Abstract:
A method and circuit for attenuating positive feedback in a comparator in one embodiment includes an amplifier configured to compare a first input signal with a second input signal and to provide an output based upon the comparison, a non-linear function with a first input operably connected to an output of the amplifier, and a feedback loop operably connected to the output of the non-linear function and to a second input of the non-linear function, the feedback loop including a feedback limiting circuit configured to attenuate a feedback signal to the second input of the non-linear function.

Description:
FIELD 
     The present disclosure relates to comparators and more specifically to comparators with positive feedback. 
     BACKGROUND 
     Among integrated circuits, comparators are circuit blocks that produce an output signal based upon a comparison between two input voltage levels. The output signal transitions between two values depending on the relative magnitude of the input voltage levels. For instance, a comparator output may be configured to generate a “high” output voltage level when a first input voltage is greater than a second input voltage and a “low” output voltage level when the first input voltage is less than the second input voltage. An exemplary high output voltage level may be five volts and an exemplary low output voltage level may be zero volts. The output voltages selected for a particular application may be higher or lower depending on design choices. 
     Comparators are useful in a wide variety of circuit applications, including analog to digital (“A/D”) converters. Many comparators, however, exhibit slow transitions between the high and low output voltages. The slow response of some comparators is a not suitable in various circuits. Specifically, many modem electronic circuits are designed to exhibit increased speed in comparison with traditional circuits. A slowly responding comparator in such a circuit slows the device to an unacceptable speed. To keep pace with the need for increased speed, some comparators are designed to exhibit a more rapid transition between the “low” and “high” output voltage levels. High-gain amplifiers, for example, are semiconductor devices capable of transiting between voltage levels very quickly. Accordingly, amplifiers may be incorporated as a comparator in a circuit to increase the speed of the circuit. 
     The transition speed of an amplifier may be increased by the inclusion of a feedback loop as is known in the art. By way of example,  FIG. 1  depicts a circuit  10  which includes an amplifier  12  which provides an input to a non-linear function  14 . The non-linear function  14  applies positive feedback from a non-linear function  16  to the output of the amplifier  12 . 
     Application of positive feedback by the non-linear function  14  is controlled by a logic circuit  18  which senses the output of the non-linear function  16  and, when a transition in output voltage is sensed, closes a switch  20  thereby applying the output of the non-linear function  16  to an input of the non-linear function  14 . Once the output value of the non-linear function  16  is no longer changing, the control logic circuit  18  controls the switch  20  to an open position. 
     The increased transition speed of an amplifier comparator with positive feedback compared to an amplifier comparator without positive feedback is evidenced by the chart  30  in  FIG. 2  which includes an input portion  32 , an output portion  34 , and a power portion  36 . In the chart  30 , solid lines in the output and power portions of the chart  30  correspond to the comparator  10  with positive feedback and the dashed lines in the output and power portions of the chart  30  correspond to the comparator without positive feedback. 
     Chart  30  depicts two input signals  38  and  40  which are applied to the two comparators. From T=0 to T=1, the input signal  38  is higher than the input signal  40 . In this example, both comparators are configured to exhibit a low output signal when the input signal  38  is higher than the input signal  40 . This is reflected in the output value lines  42  and  44  in the output portion  34 . 
     At T=1, however, the value of the input signal  40  exceeds the value of the input signal  38 . Accordingly, the outputs  42  and  44  begin to transition to a high value. The transition from a low output to a high output requires expenditure of power. Accordingly, the power expenditure of the comparators, indicated by the power consumption lines  50  and  52 , begins to increase. 
     The logic circuit  18  detects the increase in the output of the non-linear function  16  and closes the switch  20  at T=2. Accordingly, the power consumed by the comparator  10  (line  50 ) exhibits a rapid increase followed at time T=3 by a rapid increase in the output level of the non-linear function  16  as feedback is provided by the non-linear function  14 . At time T=4, the output of the non-linear function  16  is at the “high” output level. This is sensed by the logic circuit  18  at time T=4, and shortly thereafter the switch  20  is opened, resulting in a sudden drop in the consumed power (line  50 ). 
     As evidenced by a comparison of the output line  42  with the output line  44 , the comparator  10  with positive feedback achieves the final high value more quickly than the comparator without positive feedback. As evidenced by a comparison of the power consumption line  50  with the power consumption line  52 , the comparator  10  with positive feedback achieves the final high value at the expense of a power spike. The power spike extends beyond the time that the final output value is achieved because the control logic  18  is unable to open the switch  20  at the exact moment the output reaches its final value. Thus, the delay introduced by the control logic  18  generates a plateau of very high power consumption. 
     A need exists for a comparator that rapidly transitions from one output state to another output state. A rapidly transitioning comparator with low power consumption is also needed. 
     SUMMARY 
     A method and circuit for attenuating positive feedback in a comparator in one embodiment includes an amplifier configured to compare a first input signal with a second input signal and to provide an output based upon the comparison, a non-linear function with a first input operably connected to an output of the amplifier, and a feedback loop operably connected to the output of the non-linear function and to a second input of the non-linear function, the feedback loop including a feedback limiting circuit configured to attenuate a feedback signal to the second input of the non-linear function. 
     In another embodiment, a method of controlling feedback in a comparator includes establishing a first output condition at an output terminal of a comparator circuit, determining that a first input signal to the comparator circuit has a value greater than the value of a second input signal to the comparator circuit, applying a positive feedback signal in the comparator in response to the determination, attenuating the positive feedback signal, and achieving a second output condition at the output terminal of the comparator circuit using the attenuated positive feedback signal. 
     In yet another embodiment, a comparator circuit includes an output, a positive feedback circuit including a MOSFET with a gate operably connected to the output, and a capacitor in direct electrical communication with the MOSFET. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates, in block diagram form, a prior art high-gain amplifier configured as a comparator with a positive feedback loop; 
         FIG. 2  depicts a graph that demonstrates the relationship between the input signals, the output signal, and the power consumed by a prior art amplifier with a positive feedback loop and a prior art amplifier without a positive feedback loop; 
         FIG. 3  illustrates, in block diagram form, an amplifier having a positive feedback loop and a feedback limiter device; 
         FIG. 4  depicts a graph that demonstrates the relationship between the input signals, the output signal, and the power consumed by the amplifier of  FIG. 3 ; 
         FIG. 5  depicts an exemplary schematic view of a comparator circuit incorporating a feedback limiting device in a positive feedback loop; 
         FIG. 6  depicts the comparator circuit of  FIG. 5  with the feedback limiting device located at an alternative location within the circuit; and 
         FIG. 7  depicts the comparator circuit of  FIG. 5  modified to provide different functionality. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 3 , a circuit  100  includes an amplifier  102  with an input  104  and an input  106 . The amplifier  102  includes an output  108  connected to a non-linear function  110  at an input  112 . The non-linear function  110  includes a second input  114  and an output  116  which is provided to a non-linear function  118 . An output  120  of the non-linear function  118  is connected to a feedback loop  122 . 
     The feedback loop  122  receives input from the output  120  of the non-linear function  118 . The input signal is applied to a control logic circuit  124  and a feedback limiting device  126 . The control logic circuit  124  controls a switch  130  that includes one terminal  132  connected to the feedback limiting device  126  and another terminal  134  connected to the terminal  114  of the non-linear function  110 . The control logic circuit  124  is also connected to the feedback limiting device  126 . 
     Operation of the circuit  100  is described with reference to  FIG. 4 . In  FIG. 4 , a graph  140  includes an input portion  142 , an output portion  144 , and a power portion  146 . Initially, exemplary input signals  150  and  152  are applied to the inputs  104  and  106 , respectively. At time T=0, the voltage  152  has a larger value than the voltage  150 . In this embodiment, the amplifier  102  is configured to amplify the difference between signals on the input  104  and  106 . The low signal is felt at the input  112  of the non-linear function  110 . Because the switch  130  is open, there is no signal at the input  114 . Accordingly, the output of the non-linear function  110  is low. 
     Since the output  116  of the non-linear function  110  is low, the signal provided to the non-linear function  118  is low and a low output signal is maintained at the output  120 . The control logic circuit  124  senses a stable low signal at the output  120 , and maintains the switch  130  in the open position. 
     At time T=1, the voltage  150  at the input  104  exceeds the voltage  152  at the input  106 . Accordingly, the voltage at the output  108  begins to increase. The increase in voltage at the output  108  is provided as an input to the input  112  of the non-linear function  110 . Accordingly, the output  116  of the non-linear function  110  begins to increase. 
     The increase of the output  116  of the non-linear function  110  is provided to the non-linear function  118  and the output  120  of the non-linear function  118  begins to increase as indicated by the output line  154  immediately after time T=1. The increase is accomplished by an increase in power consumption as indicated by the power consumption line  156  in the power portion  146  after time T=1. 
     The increased voltage at the output  120  is detected by the logic circuit  124  and the switch  130  is closed at time T=2. Closing of the switch  130  causes a sharp increase in the power consumed by the circuit  100  as indicated by the power consumption line  156  in the power portion  146 . Shortly after switch  130  closes, a feedback signal from the terminal  134  is felt at the input  114 . Accordingly, the non-linear function  110  adds the feedback signal to the input  112  received from the amplifier  102 . This causes a rapid increase in the output  116  of the non-linear function  110 . The rapid increase of the output  116  of the non-linear function  110  is provided to the non-linear function  118  and the output  120  of the non-linear function  118  begins to increase rapidly as indicated by the output line  154  immediately after time T=3. 
     Once the switch  130  is closed, the signal passing from the output  120  of the non-linear function  118  to the input  114  of the non-linear function  110  through the switch  130  also begins to be attenuated by the feedback limiting device  126 . Attenuation is accelerated as the rapid increase in the output signal (line  154 ) occurs after time T=3. Accordingly, the positive feedback signal at the input  114  is decreased, and the power consumed by the circuit  100  is rapidly reduced (see line  156  at time T=3 + ). When the signal at the output  120  is at the high output voltage level, the control circuit  124  opens the switch  130  at time T=4. 
     Accordingly, as the output of the circuit  100  (line  154 ) approaches the final output value, by proper selection of the feedback limiting device  126 , the power consumed by the circuit  100  (line  156 ) approaches zero since the feedback signal at the input  114  to the non-linear function  110  becomes significantly attenuated. 
     The circuit  100  thus provides a significant increase in response time as compared to a comparator circuit incorporating an amplifier with no positive feedback (line  44  of  FIG. 4 ) while consuming significantly less power than the circuit  10  (line  50 ). 
     In some circuits, the feedback limiting device  126  may be embodied as an energy storing device. Specifically, in circuits where signal processing is done in the voltage domain, the feedback limiter device  126  may be a capacitor. One such example is the circuit  170  depicted in  FIG. 5 . 
     The circuit  170  is a comparator which includes the pre-amplifier. The circuit  170  includes a current source  172  connected to the source of a P-channel MOSFET  174 . The gate of the MOSFET  174  is connected to an input terminal  176 . The drain of the MOSFET  174  is connected to the source of an N-channel MOSFET  178 . The substrate of the MOSFET  174  is connected to the substrate of a P-channel MOSFET  180 . 
     The source of the MOSFET  180  is connected to the current source  172 . The gate of the MOSFET  180  is connected to an input terminal  182 . The drain of the MOSFET  180  is connected to the source of an N-channel MOSFET  184 . The substrate and drain of the MOSFET  184  are connected to circuit ground. The gate of the MOSFET  184  is connected to the gate of an N-channel MOSFET  186 . The substrate and drain of the MOSFET  186  are connected to circuit ground. The source of the MOSFET  186  is connected to an output terminal  188  and to the drain of a P-channel MOSFET  190 . 
     The substrate and source of the MOSFET  190  are connected to a supply voltage (not shown). The gate of the MOSFET  190  is connected to the gate and drain of a P-channel MOSFET  192 . The substrate and source of the MOSFET  192  are connected to the supply voltage (not shown). The drain of the MOSFET  192  is connected to the source of an N-channel MOSFET  196 . The substrate and drain of the MOSFET  196  are connected to circuit ground. The gate of the MOSFET  196  is connected to the gate of the MOSFET  178 . The substrate and drain of MOSFET the  178  are connected to circuit ground. 
     The circuit  170  also includes a positive feedback loop  200 . The positive feedback loop  200  includes an N-channel MOSFET  204 , and a feedback limiting circuit  206 . The gate of the MOSFET  204  is connected to the source of the MOSFET  186 , which is coupled to the output voltage terminal  188 . The source of the MOSFET  204  is connected to the source of the MOSFET  196 . The substrate of the MOSFET  204  is connected to circuit ground. 
     Finally, the drain of the MOSFET  204  is connected to the feedback limiting circuit  206  which in this embodiment includes a capacitor  208 , a current source  210 , and a switch  212 . Each of the capacitor  208 , the current source  210 , and the switch  212  are connected to the drain of the MOSFET  204  and to circuit ground. 
     The circuit  170  of  FIG. 5  generates an output voltage signal on the terminal  188  that transitions between close to the supply voltage (“high”) and close to zero volts (“low”) depending on the relative magnitude of the input signals applied to terminals  176  and  182 . During a transition from low to high voltage, the signal at the output terminal  188  is applied to the gate of the MOSFET  204 , allowing current to flow from the source to the drain of the MOSFET  204 . The current flowing through the MOSFET  204  represents the positive feedback signal applied to the terminal  114  of  FIG. 3  discussed above. 
     The feedback limiting circuit  206  reduces the magnitude and duration of the positive feedback signal by attenuating the current flowing through the MOSFET  204 . Specifically, as the feedback current flows through the MOSFET  204 , the current also flows through the capacitor  208 . When the capacitor  208  does not contain any stored charge it offers substantially zero impedance to the flow of current. Accordingly, the capacitor  208  initially looks like a short in the feedback circuit  200  allowing maximum feedback current to flow therethrough. This allows the output signal at the terminal  188  to reach the final output value very quickly. 
     As the feedback current flows through the capacitor  208 , the capacitor  208  becomes charged. Once the capacitor  208  beings to store charge, the conductance of MOSFET  204  decreases to a very low level and the feedback current through the feedback circuit  200  is thus reduced, thereby attenuating the positive feedback signal. Additionally, the power consumption rate of the circuit  170  is rapidly reduced. 
     Once the output terminal  188  is at the desired level, the MOSFET  204  stops passing current from the source to the drain of the MOSFET  204  and current flow through the feedback loop  200  ceases. 
     Once current is no longer flowing through the feedback loop  200 , the current source  210  or the switch  212  is used to drain the charge from the capacitor  208  in preparation for another transition. Since either the current source  210  or the switch  212  can drain the charge from the capacitor  208 , in alternative embodiments, only one of the current source  210  or the switch  212  may be incorporated. 
     Thus, by proper selection of the capacitance of the capacitor  208 , the feedback loop  200  may be controlled to reduce the amount of power used by the circuit  170  while providing a rapid transition in the output of the circuit  170 . 
     Various circuits may incorporate a feedback limiting device and the feedback limiting device may be located differently in various circuits. By way of example, the circuit  170 ′ of  FIG. 6  is similar as the circuit  170  of  FIG. 5 . In the circuit  170 ′, however, the feedback loop  200 ′ and the feedback limiting circuit  206 ′ are modified from the feedback loop  200  and the feedback limiting circuit  206  of  FIG. 5 . Specifically, the feedback limiting circuit  206 ′ does not include a current source and the feedback limiting circuit  206 ′ is positioned between the source of the MOSFET  204  and the drain of the MOSFET  192 . Operation of the circuit  170 ′, however, is similar to the operation of the circuit  170 . 
     The circuit  170 ″ of  FIG. 7  is also substantially the same as the circuit  170  of  FIG. 5 . In the circuit  170 ″, however, cross-coupled transistors  220  and  222  have been incorporated into the input stage of the circuit  170 ″. Thus, while the functionality of the circuit  170 ″ has been modified for a particular application, the feedback loop  200  continues to be operable for providing positive feedback and the feedback limiting circuit  206  continues to limit the amount of feedback current during a voltage transition in the same manner as described above with respect to the circuit  170 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.