Patent Publication Number: US-10310539-B2

Title: Proportional to absolute temperature reference circuit and a voltage reference circuit

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to a proportional to absolute temperature (PTAT) reference circuit and a voltage reference circuit. In particular, it relates to a PTAT reference circuit and a voltage reference circuit which compensate for transistor base currents. 
     BACKGROUND 
     Electronic circuits typically require voltage or current references in order to operate effectively. Voltage references may be required that are temperature independent. This may be useful in circuits that require a fixed voltage reference. Voltage reference may also be required that are temperature dependent. Such references may be used as temperature sensors. One circuit arrangement commonly used to provide a temperature dependent voltage reference utilises a pair of bipolar junction transistors (BJTs). It is possible to generate a voltage reference that is proportional to absolute temperature (PTAT) by using two BJTs with different collector current densities. The difference in the base-emitter voltages of each BJT can be reflected across a resistor in order to produce a PTAT voltage reference. By combining a PTAT voltage reference with a complimentary to absolute temperature (CTAT) component a voltage reference that is independent of temperature may be provided. 
     A problem with BJT-based voltage references is that the output is affected by the BJT current gain factor. This is particularly the case in some types of processing, such as CMOS, where BJTs have a low current gain factor. There is therefore a need for voltage reference circuits in which the output voltage reference is not affected by the BJT base currents. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure relates to a PTAT voltage reference circuit and a temperature independent voltage reference circuit in which the effect of transistor base currents on the circuit output is compensated for. This is achieved by a pair of compensation resistors. The base current from one of the pair of transistors is used to increase the voltage drop across one of the compensation resistors. The base current from the other of the pair of transistors is used to decrease the voltage drop across another of the compensation resistors, by an equal amount. The compensation resistors are connected in series with the resistor which reflects the difference in base-emitter voltage (ΔV BE ). The circuit output is measured across the series connected resistors. As such, the base currents are compensated for at the output. 
     In certain embodiments the disclosure provides a proportional to absolute temperature, PTAT circuit, the circuit comprising: a first bipolar transistor arranged to generate a first base-emitter voltage and a first base current and a second bipolar transistor arranged to generate a second base-emitter voltage and a second base current; and a plurality of passive components, coupled to the first and second bipolar transistors; wherein the circuit is configured to generate a PTAT output voltage, across the plurality of passive components, which is dependent on a difference in the first and second base-emitter voltages; and the plurality of passive components are configured to compensate for the first and second base currents. 
     In certain embodiments the disclosure provides a temperature independent voltage reference, the circuit comprising: a first bipolar transistor arranged to generate a first base-emitter voltage and a first base current and a second bipolar transistor arranged to generate a second base-emitter voltage and a second base current; a plurality of passive components, coupled to the first and second bipolar transistors; and a complementary to absolute temperature, CTAT, component, coupled to the plurality of passive components; wherein the circuit is configured to generate a temperature independent output voltage, across the plurality of passive components and the CTAT component; and the plurality of passive components are configured to compensate for the first and second base currents. 
     In certain embodiments the disclosure provides a method of generating a proportional to absolute temperature, PTAT, voltage, the method comprising: providing a circuit comprising a first bipolar transistor, a second bipolar transistor and a plurality of passive components, coupled to the first and second bipolar transistors; generating, at the first bipolar transistor, a first base-emitter voltage and a first base current and, at the second bipolar transistor, a second base-emitter voltage and a second base current; generating a PTAT output voltage, across the plurality of passive components, which is dependent on a difference in the first and second base-emitter voltages; and compensating for the first and second base currents, using the plurality of passive components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: 
         FIG. 1  is a PTAT circuit in accordance with a first embodiment of the disclosure; 
         FIG. 2  is a voltage reference circuit in accordance with a second embodiment of the disclosure; 
         FIG. 3  is a PTAT circuit in accordance with a third embodiment of the disclosure; 
         FIG. 4  is a voltage reference circuit in accordance with a fourth embodiment of the disclosure; 
         FIG. 5  is a chart showing a simulation of voltage drop across various resistors of the circuit of  FIG. 3 ; and 
         FIG. 6  is a chart showing a simulation of output voltage versus temperature for the circuit shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a PTAT voltage reference circuit and a temperature independent voltage reference. In the PTAT circuit, the difference in the voltage between the base-emitter voltage of one transistor of a pair of transistors, and the base-emitter voltage of another transistor of the pair, is reflected across a resistor coupled between the two transistor bases. This voltage is proportional to absolute temperature and depends on the collector current density ratio of the two transistors. If this resistor were connected to an output and ground, the output would be affected by the base currents of the transistors. This is because the base current of one of the transistors is directed to ground, while the base current of the other transistor passes through the resistor. To compensate for this, two compensation resistors are provided in series with the PTAT resistor. One of the resistors is coupled to ground. The other is coupled to the output. As a result of this, the current through one of the resistors is the current through the PTAT resistor plus the base current of one of the transistors. The current through the other resistor is the current through the PTAT resistor minus the base current of the other resistor. Assuming the compensation resistors and the base currents take the same values, then one of the resistors positively increases the voltage dropped across it by an amount equivalent to the base current, and the other negatively decreases its voltage by the same amount. As such, the output compensates for, or is independent of, the base currents. 
       FIG. 1  shows a proportional to absolute temperature voltage reference circuit  100  in accordance with an embodiment of this disclosure. The circuit  100  comprises a first PNP bipolar transistor qp 1  and a second bipolar transistor qp 2 . The collectors of each transistor are coupled to ground. The circuit  100  also includes three p-channel metal oxide semiconductor field-effect transistors (MOSFETs) mp 1 , mp 2  and mp 3 . The emitter of each bipolar transistor is coupled to a drain of a respective MOSFET. In particular, the emitter of qp 1  is coupled to the drain of mp 3 , and the emitter of qp 2  is coupled to the drain of mp 2 . The p-channel MOSFETS are used to control the emitter currents of the bipolar transistors. The source of each MOSFET is coupled to a positive supply, Vdd. 
     The bases of the bipolar transistors are coupled to respective ends a first resistor r 1 . In particular, the base of qp 1  is coupled to a first end of r 1 , and the base of qp 2  is coupled to a second end of r 1 . As will be discussed in more detail below, any difference between the base-emitter voltages of qp 1  and qp 2  will be reflected across r 1 . The first end of r 1  and the base of qp 1  are also coupled to a first end of a first compensation resistor r 2 . The second end of the first compensation resistor is coupled to ground. 
     The circuit  100  also includes an amplifier A. The amplifier  100  includes a non-inverting input (+), an inverting input (−), and an amplifier output  102 . The non-inverting input (+) is coupled to the emitter of qp 1  and the drain of mp 3 . The inverting input (−) is coupled to the emitter of qp 2  and the drain of mp 2 . During operation, the two amplifier inputs are at the same potential, and therefore ensure that the potential at the emitters of qp 1  and qp 2  are the same. As discussed in more detail below, this ensures that any difference between the base-emitter voltages of qp 1  and qp 2  is reflected across r 1 . The amplifier output  102  is coupled to the gates of mp 1 , mp 2  and mp 3 . 
     The circuit  100  also includes a PTAT output node,  104 . The PTAT output  104  is coupled to a first end of a second compensation resistor r 3 . A second end of r 3  is coupled to a base of transistor qp 2 . The PTAT output  104  is also coupled to a drain of MOSFET mp 1 . As such, the resistors r 1 , r 2  and r 3  are coupled together in series between the PTAT output  104  and ground. The values of the resistors are set such that r 2 =r 3 . r 1  may take a value different to that of r 2  and r 3 . The voltage V O  developed at the output  104  is defined by the following equation:
 
 V   O   =V   r1   +V   r2   +V   r3   (1)
 
     Here V r1 , V r2  and V r3  are the corresponding voltage drops across the three resistors. 
     The bipolar transistor qp 1  has an emitter area of unity. The bipolar transistor qp 2  has an emitter area of n times unity. As such, if qp 1  and qp 2  are fed with the same emitter current, then the base-emitter voltage of qp 2  will be lower than the base-emitter voltage of qp 1 . The amplifier A ensures that the same voltage is present at both the inverting (−) and non-inverting (+) inputs. The emitter voltages of qp 1  and qp 2  are therefore the same. As such, the difference in base-emitter voltage (ΔV BE ) is reflected across r 1 . 
     The voltage dropped across r 1  is ΔV BE , and hence is strictly dictated by the collector current density ratio of qp 1  and qp 2 . As such, the current generated in r 1  is dependent on ΔV BE  and the value of r 1 , and not on the base currents generated by qp 1  and qp 2 . The base current of qp 1  is driven through r 2 . As such, the voltage developed across r 2  is dependent on the current generated by r 1 , the base current of qp 1  and the value of resistor r 2 . The current driven through r 3  is the current driven through r 1 , less the base current of qp 2 . As such, assuming that r 2 =r 3 , the base currents effectively cancel, and V O  is dependent on ΔV BE  but independent of the base currents of qp 1  and qp 2 . 
     Following on from equation 1 above:
 
 V   O   =ΔV   BE   +I   r2   ·r 2+ I   r3   ·r 3  (2)
 
     Since I r2 =I r1 +I Bqp1  (where I Bgp1  is the base current of qp 1 ) and since I r3 =I r1 −I Bqp2  (where I Bqp2  is the base current of qp 2 ), V O  is given by:
 
 V   O   −ΔV   BE +( I   r1   +I   Bgp1 )· r 2+( I   r1   −I   Bqp2 )· r 3  (3)
 
Therefore:
 
 V   O   =ΔV   BE   +I   r1   ·r 2+ I   Bqp1   ·r 2+ I   r1   ·r 3− I   Bqp2   ·r 3  (4)
 
     Given that I Bqp1  and I Bqp2  are equal, and that r 2  is equal to r 3 , the equation can be reduced to:
 
 V   O   =ΔV   BE   +I   r1   ·r 2+ I   r1   ·r 2  (5)
 
Therefore:
 
 V   O   =ΔV   BE +2· I   r1   ·r 2  (6)
 
     Substituting I r1  for ΔV BE /r1, gives:
 
 V   O   =ΔV   BE +2·Δ V   BE   ·r 2/ r 1  (6)
 
Therefore:
 
 V   O   =ΔV   BE ·(1+2· r 2/ r 1)  (7)
 
     As such, the output  104  is dependent only on ΔV BE  and the values of resistors r 2  and r 1 . As such, the output is independent of the current gain factor of the bipolar transistors. 
     A further advantage of this circuit arrangement relates to the fact that the current flowing through r 1  is not the same as the emitter currents. As such, the current through r 1  may be much larger than the emitter currents. The larger the current through r 1  is with respect to the base currents, the greater the base current effect is attenuated. This also helps with reducing the wide band noise, which is dominated by the r 1  value. 
       FIG. 2  shows a circuit  200  in accordance with an embodiment of the disclosure. Many of the components of circuit  200  are the same as the components of circuit  100 . These elements are referred to using the same references, and will not be described again here. The only difference between circuit  100  and circuit  200 , is that circuit  200  includes a further bipolar transistor qp 3 . The emitter of qp 3  is coupled to the second end of the first compensation resistor r 2 . The base and the collector of qp 3  are coupled to ground. qp 3  produces an output voltage that is a complimentary to absolute temperature (CTAT). As such, the circuit output  104  can be set independent of temperature, and may be used as a temperature independent voltage reference. 
     The output voltage  104  of circuit  200  is given by:
 
 V   O   =V   BEqp3   +V   r1   +V   r2   +V   r3   (8)
 
     As such, the PTAT voltage developed across V r1 , V r2  and V r3  is combined with the CTAT voltage developed across qp 3  to produce an output voltage that is temperature independent. The emitter current of qp 3  is the same as the current in r 2 . I r2  is given by:
 
 I   r2   =ΔV   BE   /r 1+ I   Bqp1   (9)
 
     Assuming that the aspects ratios of mp 1 , mp 2  and mp 3  are the same then the base current of qp 3  is the same as the base current of qp 1 , and as such the collector current of qp 3  becomes:
 
 I   Cqp3   =ΔV   BE   /r 1  (10)
 
     As such, the base current is also compensated for in qp 3 . 
       FIG. 3  shows a PTAT circuit  300  in accordance with a further embodiment of the disclosure. Many of the components of circuit  300  are the same as the components of circuit  100 . These elements are referred to using the same references, and will not be described again here. The PTAT circuit  300  includes a stack architecture. In particular, in addition to bipolar transistors qp 1  and qp 2 , the circuit  300  includes bipolar transistors qp 3  and qp 4 , arranged in a stack configuration. The circuit  200  also includes additional p-channel MOSFETs mp 4  and mp 5 . 
     The bases of transistors qp 3  and qp 4  are coupled to the emitters of transistors qp 1  and qp 2  respectively. The collectors of transistors qp 3  and qp 4  are coupled to ground. The emitter of qp 3  is coupled to the non-inverting input (+) of amplifier A. In contrast to circuit  100 , the non-inverting input (+) is not coupled to the emitter of qp 1 . The emitter of qp 4  is coupled to the inverting input (−) of amplifier A. In contrast to circuit  100 , the inverting input (−) is not coupled to the emitter of qp 2 . As such, the amplifier A controls the potential at the emitters of qp 3  and qp 4 , rather than qp 1  and qp 2 . 
     The output  102  of amplifier A is coupled to the gates of mp 4  and mp 5 . The drains of mp 4  and mp 5  are coupled to the emitters of qp 3  and qp 4  respectively. The sources of mp 4  and mp 5  are coupled to the positive supply, Vdd. 
     The bipolar transistor qp 3  has an emitter area of unity. The bipolar transistor qp 4  has an emitter area of n times unity. As such, if qp 3  and qp 4  are fed with the same emitter current, then the base-emitter voltage of qp 4  will be lower than the base-emitter voltage of qp 3 . 
     In this circuit arrangement, the voltage developed across r 1  is the combination of the difference in base-emitter voltages of two pairs of transistors. As such, V r1  is double V r1  in circuit  100 . As such, the amplifier offset voltage effect on the base-emitter voltage difference is reduced. Furthermore, because Vr 1  is double Vr 1  in circuit  100 , the gain factor (the ratio of r 2  to r 1 ) can be half that in circuit  100  to achieve the same output voltage. 
       FIG. 4  shows a circuit  400  in accordance with an embodiment of the disclosure. Many of the components of circuit  400  are the same as the components of circuit  300 . These elements are referred to using the same references, and will not be described again here. The only difference between circuit  300  and circuit  300 , is that circuit  300  includes a further bipolar transistor qp 5 . This is a similar arrangement to that shown in  FIG. 2 . The emitter of qp 5  is coupled to the second end of the first compensation resistor r 2 . The base and the collector of qp 5  are coupled to ground. qp 5  is a complimentary to absolute temperature (CTAT) component, and as such, the circuit output is independent of temperature. 
     The effectiveness of the above-described circuit arrangements for the purposes of compensating for base currents will now be described with reference to circuit  300  and  FIG. 3 . The circuit  300  was simulated using CMOS processing using substrate bipolar transistors having a “beta” factor of around 25 at ambient temperature. qp 1  and qp 3  were set to have emitter areas of 5 μm*5 μm. qp 2  and qp 4  were formed from 26 identical bipolar transistors, connected in parallel, in order to simulate an N of 26. Resistors r 1 , r 2  and r 3  were given a value of 17 kΩ. The emitter currents at ambient temperature of the four bipolar transistors qp 1  to qp 4  were set to 0.28 μA and the current through r 1 , r 2  and r 3  were set to approximately 10 μA. 
       FIG. 5  shows a simulation plot of the voltage drops across each resistor r 1  to r 3  against temperature, assuming that the three resistors have the same value. As can be seen, the voltage drop across r 2  is slightly higher than r 1  owing to the base current of qp 1 . The voltage drop across r 3  is lower than that across r 1  by the same amount, owing to the base current of qp 2 . As such, the output voltage is exactly three times the voltage across r 1 , i.e. three times ΔV BE . Therefore, the base currents are compensated. 
       FIG. 6  is a chart showing the simulated voltage at the output of circuit  200 . As can be seen, the voltage has very little variation from −40° C. to 125° C. 
     The circuits  200  and  400  may be used for one of three functions. By connecting the emitter of qp 3  (in  FIG. 2 ) or qp 5  (in  FIG. 4 ) to ground, the circuit performs the same PTAT functions as circuits  100  and  300 . When the emitter of qp 3  or qp 5  is not coupled to ground, the circuit provides a temperature independent reference voltage. Finally, the circuits may act as a PTAT current generator by mirroring the bias current of mp 1 .