Abstract:
A comparator having a differential amplifier with a signal input to receive an input signal and a reference input to receive a reference voltage, and having a controllable bias current source for supplying to the differential amplifier a bias current that has a low quiescent current or a higher active current as a function of whether the input signal is constant or variable. The controllable bias current source is further configured to adjust the bias current in accordance with the rate of change of the input signal.

Description:
TECHNICAL FIELD 
     This invention relates to a comparator having a differential amplifier, a signal input to which an input signal is applied, a reference input to which a reference voltage is applied, and a controllable bias for adjustment of a predetermined operating point of the differential amplifier. 
     BACKGROUND OF THE INVENTION 
     Examples of comparators are shown in U.S. Pat. Nos. 4,384,219 and 4,529,891. In both cases a differential amplifier stage with two transistors is connected on the high potential side via a constant current source with a supply voltage source and on the low potential side via a current mirror with a ground lead. 
     The speed of a comparator is known to be higher the greater the bias current. That is, the speed at which an effect occurs at the comparator output when the input signal thereof passes through the comparator threshold depends on the strength of the bias current. From the point of view of the speed of the comparator it would therefore be desirable to supply a bias current with high current strength. 
     A high bias current leads to accordingly high power consumption, however, which is undesirable, especially when power is supplied from a battery. Conventionally, one must therefore make a compromise between a tolerable power consumption and a tolerable speed of the comparator. The invention addresses the problem of providing a comparator which is not subject to this compromise but permits both high speed and low power consumption. 
     SUMMARY OF THE INVENTION 
     The embodiments disclosed herein provide a comparator that has a controllable bias current source for supplying a bias current having a low quiescent current strength or a higher active current strength as a function of whether the input signal is constant or variable at the moment. 
     In very many applications, in particular in the digital area, speed of the comparator is only required during times when the input signal varies, while in the times therebetween when the input signal remains constant the comparator can be indefinitely slow. During the times of a constant input signal a very low bias current is thus required and supplied according to the invention. During times of a varying input signal when speed of the comparator is important, the bias current is increased in order to ensure the speed of the comparator then desired. In the disclosed embodiments, a high bias current only flows during times of a varying input signal, while a very low bias current flows during the remaining times of a constant input signal, which are generally greatly predominant. Thus, the invention achieves a relatively low mean power consumption while nevertheless ensuring a high speed of the comparator at the crucial times. 
     In one embodiment of the invention, the controllable bias current source contains two current sources whose sum current is supplied to the differential amplifier as the bias current. One of said current sources is a constant current source which supplies a very low quiescent current, preferably in the range of a few microamperes. The other current source is an additional current source which supplies an additional current only during those times when the input signal varies. 
     Considering as an input signal, for example, a digital signal which changes between a constant low potential and a constant high potential, the transitions between low potential and high potential being formed by leading or trailing edges with a certain edge slope, the additional current source supplies an additional bias current only during the times of leading and trailing edges, while the additional bias current is zero between the edges, ie., during the time segments of constant low or constant high potential, and only the quiescent current from the constant current source is supplied as the bias current during such times. During the signal edges the comparator is fast by reason of the additional bias current, while it is slow, and only needs to be slow, during the time segments of constant potential. 
     In another disclosed embodiment, the level of the additional current is made dependent on the rate of the input signal variation during times when the input signal varies. This causes the comparator to be faster the steeper the input signal variation is. The speed of the comparator is thus adapted to the need for speed. If at very high speed in the comparator is required upon very fast or very steep input signal variation, it is given the necessary high bias current. However, if the comparator only needs to process less fast or less steep input signal variations, it is supplied a lower additional bias current, which keeps the mean power consumption especially low. 
     In accordance with one disclosed embodiment, an additional current source with a controllable current source and a capacitor connected between the signal input and a control input of the controllable current source is provided. The input signal is supplied to the capacitor. Designating the capacitor C, the capacitor current Ic, the input signal U and the time t, one obtains a capacitor current 
     
       
           Ic=dU/dt.   (1) 
       
     
     Ic is nonzero only when voltage U varies so that dU/dt is nonzero. 
     In an embodiment of a comparator with a capacitor in the additional current source, it is recommendable to use two current sources, each with a capacitor, for example, a first current source and first capacitor supplying an additional current only during positively directed input signal variations, and a second current source and second capacitor supplying additional current only during negatively directed input signal variations. If one and the same capacitor were used both for positively and for negatively directed input signal variations, it would draw current from the input signal source during input signal variations in one direction in the desired fashion but would have to draw a current from the quiescent current source upon input signal variations in the other direction. 
     In an embodiment of the invention with additional current source constructed with one or two capacitors, the, or each, controllable current source has a current mirror that is given capacitor current Ic in an input branch and mirrors it to the biasing current terminal of the differential amplifier. 
     The differential amplifier can be constructed with further current mirrors in a way known in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of an embodiment of an inventive comparator; 
     FIG. 2 shows an example of an input signal shape; and 
     FIG. 3 shows an example of a capacitor current of the circuit shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a circuit diagram of an embodiment of an inventive comparator with a bias current source comprising a quiescent current source and an additional current source. 
     In a way known in the art, the comparator shown in FIG. 1 has a differential amplifier stage with two transistors T 1  and T 2 . The gate terminal of said transistor T 2  is connected with signal input Si of the comparator, which is supplied with an input signal U. A detail of an example of such an input signal U is shown in FIG.  2 . The gate terminal of transistor T 2  is connected with reference voltage source REF, which supplies reference voltage Vref to the gate terminal of T 2 . 
     All lines in FIG. 1 which end in a circle are connected with high potential side Vs, and all lines in FIG. 1 which end in a crossline are connected with low potential side GND of a supply voltage source. 
     In the way likewise known in the art, transistors T 1  and T 2  of the differential amplifier stage are located between high potential side current mirror Sp 5  and low potential side current mirror Sp 4 . Differential amplifier stage T 1 , T 2  and current mirrors Sp 4 , Sp 5  form a differential amplifier. The latter has biasing current terminal S which simultaneously forms a current summing point of the circuit. Connected to S is quiescent current source Q 0  which supplies quiescent current I 0  constituting a quiescent bias current to bias current terminal S. 
     Connected to bias current terminal S is furthermore an additional current source which, according to the invention, feeds an additional current into biasing current terminal S only during time segments when input signal U varies. The additional current source comprises two additional current stages. A first additional current stage comprises first capacitor C 1 , first current mirror Sp 1 , and second current mirror Sp 2 . The second additional current stage comprises second capacitor C 2  and third current mirror Sp 3 . Current mirrors Sp 1  and Sp 3  each form a controllable current source. The control of said current sources is effected with capacitor current Ic 1  of first capacitor C 1  or with capacitor current Ic 2  of second capacitor C 2 . 
     All current mirrors Sp 1  to Sp 5  are constructed in known fashion each with a diode branch in the form of a transistor wired as a diode and a transistor branch, in a known way as shown in FIG. 1, so that no detailed discussion of the structure and effect of such current mirrors is necessary here. In the way likewise known in the art, low potential side current mirror Sp 4  of the differential amplifier is constructed with, three current mirror branches. A connecting point between transistor T 2  and current: mirror Sp 4  forms output terminal So of the comparator. 
     First additional current stage C 1 , Sp 1 , Sp 2  serves to feed an additional current into bias current terminal S whenever input signal U is varying in the positive direction or in the direction of increasing potential. Second additional current source C 2 , Sp 3  serves to supply an additional current into bias current terminal S whenever input signal U is varying in the negative direction or in the direction of decreasing potential. Since in both cases the additional current is to be drawn from the signal source providing input signal U and to be supplied into bias current terminal S, the first additional current stage has two series-connected current mirrors Sp 1  and Sp 2 , Sp 1  being on the low potential side of the supply voltage source and Sp 2  on the high potential side thereof. 
     The mode of operation of the comparator circuit shown in FIG. 1 will now be explained, assuming input signal U with the signal pattern shown by way of example in FIG.  2 . 
     FIG. 2 shows input signal U having constant low and high potential patterns in time segments t 0 , t 2  and t 4  and having a leading edge and a trailing edge in time segments t 1  and t 3 , respectively. 
     The two capacitors C 1  and C 2  lead to current 
      Ic 1 =C 1 · dU/dt   (2) 
     
       
         Ic 2 =C 2 · dU/dt.   (3) 
       
     
     During time segments t 0 , t 2  and t 4 , dU/dt=0, while during time segments t 1  and t 3  dU/dt assumes a positive or negative value. This is shown in FIG. 3 for capacitor current Ic 1  of first capacitor C 1 . 
     During time segments t 0 , t 2  and t 4 , which can be termed quiescent. segments, capacitor currents Ic 1  and Ic 2  are thus both equal to zero 
     Current mirror Sp 1  is rendered conductive only by positive capacitor current Ic 1  while it blocks negative capacitor current Ic 1 . Conversely, current mirror Sp 3  is rendered conductive only by negative capacitor current Ic 2  while it block, positive capacitor current Ic 2 . 
     During time segments t 0  to t 4  the following bias current feed into bias, current terminal S thus takes place. 
     During time segments t 0 , t 2  and t 4  only quiescent current I 0  from constant current source Q 0  is fed in. During time segment t 1 , previously charged first capacitor C 1  supplies a current corresponding to Ic 1  to bias current terminal S via current mirrors Sp 1  and Sp 2 . During time segment t 3 , previously charged second capacitor C 2  supplies a current corresponds to Ic 2  to bias current terminal S via current mirror Sp 3 . 
     Put in general terms, the inventive bias current source thus works in such a way that the differential amplifier is supplied via bias current terminal S a bias current which has low quiescent current strength I 0  or a higher active current strength of about I 0 +Ic 1  or I 0 +Ic 2  as a function of whether input signal U is constant or variable at the moment. 
     If signal input Si of the comparator is for example a connection to a bus and input signal U is a bus signal that is to be evaluated by means of the comparator according to FIG. 1, the time fractions when edges occur are relatively small on the average, in particular for bus systems having relatively long quiescent times between active times. The invention has realized the goal of achieving very low power consumption of the comparator during such quiescent times, which constitute the greatest time fraction in practical cases. It is virtually irrelevant that higher power consumption occurs during the active bus phases, i.e., at slope times of input signal U, in view of the small mean time fraction of such slopes. The invention thus makes it possible to keep mean power consumption of the comparator very low by reducing quiescent bias current  10  to a few microamperes for example, while nevertheless making the comparator fast during the crucial time segments of input signal edges. 
     The embodiment of an inventive comparator with capacitors C 1  and C 2  as shown in FIG. 1 has the additional advantage that capacitor current Ic 1  or Ic 2  is higher the steeper the particular edge of input signal U is. That is, the steeper the edge the faster the comparator becomes. This is because with increasing edge steepness, bias, current terminal S is supplied accordingly increasing additional current from the additional current source. 
     In the embodiment of the comparator shown in FIG. 1, all transistor, are formed as MOS transistors. The type of channel which the individual transistors have depends on the polarity of the supply voltage terminals. However, it is also possible to construct a corresponding circuit completely or at least partly with bipolar transistors.