Abstract:
A load testing circuit a circuit tests the load impedance of a load connected to an amplifier. The load impedance includes a first terminal and a second terminal, the load testing circuit comprising a signal generator providing a test signal of a defined bandwidth to the first terminal of the load impedance, an energy-storing element being connected to the second terminal of the load impedance and providing an output signal, and a measuring unit that measures the output signal or compares the output signal with a reference.

Description:
CLAIM OF PRIORITY 
       [0001]    This patent application claims priority to European Patent Application serial number 07 010 276.9 filed on May 23, 2007. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a load testing circuit, and in particular to a load testing circuit for detecting the presence of a defined load impedance connected to the output of a power amplifier. 
       RELATED ART 
       [0003]    Many amplifier circuits require overload protection for protecting their output-stages against destruction due to inappropriate loads attached thereto. Suitable loudspeakers (or other electro-acoustic transducers) are a prerequisite for a correct function of the associated amplifiers. 
         [0004]    Modern amplifiers are often controlled by microcontrollers that perform many different tasks, such as selecting signal sources, processing user input, and so on. Microcontrollers are also useful for fault-detection. There is a need for a test circuit that allows for easily interfacing with a microcontroller and detects inappropriate electro-acoustic transducers attached to an output-stage of a power amplifier in order to protect the output stage. 
       SUMMARY OF THE INVENTION 
       [0005]    A test circuit for detecting a defined load of an electro-acoustic transducer comprises a signal generator that provides a test signal of a defined bandwidth, an electro-acoustic transducer having a first and a second terminal, the first terminal being connected to the signal generator for receiving the test signal, an energy-storing element being connected to the second terminal of the electro-acoustic transducer, and providing an output signal, and measuring unit that measures the output signal or compares the output signal with a reference. 
         [0006]    A method for testing an electro-acoustic transducer comprises providing a test signal of a defined bandwidth, supplying the test signal to a first terminal of an electro-acoustic transducer, thereby providing an output signal at the second terminal of the electro-acoustic transducer, measuring the output signal or for comparing the output signal with a reference. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention can be better understood with reference to the following drawings and the description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0008]      FIG. 1  is a block diagram illustration of a load testing circuit; 
           [0009]      FIG. 2  is a more detailed circuit diagram of the load testing circuit illustrated in  FIG. 1 ; 
           [0010]      FIG. 3  is a timing diagram illustrating a test signal before low-pass filtering; 
           [0011]      FIG. 4  is a timing diagram illustrating different output signals corresponding to different impedance values of the load; and 
           [0012]      FIG. 5  is a circuit diagram of another example of the load testing circuit. 
       
    
    
     DETAILED DESCRIPTION  
       [0013]    Referring to  FIG. 1 , a pulse signal S P  on a line  12  is provided, for example, by a microcontroller (not shown). The pulse may be generated at an I/O-pin of an I/O-port of the micro-controller. Alternatively, a digital-to-analog converter output of the microcontroller may be used to provide the pulse signal S P  on the line  12 . The pulse signal comprises at least one pulse which is, for example, rectangular, and has a spectral bandwidth that may comprise at least parts of the spectral range audible by the human ear. 
         [0014]    A test signal S T  on a line  14  is derived from the pulse signal S P  on the line  12  by a signal shaping circuit  20 . The test signal ST has a defined bandwidth determined by a transfer function of the signal shaping circuit  20 . The bandwidth of the low-pass and/or the band-pass usually depends on the bandwidth of the electro-acoustic transducer (e.g., a loudspeaker) that forms load  30 . As an example, the bandwidth of the band-pass may correspond to the bandwidth of the human ear, which is about 20 kilohertz starting from approximately 20 Hertz. Alternatively, the signal shaping may already be performed by the above-mentioned analog-to-digital converter by converting an arbitrary synthesised signal of the desired bandwidth. 
         [0015]    The signal shaping circuit  20  may also include a amplifier  22  to provide the test signals S T  on the line  14  having higher levels than the original pulse signal S P  or to perform an impedance conversion providing a low-output resistance of the signal shaping circuit  20 . The signal shaping circuit  20  is connected to a first terminal of the load  30  (e.g., the electro-acoustic transducer) supplying the test signal S T  to the load  30 . A second terminal of the load  30  is connected to an energy storing element  36  which may be, for example, a capacitor C OUT2  as illustrated in  FIG. 2 . Capacitors are usually connected parallel to the output of an amplifier for electromagnetic compatibility (EMC) and electrostatic discharge (ESD) protection. These capacitors can be used as capacitors C OUT1  and C OUT2  illustrated in  FIG. 2 . 
         [0016]    The energy storing element  36  provides an output signal S O  on a line  38  to a comparator  40 . The energy storing element is connected to the load  30 , such that the load&#39;s impedance and energy storing element form a filter circuit disposed downstream of the signal forming circuit  20 . This filter circuit may represent a low-pass as illustrated in  FIG. 2  or a band-pass, but the use of other filter characteristics is of course also applicable to the inventive test circuit of the present invention. In each case, the filter characteristics can be interpreted as a representation of the (generally complex) load impedance. In the case of a low-pass filter, the cut-off frequency, and respectively the time constant, of the low-pass depends on the load impedance. 
         [0017]    The output signal S O  on the line  38  is supplied to the comparator  40 , such as for example a window-comparator or a Schmitt-trigger for comparing the output signal on the line  38  to a threshold. The output signal S O  on the line  38  essentially represents the impulse response (of a band-limited pulse of the test signal S T ) of a system formed by the load  30  and the energy storing element  36 . Consequently, the load impedance is also represented by the slope of the output signal S O , such that the lower the load impedance, the steeper the slope of the output signal and the earlier a given threshold is reached by the output signal S O  on the line  38 . That is, the time period between initiation of the pulse signal S P  (or the test signal S T ) and the triggering of the comparator  40  by the output signal So represents the load impedance. This time period can easily be measured by a microcontroller. Alternatively, the output-signal can be directly supplied to an analog-to-digital converter port (A/D-port) of the microcontroller. In this case the functionality of the comparator  40  (or any other, even more complex analysis) can be implemented in the microcontroller. 
         [0018]    If the microcontroller detects an inappropriate load impedance  30  it can initiate appropriate measures for protecting the power-amplifier output-stage to which the load-impedance is connected. Thus the above-described circuit can be used for overload protection of a output-stage of a power amplifier. For example, the microcontroller may deactivate the output-stage as long as it senses an unsuitable load impedance  30  (e.g., unsuitable loudspeakers) at the output of the power amplifier. 
         [0019]      FIG. 2  illustrates an embodiment of the circuit of  FIG. 1 . The pulse signal S P  on the line  12  can be generated by microcontroller  10 . Therefore, for example, an output pin of the microcontroller can be connected to the signal shaping circuit  20 . In the example of  FIG. 2  the pulse signal S P  is received by the gate terminal of a junction field-effect transistor  13 . It is contemplated that other types of transistor can be used for this purpose as well. The drain terminal of the transistor  13  is connected to a first supply terminal receiving a first supply potential V DD  via a resistor R d . The source terminal of the transistor  13  is connected to a second supply terminal receiving a second supply potential (e.g., ground potential GND) via resistor R s . A first capacitor C out1  is connected in parallel to the source resistor R s . The test signal S T  is provided by the source terminal of transistor  13 , which is also connected to a first terminal of the load impedance (comprising a resistance R load ). The transistor circuit within the signal shaping circuit  20  essentially forms a source-follower with source resistor R s , a drain resistor R d  and the first capacitor C out1 . The drain and source resistors R d  and R s , form, together with the first capacitor C out1 , a first low pass, thus limiting the band width of the pulses in the pulse signal S P  for providing a band limited test signal S T  to the load impedance. This band limitation has to be performed, because some loads, especially electro-acoustic-transducers react in a bad manner or even can be destroyed if too high slopes (occurring for example in rectangular pulses) are applied. 
         [0020]    The second terminal of the load  30  is connected to the second supply terminal (ground potential) via an energy storing element such as a second capacitor C out2  in the present embodiment. As it can be easily seen from  FIG. 2 , the load  30  and the second capacitor C out2  form a second low pass filter receiving the test signal S T  and providing the output signal S O  on the line  38 . This output signal can be supplied to the comparator  40  (not shown in  FIG. 2 ) or directly to an A/D-Port of a microcontroller as explained above reference to  FIG. 1 . The output stage of a power amplifier  60  is connected to the first and the second terminal of the load  30 . The circuit of  FIG. 2  can be used also for overload protection of the output stage. The microcontroller can therefore keep the output stage deactivated until the correct load impedance, i.e., an appropriate electro-acoustic transducer is detected. 
         [0021]      FIGS. 3 and 4  illustrate some exemplary experimental data.  FIG. 3  is a timing diagram showing an exemplary pulse signal S P  having a pulse width of 5 ms.  FIG. 4  shows output signals S O  for different load impedances (e.g., 1 mΩ, 4Ω, 16Ω, 1 MΩ). If the output signal S O  is supplied to a comparator, an appropriate threshold value may be, for example, 2 volts. Assuming further, the threshold level is reached within about 4 μs, then it can be concluded the load impedance is about 4Ω. If the threshold level is reached in a shorter time, the impedance is too low, whereas, if the threshold value is reached later, the impedance is too high. 
         [0022]      FIG. 5  illustrates another embodiment of a load testing circuit. By employing multiplex-switches SW 1  and SW 2  the signal shaping circuit  20  can be used for different amplifiers. In the shown example each of the three power amplifier  60 A,  60 B,  60 C has an associated load  30 A,  30 B,  30 C, respectively, connected to its output terminals. A first capacitor C OUT1  and a second capacitor C OUT2  are connected between the first output terminal of the amplifier and the reference potential (e.g., ground potential GND) and a second output terminal of the amplifier and the reference potential, respectively. As mentioned above the capacitors C OUT1  and C OUT2  may be part of the amplifier output stage. 
         [0023]    The first output terminals of each of the amplifiers  60 A,  60 B, and  60 C and the signal shaping circuit  20  are connected to the multiplex-switch SW 1  such that either the first amplifier  60 A, the second amplifier  60 B, or the third amplifier  60 C is connected to the signal shaping circuit  20 . The second output terminals of each of the amplifiers  60 A,  60 B, and  60 C and the signal shaping circuit  20  are connected to the multiplex-switch SW 2  such that either the first amplifier  60 A, the second amplifier  60 B, or the third amplifier  60 C is connected to the comparator (not shown). Of course the multiplex-switches SW 1  and SW 2  have to switch synchronous, i.e., if the first output terminal of the second amplifier  60 B is connected to the signal shaping circuit  20 , then the second output terminal of second amplifier  60 B has to be connected to the comparator. 
         [0024]    The multiplex-switches SW 1  and SW 2  may be controlled by control signals CT 1  and CT 2  generated by the microcontroller  10 . In each switching state of the multiplex-switches SW 1  and SW 2  a circuit as depicted in  FIG. 2  is effectively achieved, where the connected amplifier can be switched. One of ordinary skill will immediately recognize that variations and modifications of the circuit explained with respect to the example of  FIG. 2  are also applicable to the exemplary circuit of  FIG. 5 . The number of connectable amplifiers is of course not limited to three. 
         [0025]    It is to be understood, that the invention is not limited to detection of the load impedances of electro-acoustic transducers connected to the output stage of an amplifier, but may be employed to detect any load impedance connected to an arbitrary power supply. Dependent on the application, the capacitors C OUT1  and C OUT2  may be replaced by inductors. 
         [0026]    Although various examples to realize the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. Such modifications to the inventive concept are intended to be covered by the appended claims.