Abstract:
Apparatus to compare an input signal to a threshold level are disclosed. An example circuit described herein includes a Widlar bandgap circuit to receive the input signal, an intermediate stage coupled with the output of the Widlar bandgap circuit, and a final stage coupled with the output of the intermediate stage, the final stage to provide an output based on the input signal and the threshold level.

Description:
RELATED APPLICATION 
   This application claims priority from U.S. provisional application Ser. No. 60/877,716 filed Dec. 28, 2006, which is hereby incorporated by reference in its entirety. 

   FIELD OF THE DISCLOSURE 
   This disclosure relates generally to analog devices and, more particularly, to apparatus to compare an input voltage with a threshold voltage. 
   BACKGROUND 
   A voltage comparator circuit compares a first input voltage to a second input voltage and outputs a signal indicative of the results of the comparison. For example, when the first input voltage exceeds the second input voltage the comparator circuit may output a logical high signal. Conversely, when the first input voltage does not exceed the second input voltage, the comparator circuit may output a logical low signal. In some implementations, the second input voltage may be a reference voltage (e.g., the voltage comparator circuit only has one varying input voltage that is compared to the reference voltage). For example, the reference voltage may be an output of a bandgap reference circuit. A bandgap reference circuit provides a temperature-independent reference voltage that is typically around 1.25 volts. To provide reliable comparator performance, a comparator circuit having a bandgap reference voltage of about 1.25 volts has a minimum operating voltage of about 1.4 volts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an electrical schematic diagram of an example implementation of a comparator apparatus. 
       FIG. 2  is a graph illustrating the output characteristic of the example comparator apparatus of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is an electrical schematic diagram of an example implementation of a comparator apparatus  100 . The example comparator apparatus  100  includes a first stage  102 , an intermediate stage  104 , and a final stage  106 , each of which connects to a ground voltage potential  108 . The example comparator apparatus  100  receives an input voltage V IN  and generates an output voltage V O . The output voltage remains at a low value while the input voltage V IN  is less than a threshold voltage V th . When V IN  exceeds V th , output voltage V O  rises to approximately equal V IN . 
   The first stage  102  of the illustrated example is a Widlar bandgap circuit that supplies a current signal to the intermediate stage  104  based on input voltage V IN . No current is supplied to the intermediate stage while V IN ≦V th . When V IN &gt;V th , a current flows from the first stage  102  into the intermediate stage  104 . 
   First stage  102  of the illustrated example is implemented using a first impedance  110 , a second impedance  112 , a third impedance  114 , a first transistor  116 , and a second transistor  118 . 
   First impedance  110  connects between input voltage V IN  and a collector  116   c  of first transistor  116 , which also connects to a base  116   b  of first transistor  116  and a base  118   b  of second transistor  118 . The second impedance  112  connects between input voltage V IN  and collector  118   c  of second transistor  118 , which also connects to intermediate stage  104  at node N 2 . Third impedance  114  connects between an emitter  118   e  of second transistor  118  and ground voltage potential  108 . First impedance  110 , second impedance  112 , and third impedance  114  of the illustrated example are resistors. However, as will be appreciated by those having ordinary skill in the art, other impedances may be used in alternative implementations. 
   First transistor  116  includes collector  116   c , base  116   b , and an emitter  116   e . As previously described, collector  116   c  connects to base  116   b  and base  118   b . Base  116   b  connects to collector  116   c  and base  118   b  of second transistor  118 . Emitter  116   e  connects to ground voltage potential  108 . In the illustrated example, first transistor  116  is an NPN bipolar junction transistor. Alternatively, first transistor  116  may comprise a PNP bipolar junction transistor. 
   Second transistor  118  includes collector  118   c , base  118   b , and emitter  118   e . As previously described, collector  118   c  connects to second impedance  112  and intermediate stage  104  at node N 2 . Base  118   b  connects to base  116   b  of first transistor  116 . Emitter  118   e  connects to third impedance  114 . In the illustrated example, second transistor  118  is an NPN bipolar junction transistor. Alternatively, second transistor  118  may comprise a PNP bipolar junction transistor. 
   The intermediate stage  104  of the illustrated example is a common-emitter amplifier. The example intermediate stage  104  receives the current signal from first stage  102  at node N 2  and outputs a current signal to final stage  106  at node N 3 . Intermediate stage  104  sources an appreciable current to final stage  106  only when first stage  102  fails to source appreciable current to intermediate stage  104 . 
   Intermediate stage  104  of the illustrated example includes a fourth impedance  120  and a third transistor  122 . Fourth impedance  120  connects between input voltage V IN  and node N 3 , to which a collector  122   c  of third transistor  122  also connects. Fourth impedance  120  of the illustrated example is shown as a resistor; however, other impedances may be used in alternative implementations. 
   Third transistor  122  includes collector  122   c , a base  122   b , and an emitter  122   e . As previously described, collector  122   c  connects to fourth impedance  120  and node N 3 . Base  122   b  connects to first stage  102  at node N 2 . Emitter  122   e  connects to ground voltage potential  108 . In the illustrated example, the third transistor  122  sinks the current flowing through the fourth impedance  120  to ground when an appreciable current signal is received from the first stage  102 . The example third transistor  122  is an NPN bipolar junction transistor. Alternatively, third transistor  122  may comprise a PNP bipolar junction transistor. 
   The final stage  106  of the illustrated example is a transistor switch. The example final stage  106  connects to node N 3  and generates an output voltage V O  for the comparator apparatus  100 . The output voltage V O  substantially equals V IN  when the final stage receives no current from intermediate stage  104  at node N 3 . The output voltage V O  drops to a low value when the final stage receives appreciable current from intermediate stage  104 . 
   Final stage  106  of the illustrated example includes a current source  124  and a fourth transistor  126 . Current source  124  serves as a load for fourth transistor  126  and provides output current for comparator apparatus  100 . Current source  124  can be implemented as a depletion-mode field-effect transistor (FET), a junction field-effect transistor (JFET), or any other element operable to restrict current. Current source  124  may not be necessary in all implementations of comparator  100 . 
   Fourth transistor  126  includes collector  126   c , a base  126   b , and an emitter  126   c . As previously described, collector  126   c  connects to current source  124  and provides output voltage V O . Base  126   b  connects to intermediate stage  104  at node N 3 . Emitter  126   e  connects to ground voltage potential  108 . In the illustrated example, the fourth transistor  126  sinks the current from current source  124  to the ground voltage potential  108  when the final stage  106  receives an appreciable current signal from the intermediate stage  104 . The example fourth transistor  126  is an NPN bipolar junction transistor. Alternatively, fourth transistor  126  may comprise a PNP bipolar junction transistor. 
   The threshold voltage V th  for comparator apparatus  100  of the illustrated example approximately equals 
   
     
       
         
           
             
               
                 
                   
                     V 
                     th 
                   
                   = 
                   
                     
                       V 
                       
                         be 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         122 
                       
                     
                     + 
                     
                       
                         
                           R 
                           112 
                         
                         
                           R 
                           114 
                         
                       
                       ⁢ 
                       
                         V 
                         T 
                       
                       ⁢ 
                       
                         ln 
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 A 
                                 
                                   E 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   118 
                                 
                               
                               ⁢ 
                               
                                 R 
                                 112 
                               
                             
                             
                               
                                 A 
                                 
                                   E 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   116 
                                 
                               
                               ⁢ 
                               
                                 R 
                                 110 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 , 
               
             
             
               
                 [ 
                 1 
                 ] 
               
             
           
         
       
     
   
   where V be122  equals the voltage drop between base  122   b  and emitter  122   e  of third transistor  122 , R 110  is the resistance of first impedance  110 , R 112  is the resistance of second impedance  112 , R 114  is the impedance of third impedance  114 , A E116  is the emitter area of first transistor  116 , A E118  is the emitter area of second transistor  118 , and V T  is the thermal voltage, which equals Boltzmann&#39;s constant k times the charge on the electron q divided by the absolute temperature T, or V T =kT/q. 
   When V IN  equals the threshold voltage V th , first transistor  116 , second transistor  118 , and third transistor  122  are all based into the forward active region. In this region of operation, 
   
     
       
         
           
             
               
                 
                   
                     V 
                     BE 
                   
                   = 
                   
                     
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                       T 
                     
                     ⁢ 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           
                             I 
                             C 
                           
                           
                             
                               A 
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                             ⁢ 
                             
                               J 
                               S 
                             
                           
                         
                         ) 
                       
                     
                   
                 
                 , 
               
             
             
               
                 [ 
                 2 
                 ] 
               
             
           
         
       
     
   
   where V BE  is the voltage differential between the base and the emitter of a particular transistor, V T  is the thermal voltage, I C  is the current through the collector of this transistor, A E  is the emitter area of this transistor, and J S  is the saturation current density of this transistor. Assuming V IN =V th  and ignoring base currents, the collector current of transistor  116  equals 
   
     
       
         
           
             
               
                 
                   I 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     116 
                   
                 
                 = 
                 
                   
                     
                       V 
                       IN 
                     
                     - 
                     
                       V 
                       
                         be 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         116 
                       
                     
                   
                   
                     R 
                     110 
                   
                 
               
             
             
               
                 [ 
                 3 
                 ] 
               
             
           
         
       
     
   
   Similarly, the collector current of transistor  118  equals 
   
     
       
         
           
             
               
                 
                   I 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     118 
                   
                 
                 = 
                 
                   
                     
                       V 
                       IN 
                     
                     - 
                     
                       V 
                       
                         be 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         122 
                       
                     
                   
                   
                     R 
                     112 
                   
                 
               
             
             
               
                 [ 
                 4 
                 ] 
               
             
           
         
       
     
   
   Assuming that V be116  approximately equals V be122 , one can combine equations [2], [3], and [4] to obtain 
   
     
       
         
           
             
               
                 
                   
                     V 
                     
                       be 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       118 
                     
                   
                   - 
                   
                     V 
                     
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       116 
                     
                   
                 
                 = 
                 
                   
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                   ⁢ 
                   
                     ln 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             A 
                             
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               118 
                             
                           
                           ⁢ 
                           
                             R 
                             112 
                           
                         
                         
                           
                             A 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               116 
                             
                           
                           ⁢ 
                           
                             R 
                             110 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 [ 
                 5 
                 ] 
               
             
           
         
       
     
   
   The voltage described by equation [5] appears across resistor  114 . Again neglecting base currents, the collector current of transistor  118  equals its emitter current, 
   
     
       
         
           
             
               
                 
                   I 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     118 
                   
                 
                 = 
                 
                   
                     
                       V 
                       T 
                     
                     
                       R 
                       114 
                     
                   
                   ⁢ 
                   
                     ln 
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             A 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               118 
                             
                           
                           ⁢ 
                           
                             R 
                             112 
                           
                         
                         
                           
                             A 
                             
                               E 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               116 
                             
                           
                           ⁢ 
                           
                             R 
                             110 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 [ 
                 6 
                 ] 
               
             
           
         
       
     
   
   Combining equation [3] with equation [6] and evaluating at V IN =V th  gives 
   
     
       
         
           
             
               
                 
                   V 
                   th 
                 
                 = 
                 
                   
                     V 
                     
                       be 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       122 
                     
                   
                   + 
                   
                     
                       
                         R 
                         112 
                       
                       
                         R 
                         114 
                       
                     
                     ⁢ 
                     
                       V 
                       T 
                     
                     ⁢ 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               A 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 118 
                               
                             
                             ⁢ 
                             
                               R 
                               112 
                             
                           
                           
                             
                               A 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 116 
                               
                             
                             ⁢ 
                             
                               R 
                               110 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 [ 
                 7 
                 ] 
               
             
           
         
       
     
   
   which is identical to equation [1]. Because the base-emitter voltage of a bipolar junction transistor exhibits a negative temperature coefficient, while the thermal voltage exhibits a positive temperature coefficient, proper selection of resistors and emitter areas will result in a net cancellation of temperature coefficients within equation [7]. The threshold voltage V th  at which the output voltage V O  of comparator apparatus  100  switches states can therefore be made substantially independent of temperature. For silicon transistors, this temperature-independent threshold voltage equals approximately 1.22V, a quantity sometimes called the bandgap voltage. 
   The assumption that V be116 =V be122  used in deriving equation [5] can be approximately satisfied by selecting the resistances of first impedance  110  and fourth impedance  120  to satisfy the equation 
   
     
       
         
           
             
               
                 
                   
                     R 
                     110 
                   
                   
                     R 
                     120 
                   
                 
                 = 
                 
                   
                     A 
                     
                       E 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       122 
                     
                   
                   
                     A 
                     
                       E 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       116 
                     
                   
                 
               
             
             
               
                 [ 
                 8 
                 ] 
               
             
           
         
       
     
   
   Comparator apparatus  100  can be adjusted to produce a threshold voltage larger than the bandgap voltage while ensuring that this threshold voltage remains substantially independent of temperature by adding a first adjusting impedance  128 , a second adjusting impedance  130 , and a third adjusting impedance  132 . The adjusting impedances should satisfy the equation 
   
     
       
         
           
             
               
                 
                   
                     R 
                     110 
                   
                   
                     R 
                     128 
                   
                 
                 = 
                 
                   
                     
                       R 
                       112 
                     
                     
                       R 
                       130 
                     
                   
                   = 
                   
                     
                       R 
                       120 
                     
                     
                       R 
                       132 
                     
                   
                 
               
             
             
               
                 [ 
                 9 
                 ] 
               
             
           
         
       
     
   
   where R 110  is the resistance of first impedance  110 , R 128  is the resistance of first adjusting impedance  128 , R 112  is the resistance of second impedance  112 , R 130  is the resistance of second adjusting impedance  130 , R 120  is the resistance of fourth impedance  120 , and R 132  is the resistance of third adjusting impedance R 132 . The insertion of the adjusting impedances changes the formula for the threshold voltage to 
   
     
       
         
           
             
               
                 
                   V 
                   th 
                 
                 = 
                 
                   
                     
                       V 
                       
                         be 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         122 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                             112 
                           
                           
                             R 
                             130 
                           
                         
                       
                       ) 
                     
                   
                   + 
                   
                     
                       
                         R 
                         112 
                       
                       
                         R 
                         114 
                       
                     
                     ⁢ 
                     
                       V 
                       T 
                     
                     ⁢ 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               A 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 118 
                               
                             
                             ⁢ 
                             
                               R 
                               112 
                             
                           
                           
                             
                               A 
                               
                                 E 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 116 
                               
                             
                             ⁢ 
                             
                               R 
                               110 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 [ 
                 10 
                 ] 
               
             
           
         
       
     
   
   The inclusion of the adjusting impedances allows the achievement of any desired threshold voltage greater than the bandgap voltage while ensuring that this threshold voltage remains substantially independent of temperature. 
     FIG. 2  is a graph illustrating the output characteristic of the example comparator apparatus  100  of  FIG. 1 . In region  200 , comparator apparatus  100  lacks sufficient voltage to bias fourth transistor  126  into conduction. Current source  124  pulls output voltage V O  high within this region. Once the input voltage V IN  exceeds the base-emitter voltage V be  of fourth transistor  126 , output voltage V O  drops to nearly zero. The output voltage V O  remains nearly zero between V be  and the threshold voltage V th . The output voltage V O  switches state at approximately V IN =V th . The output voltage V O  equals the input voltage V IN  for values of the input voltage V IN  greater than or equal to V th . 
   Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers every apparatus, method and article of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.