Abstract:
An apparatus for reducing voltage surge while powering up an amplifier device, the amplifier device charging an input capacitance in at least one input line during the powering up, the charging being subject to a time constant established by the input capacitance in cooperation with an input resistance in the at least one input line, includes: a resistive load switchingly coupled with at least a portion of the input resistance in the at least one input line. The switchingly coupling is effected during at least a portion of the powering up.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to input stages of amplifier devices. The present invention may be particularly advantageously used with an input stage for an audio amplifier device. The present invention will be described in terms of an audio amplifier device solely in an illustrative manner with the understanding that the invention is useful with other amplifier devices as well.  
         [0002]     When an amplifier is powered up, or turned on, there is often a voltage surge that occurs. The voltage surge is commonly manifested as an audible “pop” or “click” that is a significant annoyance to users. The pop/click may be more than a mere annoyance. If the voltage surge is large enough the pop/click may loud enough to damage speakers coupled with the amplifier device. Speaker coils may be damaged permanently.  
         [0003]     There is a need for an apparatus and method for limiting voltage surge at amplifier start up to avoid damage to the amplifier or damage to equipment coupled with the amplifier.  
       SUMMARY OF THE INVENTION  
       [0004]     An apparatus for reducing voltage surge while powering up an amplifier device, the amplifier device charging an input capacitance in at least one input line during the powering up, the charging being subject to a time constant established by the input capacitance in cooperation with an input resistance in the at least one input line, includes: a resistive load switchingly coupled with at least a portion of the input resistance in the at least one input line. The switchingly coupling is effected during at least a portion of the powering up.  
         [0005]     A method for reducing voltage surge while powering up an amplifier device, the amplifier device charging an input capacitance in at least one input line during the powering up, the charging being subject to a time constant established by the input capacitance in cooperation with an input resistance in the at least one input line, includes the steps of: (a) providing a resistive load; and (b) switchingly coupling the resistive load with at least a portion of the input resistance in the at least one input line. The switchingly coupling is effected during at least a portion of the powering up.  
         [0006]     It is, therefore, an object of the present invention to provide an apparatus and method for limiting voltage surge at amplifier start up to avoid damage to the amplifier or damage to equipment coupled with the amplifier.  
         [0007]     Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an electrical schematic diagram of a prior art input circuit for an amplifier.  
         [0009]      FIG. 2  is a graphic representation of selected signals in the prior art input circuit illustrated in  FIG. 1 .  
         [0010]      FIG. 3  is an electrical schematic diagram of an input circuit for an amplifier configured according to the present invention.  
         [0011]      FIG. 4  is a graphic representation of selected signals in the input circuit illustrated in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]      FIG. 1  is an electrical schematic diagram of a prior art input circuit for an amplifier. In  FIG. 1 , an amplifier system  10  includes an amplifier device  111  and an input section  12 . Input section  12  includes a preamplifier unit  14  coupled with amplifier device  11  and coupled with input loci  16 ,  18  for receiving input signals.  
         [0013]     Preamplifier unit  14  receives first input signals via a first input network  20  from input locus  16 . First input network  20  includes a capacitor  24  and a resistor  26 . Preamplifier unit  14  is biased with respect to first input network  20  by a resistor  28 . Capacitor  24  has a value C 1 . Resistor  26  has a value R 1 . Resistor  28  has a value R 3 .  
         [0014]     Preamplifier unit  14  receives second input signals via a second input network  30  from input locus  18 . Second input network  30  includes a capacitor  34  and a resistor  36 . Preamplifier unit  14  is biased with respect to second input network  30  by a resistor  38 . Capacitor  34  has a value C 2 . Resistor  36  has a value R 2 . Resistor  38  has a value R 4 .  
         [0015]     Amplifier system  10  is illustratively described herein as a fully differential amplifier system. The present invention is equally useful with a non-differential, single-ended amplifier system having only one input locus  16  or  18 .  
         [0016]      FIG. 2  is a graphic representation of selected signals in the prior art input circuit illustrated in  FIG. 1 . In  FIG. 2 , a graphic plot  50  is presented with respect to a vertical axis  52  representing signal amplitude, such as voltage amplitude and with respect to a horizontal axis  54  representing time. A first curve  56  represents charge on first capacitor  24  ( FIG. 1 ). A second curve  58  represents charge on second capacitor  34  ( FIG. 1 ). Times t 0 , t 1  are indicated on axis  54 . Time t 0  represents the time at which amplifier system  10  is initially powered up. Time t 1  represents the time at which amplifier device  11  is turned on. During the interval t 0 -t 1 , only input section  12  is turned on and amplifier device  11  is not yet powered. Preamplifier unit  14  charges capacitors  24 ,  34  to a voltage reference V REF  which sets the common-mode voltage for amplifier device  11  during interval t 0 -t 1 . Certain biases and references for operating amplifier device  11  are also permitted to settle during interval t 0 -t 1  before amplifier device  11  is turned on.  
         [0017]     Time t 1  for turning on amplifier device  11  is commonly established according to standards in effect for a given product in which amplifier system  10  is employed (not shown in  FIG. 1 ). As a consequence a difference in charge level often exists between capacitors  24 ,  34 , as indicated by difference between curves  56 ,  58  at time t 1  in FIG.  2 . It is difference between charge levels of capacitors  24 ,  34  that causes a voltage surge into amplifier device  11  when amplifier device  11  is turned on at time t 1 .  
         [0018]     It is desired that first input network  20  and second input network  30  are substantially the same. In the interest of simplifying explaining the present invention and avoiding unnecessary prolixity, only first input network  20  will be described with the understanding that the principles described apply equally to second input network  30 .  
         [0019]     While charging capacitor  24  first input network  20  represents a low pass filter having an RC (resistive-capacitive) time constant T 1 : 
 
 T   1 =( R   1   +R   3 )· C   1   [1]
 
         [0020]     By way of example and not by way of limitation, if amplifier system  10  were embodied in an audio amplifier typical values for C 1 , R 1 , R 3  yield a time constant T 1  in a range from 157 msec to 863 msec. Time required for capacitors  24 ,  34  to settle to within 0.1% of their final values requires approximately 5-7 time constants. Such a delay in turning on amplifier device  11  after powering up preamplifier unit  12  is regarded as unsatisfactory in many of today&#39;s products. By way of further example and not by way of limitation, products such as personal digital assistant (PDA) devices or cellular phones frequently enter a “sleep” mode to conserve battery power. Having to wait for a second or more to reenter an active mode from such a sleep mode is irritating to a user.  
         [0021]      FIG. 3  is an electrical schematic diagram of an input circuit for an amplifier configured according to the present invention. In  FIG. 3 , an amplifier system  100  includes an amplifier device  110  and an input section  112 . Input section  112  includes a preamplifier unit  114  coupled with amplifier device  110  and coupled with input loci  116 ,  118  for receiving input signals.  
         [0022]     Preamplifier unit  114  receives first input signals via a first input network  120  from input locus  116 . First input network  120  includes a capacitor  124  and a resistor  126 . Preamplifier unit  114  is biased with respect to first input network  120  by a resistor  128 . Capacitor  124  has a value C 1 . Resistor  126  has a value R 1 . Resistor  128  has a value R 3 . First input network  120  also includes a switch  140  coupled in parallel with series-connected resistors  126 ,  128 . Switch  140  has a resistance R S1  when switch  140  is closed.  
         [0023]     Preamplifier unit  114  receives second input signals via a second input network  130  from input locus  118 . Second input network  130  includes a capacitor  134  and a resistor  136 . Preamplifier unit  114  is biased with respect to second input network  130  by a resistor  138 . Capacitor  134  has a value C 2 . Resistor  136  has a value R 2 . Resistor  138  has a value R 4 . Second input network  130  also includes a switch  142  coupled in parallel with series-connected resistors  136 ,  138 . Switch  142  has a resistance R S2  when switch  142  is closed.  
         [0024]     Amplifier system  100  is illustratively described herein as a fully differential amplifier system. The present invention is equally useful with a non-differential amplifier system having only one input locus  116  or  118 .  
         [0025]      FIG. 4  is a graphic representation of selected signals in the input circuit illustrated in  FIG. 3 . In  FIG. 4 , a graphic plot  150  is presented with respect to a vertical axis  152  representing signal amplitude, such as voltage amplitude and with respect to a horizontal axis  154  representing time. A first curve  156  represents charge on first capacitor  124  ( FIG. 3 ). A second curve  158  represents charge on second capacitor  134  ( FIG. 3 ). Times t 0 , t 1  are indicated on axis  154 . Time t 0  represents the time at which amplifier system  100  is initially powered up. Time t 1  represents the time at which amplifier device  110  is turned on. During the interval t 0 -t 1 , only input section  112  is turned on and amplifier device  111  is not yet powered. Preamplifier unit  114  charges capacitors  124 ,  134  to a voltage reference V REF  which sets the common-mode voltage for amplifier device  110  during interval t 0 -t 1 . Certain biases and references for operating amplifier device  110  are also permitted to settle during interval t 0 -t 1  before amplifier device  110  is turned on. During interval t 0 -t 1  switches  140 ,  142  are closed.  
         [0026]     It is desired that first input network  120  and second input network  130  are substantially the same. In the interest of simplifying explaining the present invention and avoiding unnecessary prolixity, only first input network  120  will be described with the understanding that the principles described apply equally to second input network  130 .  
         [0027]     While charging capacitor  124  first input network  120  represents a low pass filter having an RC (resistive-capacitive) time constant T 2 : 
 
 T   2 =[( R   1   +R   3 )∥ RS 1 ]·C   1   [2]
 
         [0028]     The preferred embodiment of switch  140  is an FET (field effect transistor) so that resistance R S1  is established substantially as the drain-to-source resistance of switch  140 . As a consequence, 
 
 R   S1 &lt;&lt;( R   1   +R   3 )  [3]
 
         [0029]     So that time constant T 2  is substantially reduced to 
 
 T   2   =RS 1 ·C   1   [4]
 
         [0030]     Time constant T 2  is thus substantially less than time constant T 1  ( FIG. 1 ) for a comparable product. In practical terms, this means that capacitors  124 ,  134  will charge much more quickly than capacitors  24 ,  34  ( FIG. 1 ) in a comparable product so that both of capacitors  124 ,  134  are similarly charged by time t 1.  There is no charge differential between capacitors  124 ,  134  at time t 1  when amplifier device  110  is turned on so there is no voltage surge passed through amplifier device  110  and manifested as a pop/click or other signal anomaly.  
         [0031]     In operation, switches  140 ,  142  are preferably only closed during interval t 0 -t 1  and are opened no later than time t 1  when amplifier device  110  is turned on. This may be effected using a timed interval during which switches  140 ,  142  are turned on following powering up of preamplifier section  114 , or by using another control arrangement (not shown in  FIG. 2 ).  
         [0032]     The present invention permits a very fast powering up of an amplifier device. By assuring that input capacitors (e.g., capacitors  124 ,  134 ) are substantially fully charged before turning on an amplifier device (e.g., amplifier device  110 ), a designer may employ less stringently toleranced capacitors in an amplifier design and thereby use less expensive components in a design using the present invention. There is less need to closely match capacitors to reduce charge difference when turning on an amplifier device because the present invention assures that the charge difference is substantially eliminated by the time the amplifier device is turned on.  
         [0033]     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: