only the second output stage transistor  102   b  can be switched to a conductive or switched-on state. The method according to the invention and the circuit arrangement according to the invention thus make it possible to produce a fault signal when a chosen maximum current is exceeded in one or both of the output stage transistors, thus providing a low inrush current threshold. 
     Furthermore, the circuit arrangement according to the invention allows use with low supply voltages and high output currents. It should be mentioned that the first and second output stage transistors  102   a ,  102   b  may be in the form of field-effect transistors (FET) or bipolar transistors. Furthermore, the two reference current devices  109   a ,  109   b  may be in the form of field-effect transistors (FET). 
     With regard to the conventional circuit arrangement as illustrated in  FIG. 2 , reference should be made to the introduction to the description. 
     Although the present invention has been described above on the basis of preferred exemplary embodiments, it is not restricted to them, but can be modified in many ways. 
     List of Reference Symbols 
     Identical reference symbols denote identical or functionally identical components or steps in the figures.
       100  Push-pull amplifier output stage     101   a  First analog push-pull signal     101   b  Second analog push-pull signal     102   a  First output stage transistor     102   b  Second output stage transistor     103   a  First control transistor     103   b  Second control transistor     104   a  First control current     104   b  Second control current     104   c  Mirrored second control current     105  Current mirror device     105   a  First current mirror transistor     105   b  Second current mirror transistor     107   a  First reference voltage level     107   b  Second reference voltage level     108   a  First reference voltage connection     108   b  Second reference voltage connection     109   a  First reference current device     109   b  Second reference current device     110  Logic circuit unit     111  Fault signal     112  Limiter transistor     113   a  First output current     113   b  Second output current     114  Output connection     115  Ground     116  Supply voltage     117  Fault signal output connection