Abstract:
A comparator comprises a cross-coupled regenerative latch, a circuit connected to the cross-coupled regenerative latch and a clocking circuit. The cross-coupled regenerative latch regenerates, during a latching mode, a signal which is indicative of a difference between two input signals. The circuit connected to the cross-coupled regenerative latch operates as a voltage follower during an acquisition mode and as a cascode amplifier stage during the latching mode. The clocking circuit switches the comparator from the acquisition mode to the latching mode and vice versa. The comparator eliminates the extraneous loading from the positive feedback when the regeneration takes place, so that a very fast regeneration time constant is obtained.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to comparator circuits. More particularly, it relates to a comparator having a latching circuit, in which the coupling of the input signal into the latch and/or the coupling of the output signal from the latch are designed to minimize the internal loading of the latch. The reduced loading allows this comparator circuit, when it is switched from acquisition (or tracking) mode to latching (or regenerative) mode, to resolve a very small input difference into a full logic level more rapidly than any previously known comparator circuit implemented with a given transistor technology.  
           [0003]    A latching comparator determines, at a particular instant, which of two voltages is larger, or equivalently, determines the sign of a voltage difference. The output of the comparator is generally a logic voltage level compatible with the inputs of some particular logic circuitry. If the voltage difference to be resolved is small compared to the logic voltage swing, some period of time is required after the comparator is clocked for the voltage to be amplified to a recognizable logic level. A small input voltage difference generally begins to grow at an exponential rate, characterized by a so-called “regeneration time constant”, when the comparator clock signal is switched. The smallest voltage difference that can be resolved by the comparator in a given period of time is limited by the value of this constant. This means that if an input difference is too small it will not produce a valid logic signal in the allotted time interval.  
           [0004]    2. Description of the Related Art  
           [0005]    The best regeneration time for prior art comparator circuits is achieved with a differential current-mode latch with the input and output coupled into taps in the collector load resistors of the latch. See, for instance, R. Van de Plassche,  Integrated Analog - to - Digital and Digital - to - Analog Converters,  Klever Academic Publishers, 1994, pp. 121-122. However, the input transistor collectors and output transistor bases still significantly load the latch.  
           [0006]    In the usual prior art implementation of a clocked latching comparator, a clock signal switches the comparator between an acquisition mode and a latching mode. In the acquisition mode the comparator has a relatively low gain and the output follows the signal input. When the comparator is clocked into the latching mode, positive feedback is enabled so that any arbitrarily small signal that is present will regenerate and drive the latch to its full output swing. When the signal is small, the rate of growth is proportional to the signal voltage present at any given time. This means that the regeneration is characteristically exponential in time, with a regeneration time constant T r . If the clock timing for the comparator allows some time T for regeneration, the input signal should be at least exp(−T/T r )·V sw  to be resolved, where V sw  is the value of a full logic swing. Smaller inputs will not be able to reach a full logic swing and may not be correctly resolved by the subsequent logic. This undesired condition is referred to as comparator (or latch) metastability.  
           [0007]    The regeneration time constant for a comparator (or, in general, any latch) is determined primarily by the gain in the positive feedback loop and by the loading of various capacitances in the circuit.  
           [0008]    [0008]FIG. 1 shows a simplified electrical circuit of a prior art differential current-mode latch. An example of this circuit can be for example found in U.S. Pat. No. 4,083,043 to D. R. Breuer and in D. R. Breuer, ‘High-speed A/D Converter Monolithic Techniques’, International Solid State Circuits Conference Proceedings, February 1972, pp. 146-147 and 228.  
           [0009]    The circuit of FIG. 1 comprises an input differential pair of transistors Q 1 -Q 2 , a latch differential pair of transistors Q 3 -Q 4 , a clock differential pair of transistors Q 5 -Q 6 , load resistors R 1  and R 2  and a current source I 1 . The clock differential pair Q 5 -Q 6  steers the current from the current source I 1  into either the input differential pair Q 1 -Q 2  when the clock input signal CLK is high, or into the latch differential pair Q 3 -Q 4  when the clock signal CLKX is high. CLK and CLKX are complementary signals; when one is low the other is high and vice versa.  
           [0010]    When the clock signal CLK is high, the input differential pair Q 1 -Q 2  operates as a transconductor which converts the input voltage difference at the bases of Q 1  and Q 2  to a difference in the collector currents of Q 1  and Q 2 . The collector currents flow into the high impedance of the load resistors R 1  and R 2  at the nodes A and B of FIG. 1 and create a voltage difference between these nodes which is an amplified replica of the input voltage difference at the bases of Q 1  and Q 2 .  
           [0011]    When the differential clock voltage is switched so that CLK goes low and CLKX goes high, the input pair Q 1 -Q 2  is deactivated and the latch pair Q 3 -Q 4  becomes active. The latch pair Q 3 -Q 4  and the load resistors R 1  and R 2  form a very high gain positive-feedback amplifier such that any voltage present between A and B will be amplified (or “regenerated”) until the latch is driven to a saturated output. The output of the comparator may be taken either directly from A and B as shown in FIG. 1 or through some sort of buffer amplifier that has its inputs connected to A and B.  
           [0012]    To use this circuit as a strobed comparator, the voltage difference to be compared is applied between the input signals IN and INX while the clock signal CLK is high. At the time the comparison is to be made, the clock signal CLKX is rapidly switched high so that the amplified input voltage differential serves as the starting point for the latch to regenerate to a saturated output level. The rate of regeneration depends inversely on the capacitive loading at the high impedance nodes A and B. In a conventional integrated circuit implementation of the comparator, this loading essentially consists of:  
           [0013]    a) collector-base and base-emitter capacitances of Q 3  and Q 4 ;  
           [0014]    b) parasitic collector-substrate capacitances of Q 3  and Q 4 ;  
           [0015]    c) substrate capacitance of the resistors R 1  and R 2 ;  
           [0016]    d) collector-base capacitance of Q 1  and Q 2 ;  
           [0017]    e) parasitic collector-substrate capacitances of Q 1  and Q 2 ; and  
           [0018]    f) the output load, which is usually at least the collector-base and base-emitter capacitances of emitter followers or of a differential pair.  
           [0019]    Items a, b, and c above are inherently part of the regenerative latch, whereas items d, e, and f are associated with the circuitry used to couple signals into and out of the latch.  
           [0020]    In order to use this circuit as a high speed latching comparator and to reduce the loading due to the above circumstances, various modifications have been made up to now. These modifications have included in particular the use of emitter followers in the latch, and a split collector load resistor. These modifications have provided a higher loop gain in the latch, a higher collector-base voltage for Q 3  and Q 4 , together with some degree of isolation of the capacitances of the input pair, Q 1  and Q 2 , and of the output load from the loop involving Q 3  and Q 4  when the regeneration takes place. However, also when taking into account all of these modifications, the value of the regeneration time constant has always been limited to some degree by the above loadings.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention solves the aforementioned prior art problems by providing a comparator, having a comparator input receiving a first input signal and a second input signal during an acquisition mode, and having a comparator output outputting a comparator output signal indicative of the largest between the first input signal and the second input signal, the comparator comprising: a cross-coupled regenerative latch for regenerating, during a latching mode, a signal which is indicative of a difference between the first input signal and the second input signal; a circuit connected to the cross-coupled regenerative latch, operating as a voltage follower during the acquisition mode and as a cascode amplifier stage during the latching mode; and a clocking circuit for switching the comparator from the acquisition mode to the latching mode and vice versa.  
           [0022]    According to a second object of the present invention, a comparator is provided, having a comparator input receiving a first input signal and a second input signal during an acquisition mode, and having a comparator output outputting a comparator output signal indicative of the largest between the first input signal and the second input signal, the comparator comprising:  
           [0023]    a cross-coupled regenerative latch for regenerating, during a latching mode, a signal which is indicative of a difference between the first input signal and the second input signal, said cross-coupled regenerative latch comprising:  
           [0024]    a first and a second resistor, each of said first and second resistor having a first end and a second end, and  
           [0025]    a first and a second bipolar transistor, wherein the collector of the first transistor and the base of the second transistor are connected to the first end of the first resistor,  
           [0026]    the collector of the second transistor and the base of the first transistor are connected to the first end of the second resistor, and the emitter of the first transistor is connected to the emitter of the second transistor.  
           [0027]    a circuit connected to the cross-coupled regenerative latch, operating as a voltage follower to couple the input during the latch during the acquisition mode and as a cascode amplifier stage to couple a latch state during the latching mode, said circuit comprising:  
           [0028]    a third and a fourth resistor, each of said third and fourth resistor having a first end and a second end, and  
           [0029]    a third and a fourth bipolar transistor, wherein the collector of the third transistor is connected to the second end of the third resistor and the collector of the fourth transistor is connected to the second end of the fourth resistor.  
           [0030]    a clocking circuit for switching the comparator from the acquisition mode to the latching mode and vice versa.  
           [0031]    According to a further object of the present invention, a comparator is provided, having a comparator input receiving a first input signal and a second input signal during an acquisition mode, and having a comparator output outputting a comparator output signal indicative of the largest between the first input signal and the second input signal, the comparator comprising:  
           [0032]    a cross-coupled regenerative latch for regenerating, during a latching mode, a signal which is indicative of a difference between the first input signal and the second input signal, said cross-coupled regenerative latch comprising:  
           [0033]    a first and a second resistor, each of said first and second resistor having a first end and a second end, and  
           [0034]    a first and a second field effect transistor, wherein the drain of the first transistor and the gate of the second transistor are connected to the first end of the first resistor, the drain of the second transistor and the gate of the first transistor are connected to the first end of the second resistor, and the source of the first transistor is connected to the source of the second transistor.  
           [0035]    a circuit connected to the cross-coupled regenerative latch, operating as a voltage follower to couple the input during the latch during the acquisition mode and as a cascode amplifier stage to couple a latch state during the latching mode, said circuit comprising:  
           [0036]    a third and a fourth resistor, each of said third and fourth resistor having a first end and a second end, and  
           [0037]    a third and a fourth field effect transistor, wherein the drain of the third transistor is connected to the second end of the third resistor and the drain of the fourth transistor is connected to the second end of the fourth resistor.  
           [0038]    a clocking circuit for switching the comparator from the acquisition mode to the latching mode and vice versa.  
           [0039]    According to a preferred embodiment of the present invention, the cross-coupled regenerative latch comprises a first and a second resistor, each of the first and second resistor having a first end and a second end, and a first pair of three-terminal devices connected therebetween and to the first end of the first and second resistors.  
           [0040]    Moreover, according to this preferred embodiment, the circuit comprises a second pair of three-terminal devices (for example bipolar transistors or field effect transistors) connected to the second end of the load resistors, the second pair of three-terminal devices operating as voltage followers to couple the first input signal and the second input signal into the second end of the load resistors and operating as cascode amplifier stages to couple the currents in the load resistors to the comparator output.  
           [0041]    In particular, the comparator circuit according to the present invention eliminates all extraneous loading from the positive feedback when the regeneration takes place, so that a faster regeneration rate (shorter regeneration time constant) can be achieved. More particularly, all the extraneous loading associated with the input circuitry or the output circuitry is eliminated.  
           [0042]    The shorter regeneration time constant of the comparator circuit according to the present invention allows the comparator circuit to resolve a smaller voltage difference in a given time, or resolve a given voltage difference more quickly or to reduce the probability that the comparator output will not be a valid logic signal to properly drive subsequent circuitry. In particular, the regeneration time constant is about the half of the regeneration time constant of the best prior art comparators.  
           [0043]    The improved regeneration time constant of the disclosed comparator circuit will be the enabling technology for improved resolution or lowered noise floor in several types of high speed analog to digital converters. Also, particularly for parallel architectures that use large numbers of comparators, the comparator according to the present invention will allow a given speed and resolution to be achieved with lower power dissipation.  
           [0044]    The present invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]    [0045]FIG. 1, already discussed in detail, shows a simplified electrical circuit of a prior art differential current-mode latch.  
         [0046]    [0046]FIG. 2 is a circuit diagram showing the comparator circuit according to a first embodiment of the present invention.  
         [0047]    FIGS.  3 - 6  are circuit diagrams showing the comparator circuit according to further embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]    [0048]FIG. 2 is a circuit diagram showing the comparator circuit according to the present invention, in its most basic form.  
         [0049]    The basic regenerative loop, or latching circuit, comprises transistors Q 3 , Q 4  and resistors R 1  and R 2 . The input signals IN and INX are input into the bases of the transistors Q 7  and Q 8 , respectively. The output signals OUT and OUTX are taken from the collectors of Q 7  and Q 8 , respectively. The input voltage difference at the bases of Q 7  and Q 8  is coupled into resistors R 1  and R 2  to become the initial input voltage of the latching circuit comprising transistors Q 3  and Q 4  when the clock inputs CLK and CLKX are switched to steer current from the current source I 1  into the latch and cause the positive feedback to become active.  
         [0050]    In the circuit of FIG. 2, the rate of regeneration when the positive feedback is enabled is determined by the rate at which the voltage difference between the collector terminals of Q 3  and Q 4  can change. This rate in turn is determined by the current available to drive the collector terminals of Q 3  and Q 4  divided by the total capacitance at the collector terminals of Q 3  and Q 4 .  
         [0051]    The available current, determined by the voltage difference present across the base terminals of the differential pair Q 3 -Q 4 , is the same for both the comparator according to the present invention and an equivalent (same integrated circuit technology, and operating at the same bias current) prior art comparator like the one shown in FIG. 1. However, in the comparator according to the present invention as shown in FIG. 2, the collector of Q 3  is only connected to the base of Q 4  and to resistor R 1 . Similarly, the collector of Q 4  is only connected to the base of Q 3  and to resistor R 2 . As already pointed out with reference to the loading of the transistors Q 3  and Q 4  of FIG. 1, each element connected to the collector of Q 3  or Q 4  contributes to the total parasitic capacitance at that collector and is thus part of the limitation on the rate of regeneration. From a comparison between the connections of the collectors of Q 3  and Q 4  in FIG. 2, it follows that the rate of regeneration in the prior art comparator is inherently smaller than the rate of regeneration in the comparator according to the present invention. More particularly, the loading of the latch transistor pair according to the present invention consists only of:  
         [0052]    a) collector-base and base-emitter capacitances of Q 3  and Q 4 ;  
         [0053]    b) parasitic collector-substrate capacitances of Q 3  and Q 4 ; and  
         [0054]    c) substrate capacitance of the resistors R 1  and R 2 .  
         [0055]    The comparator according to the present invention achieves its advantage because transistors Q 7  and Q 8  in FIG. 2 act as voltage followers (high input impedance, low output impedance, and near-unity gain) to couple the input voltage difference into the latch through the low impedance ends C and D of the load resistors R 1  and R 2 , without loading the high impedance nodes A and B, while at the same time acting as cascodes (low input impedance, high output impedance, and near-unity current gain) to pass the current in the load resistors R 1  and R 2  through the output with substantially unity current gain, again without loading the high impedance nodes A and B of the latch. If the output current is coupled into load resistors, shown as R 3  and R 4  in FIG. 2, the output is then converted to a voltage. Alternatively, the output current from the collectors of Q 3  and Q 4  could be used directly to drive a current-operated circuit such as a current mirror.  
         [0056]    [0056]FIG. 3 shows a preferred embodiment of the present invention, where further refinements are added to the basic circuit of FIG. 2. In the circuit of FIG. 3, R 1 =R 2 , R 3 =R 4 , R 5 =R 6 , R 7 =R 8  and I 4 =I 5 . The circuit of FIG. 3 is likely to be preferred in an application where the shortest possible regeneration time is required and minimum power dissipation is not critical. On the contrary, in a parallel, or flash, analog to digital converter that uses a very large number of comparators so that the size and power consumption of the comparator are important, the simpler comparator of FIG. 2 may be preferred if it provides an adequately short regeneration time.  
         [0057]    In FIG. 3, the difference between the input signals IN and INX is buffered through a differential pair Q 1  and Q 2 , such that the input is isolated from the latch when the circuit is clocked into latching mode. Moreover, the latch Q 3 -Q 4  includes emitter followers Q 13  and Q 14  to allow a larger voltage swing and thus a higher regeneration gain, and also so that the current gain of the emitter followers will reduce the loading of the collectors of Q 3  and Q 4  from the bases of Q 4  and Q 3 , respectively.  
         [0058]    The current that is clocked into the latch is split into two parts, I 2  and I 3 . During acquisition mode, I 2  is steered into the input pair Q 1  and Q 2 , while I 3  is steered through the resistors R 7  and R 8  to provide a bias current for transistors Q 7  and Q 8 . The junction of R 7  and R 8  is a virtual ground for the differential signal, so that the collector capacitance of Q 11  does not load the latch.  
         [0059]    The voltage difference between CLK and CLKX is the ‘clock voltage’. Similarly to what explained with reference to FIGS. 1 and 2, the clock voltage controls whether the comparator is sensing the voltage difference at the inputs IN and INX (acquisition mode) or has the positive feedback enabled so that it regenerates to a full logic swing at the outputs (latching mode).  
         [0060]    When the clock voltage is high (positive) the comparator is in acquisition mode. This means that the current from the current source I 2  is steered through transistor Q 9  into the input pair Q 1 -Q 2 . The current from the current source I 3  is steered through the transistor Q 11  and split by the resistors R 7 +R 1  and R 8 +R 2  into the emitters of Q 7  and Q 8 . In this acquisition mode, the input voltage between IN and INX is amplified by the differential amplifier Q 1 -Q 2 +R 5 -R 6  and the amplified signal is applied to the bases of Q 7  and Q 8 . The amplified signal voltage difference at the bases of Q 7  and Q 8  is approximately replicated at the emitters of these transistors. This amplified signal then appears at the collectors of the latch transistors Q 3  and Q 4  after being attenuated by a factor of R 7 /(R 1 +R 7 ). In the acquisition mode the gain from the bases of Q 7  and Q 8  to the output is approximately −R 3 /(R 1 +R 7 ).  
         [0061]    In regeneration mode, when the clock voltage is low, the transistors Q 3  and Q 4 , together with the load impedance formed by the combination of resistors R 1 , R 2 , R 7 , R 8 , and the emitter impedances of Q 7  and Q 8 , form a differential amplifier. The output of the amplifier at the collectors of Q 3  and Q 4  is connected in positive feedback to the inputs at the bases of Q 4  and Q 3  respectively through the emitter followers Q 13  and Q 14  to form a regenerative latching circuit.  
         [0062]    When the clock voltage is switched low the currents from the current sources I 2  and I 3  are steered through transistors Q 10  and Q 12  into the node that is common to the emitters of Q 3  and Q 4 . This causes the regenerative circuit containing the transistors Q 3 , Q 4 , Q 13  and Q 14  to become active so that the voltage difference between the collectors of Q 3  and Q 4  will be regeneratively amplified to the point that nearly all of the current flows through one of the transistors Q 3  or Q 4  and into the load, where it is coupled through Q 7  and Q 8  into R 3  and R 4  to generate the output voltage. As seen by the output current, transistors Q 7  and Q 8  act as cascodes. The cross-coupling in the load provided by resistors R 7  and R 8  ensures that both transistors Q 7  and Q 8  are biased on, even when the collector current of one of transistors Q 3  or Q 4  is nearly zero. This limits the voltage step at the emitters of Q 3  and Q 4  when the comparator is switched to acquisition mode and thus shortens the time required for the input to settle.  
         [0063]    It is worth noting that resistors R 7  and R 8  could alternatively be connected to the top of resistors R 1  and R 2  rather than the bottom, or alternatively, rather than being tied only to the top or bottom of R 1  and R 2 , R 7  and R 8  could be tied to taps somewhere in the middle of each of R 1  and R 2  without substantially altering the function of the circuit.  
         [0064]    The rate of regeneration is limited by the parasitic capacitive loading seen by the differential signal between the collectors of Q 3  and Q 4 . The parasitic capacitance of the inventive circuit of FIG. 3 consists of:  
         [0065]    a) the base-collector and base-emitter capacitances of the emitter followers Q 13  and Q 14 ;  
         [0066]    b) the collector-base and collector-substrate capacitances of Q 3  and Q 4 ;  
         [0067]    c) the parasitic capacitances of the resistors R 1 , R 2 , R 7 , and R 8 , and  
         [0068]    d) any parasitic capacitance of the interconnections between these components.  
         [0069]    Prior art clocked regenerative comparators have all of these same parasitic capacitances (except for the capacitance of R 7  and R 8  which is usually very small compared to transistor capacitances) and have in addition the capacitances of the transistors used to couple the signal into the regenerative amplifier, and the capacitance of a load connected to the output of the amplifier. During the process of regeneration the voltage at the junction of R 7 , R 8 , and the collector of Q 11  does not change in response to changes in the differential voltage between the collectors of Q 3  and Q 4 . It is a virtual ground for the differential signal. As such, the capacitance of the collector of Q 11  does not load the differential signal during regeneration.  
         [0070]    The circuit shown in FIG. 3 has been simulated with models for an InP HBT process This process provides npn transistors having an InP substrate, AlInAs emitters and GaInAs bases and collectors. The F t  frequency of these transistors is about 80 GHz and the F max  frequency is about 130 GHz. The minimum emitter size is 2 microns square, and the minimum metal pitch is 6 microns. This process normally has two levels of interconnect metal, and tantalum nitride thin film resistors. The regeneration time constant (i.e. the growing rate of the input voltage difference) obtained is of about 4.7 psec which is two times faster than a regeneration time constant of a conventional latch with split collector load resistors.  
         [0071]    According to the embodiment of FIG. 3, the input signal is isolated from the bases of Q 7  and Q 8  when the clock is low, so that a change in the input voltage will not disturb the state of the latch. This results in the base currents of Q 1  and Q 2  being switched on and off at the clock rate, which may be undesirable for some applications.  
         [0072]    One alternative method of isolating the latch from the inputs without introducing an additional buffer stage is that of clocking current into clamping diodes wired between the collectors of transistors Q 1  and Q 2 , such that the differential voltage swing at the collectors of Q 1  and Q 2  is limited to a value substantially smaller than the latch voltage swing when the clock is low. The tail current into the emitters of Q 1  and Q 2  can then be kept constant, so that very little of the clock signal is coupled back through the bases of transistors Q 1  and Q 2 .  
         [0073]    A circuit implementing this alternative method is shown in FIG. 4. In this embodiment, transistors Q 1  and Q 2  are biased with a constant current from current source I 6 . A pair of diode-connected transistors Q 15  and Q 16  are connected across the base leads of transistors Q 7  and Q 8 . During the acquisition phase, when the signal CLK is high, no current flows in the transistors Q 15  and Q 16  so that they have a high impedance and have practically no effect on the circuit operation. When the signal CLK goes low and the comparator latches the current from the current source I 7 , the current is steered through transistor Q 10  into transistors Q 15  and Q 16 . In this way transistors Q 15  and Q 16  present a low impedance, shunting the resistors R 1  and R 2 . This will prevent any large signal voltage from reaching the bases of Q 7  and Q 8  so that a change in the voltage between the signals IN and INX will not affect the state of the latch. With this form of clocking there will be essentially no modulation of the base currents of Q 1  and Q 2  by the clock.  
         [0074]    [0074]FIG. 5 shows a further enhancement to the embodiment of FIG. 4. In FIG. 5, cascode transistors Q 17  and Q 18  are added to the input amplifier Q 1 -Q 2 −R 5 -R 6 .  
         [0075]    Transistors Q 17 -Q 18  will provide isolation between the bottoms of R 5  and R 6  and the collector-base capacitances of transistors Q 1  and Q 2  so that the voltage swings at these resistors as the comparator is clocked are not coupled back through the bases into the circuitry that drives the comparator. Coupling of a signal voltage back through the collector-base capacitance of the input transistors of a strobed comparator as the comparator is clocked is known as “kickback”, and is known to be a cause of problems in some circuit applications.  
         [0076]    [0076]FIG. 6 shows an alternative embodiment to FIG. 5. The circuit in FIG. 6 performs the same function as the circuit in FIG. 5, except it is constructed using FET&#39;s. In both FIG. 5 and FIG. 6, when the comparator is switched to regeneration mode, the circuitry connected between the bases of Q 7  and Q 8  is switched to a low impedance state. This limits the voltage between the bases of Q 7  and Q 8  to less than the value required to change the state of the regenerative latch. In the version of circuitry shown in FIG. 5 the low impedance path is through the diode-connected transistors Q 15  and Q 16  when the current from I 7  is steered through Q 10  into those transistors. When FET&#39;s are used as in FIG. 6 it is more appropriate to use a single transistor Q 19  connected as shown, so as to under the control of the clock signal, create the desired low impedance to effectively isolate the regenerative latch, i.e. limit the voltage between the bases of Q 7  and Q 8 . A signal LSCLK which controls the operation of transistor Q 19  may in some instances be identical to CLK, or depending on the threshold voltages and/or breakdown voltages of the transistor Q 19  available in the particular process used to fabricate the circuit, might differ from CLK by a constant voltage difference.  
         [0077]    The embodiments according to the present invention have been illustrated in terms of npn bipolar transistors and field effect transistors. It will be obvious to the person skilled in the art that equivalent circuits can be implemented with pnp transistors, either polarity of field effect transistors, or a mixture of these device types. Moreover, Q 1  and Q 2  of FIG. 2, or Q 7  and Q 8  of FIGS. 3, 4,  5 , or  6  could be any type of three-terminal device for which a voltage between terminals  1  and  2  controls a current between terminals  2  and  3 . Still moreover, the circuit block consisting of R 1 , R 2 , Q 3  and Q 4  is a three terminal block with the property that a current into one terminal creates a negative resistance between the other two terminals. Although in a preferred embodiment the output is taken at the collectors of transistors Q 1  and Q 2  (FIG. 2), Q 7  and Q 8  (FIGS.  3 - 5 ), and the drains of transistors Q 7  and Q 8  (FIG. 6) it is also possible to take the output at the collectors of transistors Q 3  and Q 4 , the emitters of Q 13  and Q 14 , the drains of Q 3  and Q 4 , and the sources of Q 13  and Q 14 , although either of these output connections will add some loading to the high impedance nodes of the regenerative latch and partly negate the advantage of this circuit.  
         [0078]    The circuit according to the present invention can be used also in asynchronous logic, due to its potential of greatly reducing the probability of metastability in a latch. At the interface between asynchronous logic and conventional clocked logic it is generally impossible to guarantee that an asynchronous logic transition will not occur just at the clock transition such as to leave a latch in a metastable state. If sufficient time is left before the latch must be read out, the probability of its remaining in a metastable state can become very small. The time required to reach a given probability is a multiple of the regeneration time constant of the latch. If the comparator circuit according to the present invention is used as a latch, it will require a shorter time to reach a given probability of no error than would be required for any previously known latch. For a reference on metastability see Veendrick, H. J. M., “The Behavior of Flip-Flops Used as Synchronizers and prediction of Their Failure Rates”, IEEE J. Solid-State Circuits, vol. SC-15, No. 2, April 1980.  
         [0079]    Finally, it is to be understood that the comparator circuit according to the present invention also can take into account usual tradeoffs that can be made in the detailed design of the new comparator in order to optimize it for a particular application implemented in any given integrated circuit technology. For example, these tradeoffs may be related to optimizing the circuit performance with respect to device characteristics peculiar to a particular integrated circuit technology, trading power consumption against circuit speed, trading circuit performance against the area occupied by the circuit, etc. One example would be to increase the gain of the positive feedback loop by increasing the voltage drop across the load resistors, which then necessitates the use of emitter follower level shifters with a concomitant increase in power dissipation and in area occupied by the circuit. In any case, for a given set of design tradeoffs and in a given integrated circuit technology, the new comparator circuit will always have a shorter regeneration time than a prior art comparator.  
         [0080]    While several illustrative embodiments of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.