Abstract:
The present invention relates to a low impedance band-gap voltage reference circuit which comprises a band-gap reference circuit, a buffer circuit to reduce the impedance and related noise associated with band-gap references electronically coupled with the band-gap voltage reference circuit and a voltage pull-up device electronically coupled with both the band-gap reference circuit and the buffer circuit. The voltage pull-up device acts to reduce the supply voltage required to maintain a stable, low Z band-gap reference voltage.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of integrated circuit design. 
   BACKGROUND OF THE INVENTION 
   In the arena of complex integrated circuits, there are sometimes portions of circuits that require voltage references for proper functioning. A voltage reference provides a precise output voltage, one that is much more accurate than can be produced by a voltage regulator. Its output voltage is compared to other voltages in a system and, usually, adjustments are made to those other voltages based on the reference difference. References are similar to regulators in how they function, but they are used much differently. While regulators are used to deliver power to a load, references are normally used with a small, stable load (if any) to preserve their precision. Only a few of the existing reference designs have the capability to deliver a load greater than a few milliamps while maintaining a precision output voltage. A reference is not used to supply power but to provide a system with an accurate analog voltage for comparison purposes. The band-gap reference circuit has long been used in integrated circuits for that purpose. 
   A band-gap reference takes advantage of the electro-chemical properties of a material. In a semiconductor, the amount of energy which allows the material to become conductive, i.e. move current in the presence of a voltage, is known as the band gap energy. The band gap energy is different for a variety of materials. However, silicon, the foundation material for a preponderance of integrated circuits, has a predictable band-gap energy that changes little with temperature over most of the temperature range of normal integrated circuit operations. 
   The band-gap reference is widely used in almost every application of IC technology. One common method of band-gap implementation is use of current generated by the delta V be  of a pair of unijunction transistors which essentially function as diodes. The current then flows through a diode chain to achieve a constant reference band-gap voltage. A significant problem with such simple reference circuits is a high output impedance which can change the reference behavior if the band-gap reference circuit were connected to a high noise stage. 
   Some early band-gap reference circuits used conventional junction-isolated bipolar-IC technology to make relatively stable low-voltage references. This type of reference became popular as a stable voltage reference for low-voltage circuits, such as in 5-volt data acquisition systems where zener diodes were not suitable. 
   A common failing in band-gap reference circuits, as mentioned above, is a characteristically high impedance that results in a noisy circuit. Because the demands on a reference get ever tighter with higher precision circuits, a stable low-noise performance is crucial. 
   Another common failing of band-gap circuits is the requirement for a relatively high VCC, substantially higher than the reference voltage. Since a band-gap voltage is almost always very close to 1.2 volts, a minimum value for VCC is usually somewhere around 2 volts. Since modern digital ICs using 1 volt technology are becoming daily more common, the requirement for a higher VCC can be a design limitation. 
   What is needed, then, is a band-gap reference circuit that has an innate low impedance to allow for stable low-noise operation. A further need exists for a band-gap reference circuit that can produce a usable reference voltage while being powered by a low supply voltage. 
   SUMMARY OF THE INVENTION 
   Presented herein is a band-gap reference circuit that has an innate low impedance to allow for stable low-noise operation. This novel band-gap reference circuit can produce a usable, low noise, reference voltage while being powered by a low supply voltage. 
   The present invention relates to a low impedance band-gap voltage reference circuit which comprises a band-gap reference circuit, a buffer circuit to reduce the impedance and related noise associated with band-gap references electronically coupled with the band-gap voltage reference circuit and a voltage pull-up device electronically coupled with both the band-gap reference circuit and the buffer circuit. The voltage pull-up device acts to reduce the supply voltage required to maintain a stable, low Z band-gap reference voltage. 
   These and other objects and advantages of the present invention will become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The operation and components of this invention can be best visualized by reference to the drawing. 
       FIG. 1  illustrates an implementation of a band-gap reference circuit. 
       FIG. 2  illustrates an implementation of a band-gap reference circuit with an impedance reducing buffer consistent with the conventional art and with embodiments of the present invention. 
       FIG. 3  illustrates a low-Z, low voltage, band-gap reference circuit in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
   The embodiments of the present invention discussed herein relate to the electronic characteristics of the semiconductor material from which integrated circuit devices are formed. Modern integrated circuit devices are typically very small and work in very low voltages. Most modern integrated require a stable voltage reference. In some cases, modern digital devices can draw a logic distinction between voltages differing by fractions of volts. Some analog or hybrid devices, such as ADCs (analog to digital converters) or DAC s (digital to analog converters), however, can be required to make much smaller determinations. 
   Another type of hybrid IC is family of chips employing digital signal processing (DSP). The explosion in telecommunications technology has driven a tremendous amount of progress in DSP chips and the speed demands have driven voltages downward just as in other types of processing. As the voltages have gotten smaller, the impact of noise in ICs, particularly in an environment where an acoustic signal the focus, has steadily gotten more important. One source of noise exacerbation is the innate high impedance of common voltage references. 
   One method of reducing noise in a reference circuit is by adding a buffer to the output of a band-gap reference. However, the addition of a buffer increases the power demand and can drive up the supply voltage required in order to maintain the band-gap voltage.  FIG. 1  illustrates a basic band-gap reference circuit and  FIG. 2  illustrates a reference with a buffer for noise suppression. 
     FIG. 1  is an illustration of a common Implementation of a band-gap reference circuit. The band-gap voltage at  100  is the sum of the current through transistor  107 , multiplied by the resistance of resistor  105 , and the base-emitter voltage (V BE ) of transistor  103 . The current through transistor  107  is controlled by both its gate voltage, which is a function of the action of transistors  106  and  108 , and the current diverted through resistor  104 , which is controlled by the action of transistors  101  and  102 . Transistors  106 ,  107  and  108  are connected in common at their gates with drains to supply voltage, V CC . The gate to drain shunt of transistor  106  acts to regulate the gate voltages and the current of transistors  108  and  107 . 
   Transistors  101  and  102  are both implemented as bipolar devices in this illustration. With its common base and collector, transistor  102  effectively acts as a base-emitter diode. Transistor  103  is also connected in a common base-collector form and also acts as a base-emitter diode. 
   It is the difference in currents between transistors  106  and  107  that produces the stable band-gap voltage. If I 106  is the current through transistor  106 , that same current is through transistor  101  and resistor  104 . In that case by Ohm&#39;s law, I 106  times R 104  equals the base-emitter voltage of transistor  102  minus the base-emitter voltage of transistor  101 , i.e.:
 
 I   106   ·R   104   =VBE   102   −VBE   101  
 
then
 
 I   106   ·R   104 =( V   T  ln  m )/ R   104  
 
where: m is the relationship between transistor  101  and transistor  102  and m is larger than unity which means that transistor  101  is “bigger” than transistor  102 . This in turn means that, for the same base-emitter voltage and the same emitter-collector voltage, transistor  101  will pass m times as much current as transistor  102 .
 
   The similar relationship between transistor  106  and transistor  107  is n. Transistors  106  and  107  are implemented as field effect transistors (FET) in this illustration. Transistor  107  will pass n times as much current as transistor  106  at the same gate-source voltage which is the constant state in the circuit illustrated because transistors  106  and  107  have common sources and common gates. If i 2  is the current through transistor  107  and i 1  is the current through transistor  106  and therefore transistor  101 , n=i 2 /i 1  and n is greater than or equal to 1. The current through transistors  108  and  102  is i 3 . 
   The band-gap voltage at  100 , then, is:
 
 V   BG   =I   2   ·R   105   +V   BE   103  
 
   Note that, since transistor  103  is connected with a common base-emitter, it functions as a diode with an innate resistance. 
   Then:
 
 V   BG   =ni   1   R   105   +V   BE   103  
 
 V   BG   =[n ( V   T  ln  m )/ R   104   ]·R   105   +V   BE   103  
 
 V   BG   =[n ( V   T  ln  m )/ R   104   ]·R   105   +V   T  ln( ni   1   /i   s )
 
   It must be noted here that the gate-drain shunt of transistor  106  causes the gate voltage of transistors  106 ,  107  and  108  to seek an equilibrium. The difficulty that arises in such a simple circuit is its inherent high impedance and attendant susceptibility to noise. 
   To overcome this, a buffer can be added to the band-gap circuit as is shown in  FIG. 2 . In essence the same circuit as in  FIG. 1 , the circuitry associated with transistors  201  through  207  and resistors  211  and  212  provides the same functionality as the circuitry in  FIG. 1 . The current source shown at  214  is implemented in this illustration as a MOSFET current source. PNP transistors  203  and  204  share a common base which is shunted to the collector of transistor  203 . NPN transistors  201  and  202  also share a common base that connects V BG , the band-gap voltage at  200 . Transistor  205  has a base connected to the common collectors of transistors  202  and  204 . The collector of transistor  205  is connected to the drain of transistor  206  which shares a common gate with transistor  207 . The common gate of transistors  206  and  207  is shunted to the drain-collector connection between transistors  205  and  206 . In the implementation illustrated in  FIG. 2 , m symbolizes the relationship in current flow between transistor  201  and transistor  202 . Because their bases are common, the ratio of current flows is constant. The base-emitter voltage of transistor  201  and transistor  202  differs by the voltage across resistor  211 . 
   The circuit in  FIG. 2  differs primarily from that in  FIG. 1  in the employment of transistor  209 . Transistor  209  is implemented as an NPN bipolar device, which typically have significantly lower impedances than FETs. Transistor  209  is connected at its base to common emitters of transistors  203 ,  204  and  205  and with its collector connected to V CC . This causes transistor  209  to behave as an emitter follower and function as a buffer. It is well known in the art that an emitter follower can accept a signal at a high resistance level without significant attenuation and reproduce it at a low resistance level and with no phase shift. Therefore, in this implementation, it functions well as a buffer. However, a problem that arises in the use of a buffer is the requirement for a higher supply voltage, Vcc, in order to preserve a constant band-gap voltage. 
   In the band-gap reference circuit illustrated in  FIG. 2 , the required Vcc can be defined as:
 
 V   CC   =V   BG   +V   BE   209   +V   SOURCE   214  
 
where:
 
   V BG =1.25 V 
   V BE   209 =700 mV 
   V SOURCE   214 =300 mV 
   thus: 
   V CC ≧2.25 V 
   The embodiment of the present invention discussed here enables a low supply voltage Vcc, as is shown in  FIG. 3 , by the addition of device  320 . Device  320  is accompanied by the addition of transistor  308 , transistor  310  and current source  313 . Current source  313  can be, in many implementations of this embodiment of the present invention, functionally implemented by a metal oxide/silicon field effect transistor (MOSFET) current source with its source connected to Vcc. NPN transistor  309  is connected as an emitter follower for the emitters of transistors  203 ,  204  and  205 . The emitter of transistor  309  is connected via device  320  to the base of PNP transistor  310 . It is transistor  310  that provides the final buffering in this implementation. The collector-emitter voltage, V CE , of transistor  310  is the band-gap voltage in this embodiment. In this configuration, Vcc can be very low for a buffered band-gap circuit. The minimum V CC  here is:
 
 V   CC   =V   BG   −V   BE   310   +V   320   +V   BE   309   +V   SOURCE   314  
 
since:
 
   V BG =1.25 V 
   V BE   310 =V BE   309    
   then:
 
 V   CC   =V   BG   +V   320   +V   SOURCE   314 ≅1.8 V
 
   Note that, in this embodiment, device  320  is necessary to pull the voltage back up and prevent saturation of transistors  201  and  202 . Device  320  can be implemented, in various embodiments, as a resistor or as a transistor with less than 1 V BE . In the illustration of  FIG. 3 , device  320  is disposed between buffer  309  and the band gap reference unit. It is important to note that transistors  203 ,  204 , and  205  can be implemented as either bipolar transistors or MOS transistors. 
   Device  320 , in this embodiment, can be implemented in a number of ways. It is likely that device  320  will be found to be functional when implemented as a resistor or as a fixed gain transistor. Without regard to the actual implementation, the function of device  320  remains to be the reduction in necessary supply voltage in order to produce a functional buffer across the operating range of the band-gap reference circuit. In the implementation of device  320  illustrated in  FIG. 3 , the combination of device  320  and buffering transistor  309  acts to pull the V BE  of transistor  310  towards V CC  which means that the buffering that is done by transistor  310  can be accomplished at a lower V CC . In this fashion, the buffering necessary to achieve a low impedance is enabled yet the normally high V CC  attendant to the implementation of buffering is obviated. A low voltage, low Z, band-gap reference circuit is thus embodied. 
   A novel band-gap reference circuit has been disclosed. The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.