Abstract:
A comparator circuit having improved operational characteristics. A predetermined voltage drop device is provided, such as an exemplary embodiment Schottky diode, having an anode connected to circuit power supply voltage and an output stage of the comparator and a cathode connected to an input stage of the comparator. The predetermined voltage drop device effects a lowering of the power supply voltage for the output stage bias between said power supply voltage and said common voltage. This reduces the required swing of the output stage drivers during a comparator input signal transition and reduces propagation delay of said comparator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO AN APPENDIX 
   Not applicable. 
   BACKGROUND 
   1. Technical Field 
   The technology described herein is generally related to the field of electronic devices and more particularly to comparator circuits. 
   2. Description of Related Art 
   Data transmission speed is a crucial operational characteristic for computer and telecommunications implementations. One problem associated with comparator circuits commonly used in these types of equipment is signal propagation delay. Some prior art solutions are found in U.S. Pat. No. 4,401,901 (Ochi), U.S. Pat. No. 6,121,798 (McQuilkin) and U.S. Pat. No. 6,252,437 (Fischer et al.). 
   As computing and telecommunicating systems are continually challenged to provide increased signal processing speeds with lower power consumption, there is a need for improved circuitry. 
   While the exemplary embodiments described herein is illustrative of using semiconductor devices having a specific type, e.g., bipolar, CMOS, BiCMOS and the like elements, and specific transistor polarity implementations, it will be recognized by those skilled in the art that other implementations can be made within the scope of the invention. No limitation on the scope of the invention is intended by the exemplary embodiments and none should be implied therefrom. 
   BRIEF SUMMARY 
   The present invention relates to electrical circuitry, particularly integrated circuits. Generally, the present invention comprises a method for reducing propagation delay in a comparator circuit and for a comparator circuit itself having a reduced propagation delay characteristic. 
   The foregoing summary is not intended to be inclusive of all aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Brief Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  (Prior Art) is a circuit schematic diagram depicting a typical, exemplary, two-stage comparator. 
       FIG. 2  is a circuit schematic diagram in accordance with an exemplary embodiment of the present invention. 
       FIG. 3  is a waveform diagram illustrating the reduction of propagation delay by comparing performance of the circuit of  FIG. 1  with the circuit of  FIG. 2 . 
       FIG. 4  is a schematic block diagram in accordance with the exemplary embodiment as shown in  FIG. 2 . 
   

   Like reference designations represent like features throughout the drawings. The drawings in this specification should be understood as not being drawn to scale unless specifically annotated as such. 
   DETAILED DESCRIPTION 
     FIG. 1  (Prior Art) is a circuit diagram depicting a typical, exemplary, common mode, two-input (referred to hereinafter as “inPlus” and “inMinus”), two-stage comparator circuit  101 . The circuit includes a known manner substantially constant power supply voltage, shown as “Vcc,” and a common reference, or ground, shown as “Vee”. The comparator  101  has two inputs  104  “IN−” referred to herein more simply as the “inMinus” side, and  106  “IN+” referred to herein more simply as the “inPlus” side. Generally, when the inPlus side signal is greater than the inMinus side signal, the output, Vout, will be a digital HIGH; conversely, when the inPlus side is less than the inMinus side, Vout will be LOW. 
   The first stage  103  is composed of a differential pair of transistors Q 1 , Q 2 , a current source iBias, a PMOS current source M 2 , and a tail current source M 4 . This is a known manner “I/ 2 I” differential stage with the tail current via M 4  equal to twice the drain current of M 2 . A second stage  105  is composed of a transistor Q 3  and a current source M 5 . The differential pair Q 1 , Q 2  is balanced when the input signals are equal, inPlus=inMinus, as half of the tail current via M 4  will flow through each of the differential pair transistors Q 1 , Q 2 . In this exemplary circuit, because transistor M 2  is a MOSFET type (metal-oxide semiconductor field-effect transistor), the base of transistor Q 3  is forced to Vcc in an OFF state when inMinus is greater than inPlus. In order for there to be a switch in the output, e.g., from a LOW signal to a HIGH signal, the base of Q 3  must be pulled down to a level equal to Vcc−Vbe such that transistor Q 3  is turned ON. Drain gate connected MOSFETs M 1  and M 3  are connected form a known manner current reference for two current mirrors, where M 1  is a reference for the current source from Vcc. M 3  is a reference for a current sink to ground. 
   A propagation delay for a LOW-to-HIGH Vout output transition is determined, for the most part, by the time it takes to move the base of transistor Q 3  from the Vcc level to approximately the Vcc−Vbe level. 
     FIG. 2  is a circuit schematic diagram in accordance with an exemplary embodiment of the present invention. Circuit  201  is a two-stage  203  (hereinafter also referred to as “input stage  203 ”),  205  (hereinafter also referred to as “output stage  205 ”) comparator circuit with circuit components substantially identical to  FIG. 1 . Circuit  201  has differential inputs  204 , inMinus, and  206 , inPlus. 
   A Schottky diode  207  is provided, having and anode  209  and a cathode  211 . The anode  209  is connected to the power supply voltage Vcc and to the Vcc input side of the output stage  205 . The cathode  211  is connected to the Vcc input side of the input stage  203   
   Turning also to  FIG. 3 , the waveforms of graph  301  shows a first exemplary voltage, “inPlus,” having a constant level of 1.00V. This inPlus signal is on the plus (IN+) input  206  of comparator circuit  201 . Graph  301  shows an exemplary voltage, “inMinus,” having a changing level—going from a positive 1.10V at time t=0s to a positive 0.99V at approximately t=10 ns. This changing inMinus signal is on the comparator minus (IN−) side input  204 . 
   In operation, the addition of the Schottky diode  207  inserted between Vcc and the source of M 2  causes propagation delay to decrease. The voltage at the base of transistor Q 3  in the OFF-state is now equal to:
 
Vcc−V SCHOTTKY, 
 
rather than just Vcc. This means that switching the output from LOW to HIGH only requires that the base of transistor Q 3  move by:
 
Vbe−V SCHOTTKY, 
 
rather than a full Vbe transition fro Vcc. Because V SCHOTTKY  is smaller than Vbe, transistor Q 3  will still be properly turned OFF whenever:
 
inMinus&gt;inPlus.
 
The current through the Schottky diode  207  stays constant because the tail current of M 4  always flows through the Schottky diode no matter how it is distributed between transistor Q 1  and transistor Q 2 . The source of MOSFET M 2  effectively is acting as a secondary Vcc, also now equal to:
 
Vcc−V SCHOTTKY ,
 
rather than the actual Vcc. Connecting the emitter of transistor Q 3  to Vcc allows the circuitry to take advantage of the reduced swing amount, thus reducing propagation delay. The reduced swing amount improves the delay characteristic as illustrated by example in  FIG. 3 .
 
   Looking now to graph  302 , in accordance with a circuit  101  of the prior art, having given circuit components, a propagation delay for Vout may be expected to delay to a time of approximately t=120 ns; that is, the comparator circuit  101  propagation delay characteristic could be defined as approximately 110 ns (120 ns−10 ns). This is illustrated by the line marked “Prior Art.” In accordance with the present invention, for the circuit  201 , having the same given circuit components as circuit  101 , the line marked “New Comparator” illustrates the improvement afforded by the addition of the Schottky diode  207 . Now for the comparator circuit  201  has Vout occur at an approximate time of t=85 ns. Therefore the comparator circuit  201  propagation delay characteristic may be defined as approximately 75 ns (85 ns−10 ns), an improvement of approximately 30%. 
   In general, the inputs  104 ,  106  are referred to as a differential pair; the comparator  101  input stage  103  amplifies the difference between the input signals, IN−, IN+, to the output stage, namely, the difference between the voltage on the base of Q 1  and the voltage on base of Q 2 , switching the output between HIGH and LOW accordingly. 
   Notice that generally in such a prior art circuit  101 ,  FIG. 1 , for the output Vout to switch levels LOW to HIGH, the transistor Q 3  base-emitter voltage, Vbe, must swing from zero volts to a full Vbe value. In accordance with the exemplary embodiment present invention as shown in  FIG. 2 , the propagation delay  11  is reduced because the swing has been reduced by the amount of a predetermined voltage drop across the Schottky diode  207 . In other words, the swing is related to the power supply voltage, Vcc, less the Schottky voltage (Vcc−V SCHOTTKY ) which biases the output transistor Q 3  via MOSFET M 2  such that the swing goes from Vcc−V SCHOTTKY  volts to Vbe. 
   A more detailed comparison between the prior art in  FIG. 1  and the exemplary embodiment of  FIG. 2  illustrates the method of reducing propagation delay time. In  FIG. 1  (Prior Art), the voltage swing required to flip the output stage transistor Q 3  from OFF to ON is from Vcc down to (Vcc−Vbe Q3 ), and the time for that voltage swing establishes the propagation delay characteristic of the comparator circuit  103 . In other words, when OFF, the base of transistor Q 3  is at Vcc, and when ON, the base of transistor Q 3  is at Vcc−Vbe as established by MOSFET M 2 . 
   The present invention reduces the propagation delay characteristic through the recognition that the voltage swing can be reduced. In  FIG. 2 , when transistor Q 3  is ON, the base is still at Vcc−Vbe, but when OFF is at (Vcc−V SCHOTTKY ) where V SCHOTTKY  is less than Vbe. Thus, the required voltage swing to turn transistor Q 3  to ON is reduced by the amount of the voltage drop across the Schottky diode  207 , reducing the propagation delay in a direct relationship thereto. The reason the swing is reduced is that because of the Schottky diode connection to the power supply voltage, the drop across the MOSFET M 2  is substantially zero when the differential stage is switched in turning transistor Q 3  OFF. When a differential signal appears across transistor Q 1  and transistor Q 2  to turn OFF transistor Q 3 , all current through MOSFET M 4  flows from transistor Q 1 , none from transistor Q 2 . Therefore the drop across MOSFET M 2  is zero volts and the voltage superimposed across the base-emitter of transistor Q 3  is equal to the Schottky voltage drop when transistor Q 3  is OFF. When the differential signal is such to turn transistor Q 3  OFF, no current flows through MOSFET M 2 . The voltage drop between the base and emitter of transistor Q 3  is substantially now the Schottky voltage drop. Therefore, the present invention takes advantage of a speed increase achieved from causing transistor Q 3  to not have to swing the full range as in the prior art from Vcc to Vbe to turn ON, but now only requiring a swing from V SCHOTTKY  up to Vbe. 
   After switching, namely when the comparator  201  has settled with the output in either a HIGH or LOW state, the current through the Schottky diode  207  is constant. Therefore, the voltage that appears at the cathode  211  is substantially constant. It will be recognized by those skilled in the art, that other components may be substituted for the Schottky diode  207  in the form of a more generic power supply voltage drop device  407 , as illustrated in  FIG. 4 . For example, a resistor having a given voltage drop across it, appropriately sized to the transistors used in the input stage  403  and output stage  405  of comparator  401 , achieves the same effect. 
   Similarly, circuits in accordance with the present may be implemented by those skilled in the art in full CMOS design such as by substituting PMOS transistors, sized to have appropriate Vgs, for the bipolar transistors as shown in  FIG. 2 . There is no intent to limit the scope of the invention to the exemplary embodiment thereof nor should any such intent be implied therefrom. 
   It will be recognized by those skilled in the art that the invention can be extended to be implemented for any multistage comparator wherein a Schottky diode is provided between Vcc and each input side stage. 
   The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . ”