Patent Publication Number: US-2023139284-A1

Title: Bandgap reference circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application incorporates by reference, in its entirety, India Provisional Application No. 202041019319, filed May 6, 2020, entitled “Method for High Precision in Bandgap Reference at Low Area.” 
     BACKGROUND 
     Many circuits and devices (e.g., analog-to-digital converters), need a precise reference voltage to operate. A bandgap reference circuit may be used to generate such a reference voltage. Bandgap voltage reference circuits generate a temperature-stable voltage by combining a p-n junction voltage with a thermal voltage. A bandgap reference circuit generates a complementary-to-absolute-temperature (CTAT) voltage and a proportional-to-absolute-temperature (PTAT) voltage. The CTAT voltage decreases with increasing temperature (i.e., the CTAT voltage has a negative temperature coefficient), and the PTAT voltage increases with increasing temperature (i.e., the PTAT voltage has a positive temperature coefficient). The bandgap reference circuit combines the PTAT and CTAT voltages such that their respective temperature coefficients cancel each other out to produce a temperature stable voltage. 
     SUMMARY 
     In one example, a bandgap reference circuit includes an amplifier, a first transistor, a second transistor, and a third transistor. The amplifier includes a first input and a second input. The first transistor includes a first current terminal, a second current terminal, and a control terminal. The first current terminal is coupled to the first input of the amplifier. The second current terminal is coupled to ground. The second transistor includes a first current terminal, a second current terminal, and a control terminal. The first current terminal of the second transistor is coupled to the control terminal of the first transistor and the second input of the amplifier. The second current terminal of the second transistor is coupled to the second current terminal of the first transistor. The control terminal of the second transistor is coupled to the second input of the amplifier. The third transistor includes a first current terminal, a second current terminal, and a control terminal. The first current terminal of the third transistor is coupled to the first input of the amplifier. The second current terminal of the third transistor is coupled to the second current terminal of the first transistor. The control terminal of the third transistor is coupled to the control terminal of the second transistor. 
     In another example, a bandgap reference circuit includes an amplifier, a first transistor, a second transistor, a third transistor, a first resistor, and a second resistor. The amplifier is configured to generate a bandgap voltage. The first transistor is coupled to the amplifier, and is configured to pass a first proportional to absolute temperature (PTAT) current. The second transistor is coupled to the amplifier, and is configured to pass a second PTAT current. The first resistor is coupled to the amplifier and the second transistor, and is configured to pass the second PTAT current to the second transistor. The third transistor is coupled to the amplifier, and is configured to pass a third PTAT current that bypasses the first resistor and the second transistor. The second resistor is coupled to the first transistor, the second transistor, and the third transistor, and is configured to pass the first PTAT current, the second PTAT current, and the third PTAT current. 
     In a further example, a data acquisition system includes an analog-to-digital converter (ADC), and a reference voltage circuit. The reference voltage circuit is coupled to the ADC. The reference voltage circuit includes a bandgap reference circuit. The bandgap reference circuit includes a first transistor, a second transistor, a third transistor, a first resistor, and a second resistor. The first transistor is configured to pass a first PTAT current. The second transistor is configured to pass a second PTAT current. The first resistor is coupled to the second transistor, and is configured to pass the second PTAT current to the second transistor. The third transistor is configured to pass a third PTAT current that bypasses the first resistor and the second transistor. The second resistor is coupled to the first transistor, the second transistor, and the third transistor, and is configured to pass the first PTAT current, the second PTAT current, and the third PTAT current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic level diagram for an example bandgap reference circuit. 
         FIG.  2    is a graph of β (or H FE ) variation of a transistor with current density. 
         FIG.  3    is a schematic level diagram for an example bandgap reference circuit that includes current mirrors to increase proportional to absolute temperature (PTAT) current. 
         FIG.  4    is a schematic level diagram for an example bandgap reference circuit that includes current mirroring to increase PTAT current, and a matching resistor. 
         FIG.  5    is a schematic level diagram for an example bandgap reference circuit that includes inverted bandgap pairs with current mirroring to increase PTAT current. 
         FIG.  6    is a block diagram for a data acquisition system that includes the bandgap reference circuit of  FIG.  3   . 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a schematic level diagram for an example bandgap reference circuit  100 . The bandgap reference circuit  100  includes an amplifier  102 , a transistor  106 , a transistor  104 , a resistor  108 , a resistor  110 , a resistor  112 , and a resistor  114 . The transistor  106  and the resistor  108  may be NPN bipolar junction transistors. The emitter area of the transistor  104  may be N times greater than the emitter area of the transistor  106 . For example, the transistor  104  may include N transistors that are similar or identical to the transistor  106 . A bandgap voltage (V BG ) is provided at the output of the amplifier  102 . The bandgap voltage is the sum of a complementary to absolute temperature (CTAT) voltage provided at a first input (non-inverting input) of the amplifier  102 , and proportional to absolute temperature (PTAT) voltage provided at a second input (inverting input) of the amplifier  102 . 
         V   BG   =V   BE,Q0   +V   T  ln( N )* K    
     The CTAT voltage is the base-emitter voltage of the transistor  104  (V BE,Q0 ). The PTAT voltage is the difference of the V BE  of the transistor  104  and the V BE  of the transistor  106  provided across the resistor  112 . With the same current flowing in the transistor  104  and the transistor  106 , the PTAT voltage may be expressed as: 
         V   T  ln( N )* K    
     where:
 
V T  is the thermal voltage of the transistor  104  or the transistor  106 ;
 
N is the emitter area of the transistor  104  relative to the emitter area of the transistor  106  (e.g., N=8 if the emitter area of the transistor  104  is 8 times that of the transistor  106 ); and
 
     
       
         
           
             K 
             = 
             
               1 
               + 
               
                 
                   ( 
                   
                     
                       2 
                       ⁢ 
                       R 
                       ⁢ 
                       4 
                     
                     + 
                     
                       R 
                       ⁢ 
                       2 
                     
                   
                   ) 
                 
                 
                   R 
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     where:
 
R 1  is the resistance of the resistor  112 ;
 
R 2  is the resistance of the resistor  108 ; and
 
R 4  is the resistance of the resistor  114 .
 
     In the bandgap reference circuit  100 , noise and offset from the amplifier  102  scale by K, and increasing N reduces K and reduces output noise. Thus, increasing N or reducing K, without impacting quiescent current (I q ) or increasing circuit area are desirable. N may be increased by skewing the current in the transistors  104  and transistor  106 . If 
     
       
         
           
             
               
                 
                   R 
                   3 
                 
                 
                   R 
                   2 
                 
               
               = 
               
                 N 
                 1 
               
             
             , 
           
         
       
     
     where R 3  is the resistance of the resistor  110 , then PTAT voltage is: 
     
       
         
           
             
               
                 V 
                 T 
               
               ⁢ 
               
                 ln 
                 ⁡ 
                 ( 
                 
                   N 
                   * 
                   
                     N 
                     1 
                   
                 
                 ) 
               
               * 
               K 
             
             ⁢ 
             
 
             
               
                 where 
                 : 
                 
 
                 K 
               
               = 
               
                 1 
                 + 
                 
                   
                     ( 
                     
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               1 
                               
                                 
                                   N 
                                   1 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                         ⁢ 
                         R 
                         ⁢ 
                         4 
                       
                       + 
                       
                         R 
                         ⁢ 
                         2 
                       
                     
                   
                   
                     R 
                     ⁢ 
                     1 
                   
                 
               
             
           
         
       
     
     With PTAT voltage equal to V T  ln(N*N 1 )*K, increasing N*N 1  is one method for reducing noise. However, increasing N*N 1  increases the current density (Jc) in the transistor  106 . There is a limit to increasing the current density as the transistor  106  enters non ideal operation and β=I C /I B  reduces. This problem also exists in low quiescent current designs where current density in the transistor  106  is kept fixed and current density in the transistor  104  is reduced.  FIG.  2    is a graph of β (or H FE ) variation of a BJT with current density. If a transistor (e.g., the transistor  104  or the transistor  106 ) is biased outside the relatively flat plateau of the curve shown in  FIG.  2   , then errors are introduced in the bandgap reference. For example, the PTAT nature of biasing in the bandgap reference circuit  100  causes current density to vary with temperature. As a result, β related errors vary with temperature and introduce temperature drift error in the output voltage. Temperature drift is an important specification for a precision reference. 
     Examples of bandgap reference circuits that increase PTAT current using mirror circuitry, and without increasing the size of the transistor  104  will now be described. Increasing the PTAT current reduces the scale factor value K, which reduces noise in the bandgap voltage output. The bandgap reference circuits that include mirror circuitry are smaller than circuits that provide an equivalent increase in PTAT current by increasing the size (emitter area) of the transistor  104 . 
       FIG.  3    is a schematic level diagram for an example bandgap reference circuit  300  that includes current mirrors to increase PTAT current. The bandgap reference circuit  300  includes the amplifier  102 , the transistor  106 , the transistor  104 , the resistor  108 , the resistor  110 , the resistor  112 , the resistor  114 , the mirror transistor  304 , and the mirror transistor  306 . Each of the mirror transistor  304  and the mirror transistor  306  may be formed as M/2 copies of the transistor  106 . The bandgap voltage is provided at the output of the amplifier  102 . Addition of the mirror transistor  304  and the mirror transistor  306  splits bandgap PTAT current into a mirrored branch. The mirror transistor  304  and the mirror transistor  306  mirror and scale up the PTAT current flowing through the transistor  106 . Net PTAT current flowing through the resistor  114  is: 
       ( M+ 2)* I   1 . 
     where:
 
M is the combined emitter area of the mirror transistor  304  and the mirror transistor  306  relative to the emitter area of the transistor  106 ; and
 
I 1  is PTAT current flowing through the transistor  106  also flows through the transistor  104 ).
 
     Addition of the mirror transistors provides larger PTAT current for a given resistance of the resistor  112  and transistor area, and provides a substantial reduction of the scaling factor K relative to increasing the emitter area of the transistor  104 . For example, assume that emitter size N of the transistor  104  is increased to N+M, then K reduces by: 
     
       
         
           
             
               
                 ln 
                 ⁡ 
                 ( 
                 
                   
                     N 
                     * 
                     
                       N 
                       1 
                     
                   
                   + 
                   M 
                 
                 ) 
               
               
                 ln 
                 ⁡ 
                 ( 
                 
                   N 
                   * 
                   
                     N 
                     1 
                   
                 
                 ) 
               
             
             . 
           
         
       
     
     Instead, with addition of the mirror transistor  304  and the mirror transistor  306 , providing emitter size M, K reduces by: 
     
       
         
           
             
               
                 ( 
                 
                   
                     ( 
                     
                       1 
                       + 
                       
                         1 
                         
                           N 
                           1 
                         
                       
                     
                     ) 
                   
                   + 
                   M 
                 
                 ) 
               
               
                 ( 
                 
                   1 
                   + 
                   
                     1 
                     
                       N 
                       1 
                     
                   
                 
                 ) 
               
             
             . 
           
         
       
     
     This implies that for the same total transistor area of N+M, use of mirroring as in the bandgap reference circuit  300  provides a ratio of improvement of K of: 
     
       
         
           
             
               ( 
               
                 
                   1 
                   + 
                   
                     1 
                     
                       N 
                       1 
                     
                   
                   + 
                   M 
                 
                 
                   1 
                   + 
                   
                     1 
                     
                       N 
                       1 
                     
                   
                 
               
               ) 
             
             ⁢ 
             
               ( 
               
                 
                   ln 
                   ⁢ 
                   N 
                   * 
                   
                     N 
                     1 
                   
                 
                 
                   
                     ln 
                     ⁢ 
                     N 
                     * 
                     
                       N 
                       1 
                     
                   
                   + 
                   M 
                 
               
               ) 
             
           
         
       
     
     relative to conventional techniques. For example, if N=24, N 1 =2, and M=24, the improvement by addition of the mirror transistors is 13.8×. If N=24, N 1 =2, and M=4, the improvement by addition of the mirror transistors is 3.7×. 
     The output of the amplifier  102  is coupled to a first terminal of the resistor  108  and a first terminal of the resistor  110 . A second terminal of the resistor  110  is coupled a non-inverting input of the amplifier  102  and a first current terminal (collector) of the transistor  104 . A second current terminal (emitter) of the transistor  104  is coupled to a first terminal of the resistor  114 . A second terminal of the resistor  114  is coupled to ground. A PTAT current flows through the resistor  110 , the transistor  104 , and the resistor  114 . 
     A second terminal of the resistor  108  is coupled to a first terminal of the resistor  112 . A second terminal of the resistor  112  is coupled to a control terminal (base) of the transistor  104  and a first current terminal (collector) of the transistor  106 . A second current terminal (emitter) of the transistor  106  is coupled to the second current terminal of the transistor  104 . 
     The mirror transistor  304  mirrors the PTAT current flowing through the transistor  104 . A first current terminal (collector) of the mirror transistor  304  is coupled to the first current terminal of the transistor  104 . A second current terminal (emitter) of the mirror transistor  304  is coupled to the second current terminal of the transistor  104 . A control terminal of the mirror transistor  304  is coupled to the control terminal of the transistor  106 . PTAT current flowing through the mirror transistor  304  bypasses the transistor  104 . 
     The mirror transistor  306  mirrors the PTAT current flowing through the transistor  106 . A first current terminal (collector) of the mirror transistor  306  is coupled to the first terminal of the resistor  112 . A second current terminal (emitter) of the mirror transistor  306  is coupled to the second current terminal of the transistor  104 . A control terminal of the mirror transistor  304  is coupled to the control terminal of the transistor  106 . PTAT current flowing through the mirror transistor  306  bypasses the resistor  112  and the transistor  106 . 
       FIG.  4    is a schematic level diagram for an example bandgap reference circuit  400 . The bandgap reference circuit  400  includes the amplifier  102 , the transistor  106 , the transistor  104 , the resistor  108 , the resistor  110 , the resistor  112 , the resistor  114 , a mirror transistor  404 , and a resistor  406 . The mirror transistor  404  may be formed as M copies of the transistor  106 . The bandgap voltage is provided at the output of the amplifier  102 . Addition of the mirror transistor  404  splits bandgap PTAT current into a mirrored branch. The mirror transistor  404  mirrors and scales up the PTAT current flowing through the transistor  106 . Net PTAT current flowing through the resistor  114  is: 
       ( M+ 2)* I   1 , 
     where M is the emitter area of the mirror transistor  404  relative to the emitter area of the transistor  106 . 
     A first current terminal (collector) of the mirror transistor  404  is coupled to the output of the amplifier  102 . A control terminal of the mirror transistor  404  is coupled to the control terminal of the transistor  106 . A second current terminal (emitter) of the mirror transistor  404  is coupled to a first terminal of the resistor  406 . A second terminal of the resistor  406  is coupled to the first terminal of the resistor  114 . The resistance of the resistor  406  may be selected to produce a desired value of IPTAT current flow in the mirror branch. For example, the resistor  406  may be selected to compensate for various errors in the bandgap reference circuit  400 . 
       FIG.  5    is a schematic level diagram for an example bandgap reference circuit  500 . The bandgap reference circuit  500  includes the amplifier  102 , a bandgap pair  501 , a bandgap pair  503 , a resistor  514 , and a mirror transistor  516 . The bandgap pair  501  includes a transistor  502 , a transistor  506 , and a resistor  510 . A first current terminal (collector) of the transistor  502  is coupled to the output of the amplifier  102 . A control terminal (base) of the transistor  502  is coupled to the first current terminal of the transistor  502  (the transistor  502  is configured as a diode). A second current terminal (emitter) of the transistor  502  is coupled to a first current terminal (collector) of the transistor  506  via the resistor  510 . The first current terminal of the transistor  506  is coupled to a first input (non-inverting input) of the amplifier  102 . A second current terminal (emitter) of the transistor  506  is coupled to a first terminal of the resistor  514 . A second terminal of the resistor  514  is coupled to ground. 
     The bandgap pair  503  includes a transistor  504 , a transistor  508 , and a resistor  512 . The bandgap pair  503  is structurally inverted with respect to the bandgap pair  501 . That is, the diode connected transistor  508  is at the bottom of the bandgap pair  503 , while the diode-connected transistor  502  is at the top of the bandgap pair  501 . The emitter area of the transistor  502  may be N time greater than the emitter area of the transistor  504 . A first current terminal (collector) of the transistor  504  is coupled to the output of the amplifier  102 . A control terminal (base) of the transistor  504  coupled to the control terminal of the transistor  502 . A second current terminal (emitter) of the transistor  504  is coupled to the second input (inverting input) of the amplifier  102 , and to a first current terminal (collector) of the transistor  508 . A control terminal (base) of the transistor  508  is coupled to the first current terminal of the transistor  508  (the transistor  508  is configured as a diode). A second current terminal (emitter) of the transistor  508  is coupled to a first terminal of the resistor  512 . A second terminal of the resistor  512  is coupled to the first terminal of the resistor  514 . The emitter area of the transistor  508  may be N time greater than the emitter area of the transistor  506 . 
     The mirror transistor  516  mirrors the PTAT current flowing though the bandgap pair  501  and the bandgap pair  503 . The emitter area of the mirror transistor  516  may be M times greater than the emitter area of the transistor  506 . Addition of the mirror transistor provides larger PTAT current for a given resistance (R 1 ) of the resistors  510  and  512  and transistor area, and provides a substantial reduction of the scaling factor K relative to increasing the emitter area of the transistors  502  and  508 . A first current terminal (collector) of the mirror transistor  516  is coupled to the output of the amplifier  102 . A control terminal (base) of the mirror transistor  516  is coupled to the control terminal of the transistor  508 . A second current terminal (emitter) of the mirror transistor  516  is coupled to the first terminal of the resistor  514 . 
     With addition of the mirror transistor  516 , the PTAT current flowing in the resistor  514  is: 
       (2 +M ) I   PTAT , 
     where I PTAT  is the PTAT current flowing through the bandgap pair  501  and the bandgap pair  503 . As in all of the example bandgap reference circuits described herein, use of a mirror transistor to increase PTAT current reduces the output noise of the bandgap reference circuit while reducing the circuit area needed to produce the PTAT current relative to other techniques. 
       FIG.  6    is a block diagram for a data acquisition system  600  that includes the bandgap reference circuit with PTAT current mirroring as described herein. The data acquisition system  600  may be implemented in a variety of systems (e.g., medical imaging systems, test and measurement systems, etc.). The data acquisition system  600  includes an amplifier  602 , an analog-to-digital converter (ADC)  604  coupled to the amplifier  602 , and a reference voltage circuit  606  coupled to the ADC  604 . The amplifier  602  provides a signal to be digitized to the ADC  604 . The amplifier  602  may be an operational amplifier having single-ended or differential output. The ADC  604  may be any type of ADC that converts an input signal to a digital value. For example, the ADC  604  may be a successive-approximation ADC, a FLASH ADC, a sigma-delta ADC, etc. The reference voltage circuit  606  provides reference voltage to the ADC  604  for use in digitizing the signal received from the amplifier  602 . The reference voltage circuit  606  includes a bandgap reference circuit  608  having current mirroring that increases the PTAT current flowing in the reference voltage circuit  606  to reduce noise in the reference voltage provided to the ADC  604 . The bandgap reference circuit  608  may be an implementation of the bandgap reference circuit  300 , the bandgap reference circuit  400 , or the bandgap reference circuit  500 . 
     In this description, the term “couple” or “couples” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action: (a) in a first example, device A is coupled to device B; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A. Also, in this description, the recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, then X may be a function of Y and any number of other factors. 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.