Abstract:
A high speed comparator unit for a flash A/D converter in which a bank of comparator units compare simultaneously an analog input voltage with equally spaced reference voltages, and an encoder ROM produces digital signals based on the comparator unit&#39;s outputs. The comparator unit includes a two-stage cascode configuration and a level shifter configuration which effectively reduces the miller-effect of the comparator unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a comparator unit for a flash A/D converter in which a bank of comparator units compare simultaneously an analog input voltage with equally spaced reference voltages, and an encoder ROM produces digital signals based on the comparator unit&#39;s outputs. More particularly, the present invention relates to a high speed comparator unit including a miller-effect reducer which reduces the miller-effect of latch transistors to increase comparison speed. 
     2. Description of the Prior Art 
     The flash A/D converter is the fastest of all converter systems. Conversion speed is limited by the speed of a comparator unit. 
     As shown in FIG. 1 (PRIOR ART), a conventional flash A/D converter comprises a plurality of comparator units 5 for simultaneously comparing an analog input voltage V in  with equally spaced referenced voltages V ref . A voltage divider of resistors R n1 , R n2 , . . . R n2  m disposed between supply voltage V TOP  and V BTM  are connected to an inverting terminal (-) of each comparator CP 1 , CP 2 , . . . CP 2  m, and an analog input voltage V in  is connected to a noninverting terminal (+) of each comparator CP 1 , CP 2 , . . . CP 2  m. Output terminals of the comparators CP 1 , CP 2 , . . . CP 2  m are connected to encoder ROM 10 which converts the analog input voltage V in  to a digital signal D 1  . . . D m . Therefore, the comparators CP 1-CP   2  m compare the analog input voltage V in  with the reference voltages V ref  generated by the voltage divider and the encoder ROM 10 produces corresponding digital signals D 1  . . . D m  based upon these comparisons. As shown in FIG. 1, generally there are numerous comparators CP 1 , CP 2 , . . . CP 2  m, and thus, numerous comparator units 5 are needed for a conventional flash A/D converter. 
     FIG. 2 (PRIOR ART) shows a circuit diagram of a conventional comparator unit 5 comprising clock driven transistor Q 12  and Q 13 , transistors Q 1-Q   3 , Q 10  and Q 11 , latch transistor Q 8  and Q 9 , and resistors R 1  -R 3  and R 5 . 
     When clock CK goes high, transistor Q 13  turns on. If analog input voltage V in  is higher than reference voltage V ref , a larger portion of current I 2  flows through transistor Q 10  than transistor Q 11 , thereby producing a larger voltage drop across resistor R 2  than resistor R 3 . Accordingly, the voltage at the collector of transistor Q 10  is lower than the voltage at the collector of transistor Q 11 . Since the voltage drop across resistor R 3  is small (i.e., current I 2  is smaller along the transistor Q 11  side of the comparator), likewise the voltage drop across R 1  is small because current I 2  is small (VB1 is high, thus transistor Q 2  is on). Thus, with a small current I 2  at the collector of transistor Q 2  (or enable terminal E n+1 , which does not have current flowing from adjacent comparator CP i+1  along its corresponding transistor Q 1  and enable terminal E n-1 ), output voltage V out  at the emitter of transistor Q 3  is a high level of V CC-V   BE  (Q 3 ). 
     On the other hand, if analog input voltage V in  is lower than reference voltage V ref , a larger portion of current I 2  flows through transistor Q 11  than transistor Q 10 , thereby producing a larger voltage drop across resistor R 3  than resistor R 2 . Accordingly, the voltage at the collector of transistor Q 10  is higher than the voltage at the collector of transistor Q 11 . Since the voltage drop across resistor R 3  is large (i.e., current I 2  is larger along the transistor Q 11  side of the comparator), likewise the voltage drop across R 1  is larger because current I 2  is larger (VB1 is high, thus transistor Q 2  is on). Thus, with a larger current I 2  at the collector of transistor Q 2  (or enable terminal E n+1 ), output voltage V out  at the emitter of transistor Q 3  is a low level of V CC  -I 2  ×R 1  -V BE  (Q 3 ). 
     When CK goes high, transistor Q 12  turns on thereby allowing current I 2  to flow through latch transistors Q 8  and Q 9 . Since the collector voltages of transistors Q 10  and Q 11  are connected to the bases of latch transistors Q 8  and Q 9 , during latching the collector voltages of transistors Q 8  -Q 11  maintain their pre-state voltages, thus maintaining the voltage difference between the collectors of latch transistors Q  8  and Q 9 . If analog input voltage V in  is larger than reference voltage V ref , current I 2  flows through transistor Q 12 , latch transistor Q 9 , resistor R 2 , and transistor Q 1  to enable terminal E n-1  and decreases the collector voltage of latch transistor Q 9  by I 2  ×R 2  lower than latch transistor Q 8 . On the other hand, if analog input voltage V in  is smaller than reference voltage V ref , current I 2  flows through transistor Q 12 , latch transistor Q 8 , resistor R 3 , and transistor Q 2  and decreases the collector voltage of latch transistor Q 8  by I 2  ×R 3  lower than latch transistor Q 8 . 
     By this latching operation, when analog input voltage V in  is larger than reference voltage V ref  of comparator CP i  and smaller than reference voltage V ref  of comparator CP i+1 , only comparator CP i  provides a high level output voltage V out  while the other comparators produce low level output voltages. 
     However, in a conventional comparator unit, resistors R 2  and R 3  are connected between the emitters of transistors Q 1  and Q 2  and the collectors of latch transistors Q 8  and Q 9  causing a loop (for examples, the loop for latch transistor Q 9  comprises the collector of latch transistor Q 9 , resistor R 2 , the emitter of transistor Q 1 , the bases of transistors Q 1  and Q 2 , the emitter of transistor Q 2 , resistor R 3 , the base of latch transistor Q 9 ) to have a gain which increases the miller-capacitance between the collectors and bases of latch transistor Q 8  and Q 9 , thus lowering the speed of the comparator unit 5. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a comparator unit for a flash A/D converter comprising a miller-effect reducer which can reduce the miller-effect of a latch transistor to increase the comparison speed. 
     Accordingly, a comparator unit of the present invention comprises clock driven transistors, transistors for comparing analog input voltages with references voltages, latch transistor configuration, and a cascode transistor configuration. The cascode transistor configuration is connected to the latch transistor configuration and operates to reduce a miller-capacitance effect to increase the comparison speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The present invention itself, however, as well as other features and advantages thereof, will best be understood by reference to the following detailed description of the preferred embodiment, read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram of a conventional flash A/D converter; 
     FIG. 2 is a circuit diagram of a conventional comparator unit of a flash A/D converter; and 
     FIG. 3 is a circuit diagram of a comparator unit for a flash A/D converter according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Like elements appearing in FIGS. 2 and 3 are designated with like reference numerals. 
     As shown in FIG. 3, a comparator unit 5 of the present invention comprises clock driven transistors Q 12  and Q 13 , transistors Q 1  - 3 , Q 10  and Q 11 , latch transistors Q 8  and Q 9 , and resistors R 1  -R 3  and R 5  according to a conventional comparator unit and further comprises miller-effect reducer 15 coupled between latch transistors Q 8  and Q 9  and resistors R 2  and R 3 , and resistor R 4 . 
     Two cascode transistors Q 14  and Q 15  are connected between resistor R 2  and the collector of latch transistor Q 9  and between resistor R 3  and the collector of latch transistor Q 8 , respectively. Positive feed back signals from the collectors of transistors Q 14  and Q 15  are supplied to the bases of latch transistors Q 8  and Q 9  through a level shifter transistor configuration of transistors Q 4 , Q 6  and Q 5 , Q 7 , respectively. The remaining structure and elements correspond to those previously described in FIGS. 1 and 2. 
     When clock CK goes high, the comparator compares analog input voltage V in  with the reference voltage V ref . When analog input voltage V in  is larger than reference voltage V ref  a larger portion of current I 2  flows through transistor Q 10  than transistor Q 11 . The current flowing through transistor Q 10  flows through transistor Q 14 , resistor R 2 , and transistor Q 1  from E n-1 , while the current flowing through transistor Q 11  flows through transistor Q 15 , resistor R 3 , transistor Q 2 , and resistor R 1  from V CC . Since current I 2  is larger in transistors Q 10 , Q 14  than transistors Q 11 , Q 15 , the collector voltage of transistor Q 14  is lower than the collector voltage of transistor Q 15 . These collector voltages propagate to the emitter voltages of transistor Q 6  (base voltage of transistor Q 8 ) and transistor Q 7  (base voltage of transistor Q 9 ) via level shifter transistors Q 4 , Q 6  and Q 5 , Q 7 , respectively. Accordingly, the emitter voltage of transistor Q 6  is lower than transistor Q 7 . The remaining operation of transistor Q 1  -Q 3  and derivation of voltage V out  is the same as described in FIG. 2. 
     Similarly, when the analog input voltage V in  is smaller than reference voltage V ref , a larger portion of current I 2  flows through transistor Q 11  than transistor Q 10 . Therefore, the collector voltage of transistor Q 14  is higher than the collector voltage of transistor Q 15  and the emitter voltage of transistor Q 6  (base voltage of transistor Q 8 ) is higher than that of transistor Q 7  (base voltage of transistor Q 9 ). 
     When clock CK goes high (i.e. latching begins), current I 2  flows through transistor Q 12  thereby turning on transistors Q 8  and Q 9 . If in the prelatch state condition analog input voltage V in  is larger than reference voltage V ref  (base voltage of transistor Q 8  is lower than that of transistor Q 9 ), the voltage difference between these voltages is amplified along positive feed back loop Q 8  - Q 15  - R 3  - Q 5  - Q 7  - Q 9  - Q 14  - R 2  - Q 4  - Q 6 . Current I 2  flows through transistor Q 12 , transistor Q 9 , transistor Q 14 , resistor R 2 , and transistor Q 1  so that the base voltage of transistor Q 8  is lower than that of transistor Q 9  by I 2  x R 2 . 
     On the other hand, if the pre-state condition is such that analog input voltage V in  is smaller than reference voltage V ref  (base voltage of transistor Q 8  is higher than that of transistor Q 9 ), the voltage difference between these voltages is amplified along positive feed back loop Q 8  - Q 15  - R 3  - Q 5  - Q 7  - Q 9  - Q 14  - R 2  - Q 4  - Q 6 . Current I 2  flows through transistor Q 12 , transistor Q 8 , transistor Q 15 , resistor R 3 , transistor Q 2 , and resistor R 1  from V CC  so that the base voltage of transistor Q 8  is higher than that of transistor Q 9  by I 2  ×R 3 . 
     According to the present invention, a miller-effect reducer 15 has a level shifter transistor configuration and a two-stage cascode transistor configuration. Transistors Q 1 , Q 2  and Q 14 , Q 15  are connected in a two-stage cascode arrangement and constant voltage VB2 is applied to the bases of transistors Q 14  and Q 15 . Further, the base of transistors Q 8  and Q 9  are connected to emitters Q 6  and Q 7 , respectively (rather than the base of transistor Q 8  being connected to the collector of transistor Q 9  and vice versa). 
     The level shifter transistor configuration Q 4  -Q 7  lowers the base voltages of transistors Q 8  and Q 9 . As a result, transistors Q 8  and Q 9  are prevented from saturation because the collector voltages of transistors Q 14  and Q 15  are higher than those of transistors Q 8  and Q 9 . Thus, amplified signals along resistors R 2  or R 3  propagate through level shifter transistors Q 4 , Q 6  or Q 5 , Q 7 , respectively, thereby reducing the miller-capacitance effect and enhancing the speed of the comparator unit 5. 
     The present invention is advantageous over conventional comparator units in that a one-stage cascode transistor configuration Q 1  and Q 2  connected to latch transistors Q 8  and Q 9  through resistors R 2  and R 3  (FIG. 2) cannot reduce the miller-capacitance of latch transistors Q 8  and Q 9 . However, in the present invention, a second cascode stage transistors Q 14  and Q 15  is placed between resistors R 2  and R 3  and latch transistors Q 8  and Q 9  wherein a positive feed back loop from the collectors of transistors Q 14  and Q 15  contains the level shifter transistor configuration. This second cascode stage and the positive feed back loop effectively reduce the miller-capacitance effect and the conversion speed of the flash A/D converter can be enhanced through the enhancement of the speed of the comparator unit 5.