# Patent

## Title
Reference voltage generation circuit

## Abstract

A reference voltage generation circuit, including a first current source in series with a first bipolar transistor; a second current source in series with a first resistor; a third current source in series with a second bipolar transistor, the third current source being assembled as a current mirror with the first current source; a second resistor between the base of the second bipolar transistor and the junction point between the current source and the first resistor; and a fourth current source in series with a third resistor, the junction point between the fourth current source and the third resistor defining a reference voltage terminal.

## Background
BACKGROUND
Technical Field
The present disclosure relates to a circuit for gene-rating a reference voltage under a power supply voltage smaller than 1 V.
Description of the Related Art
FIG. 1 hereof corresponds to FIG. 3 of French patent application 2969328 of Dec. 17, 2010 (B10442). This drawing shows an example of a circuit generating a reference voltage in the order of 0.1 V. This circuit comprises, between two terminals of application of a power supply voltage VDD and ground GND:

    
    
        a MOS transistor M1 in series with a bipolar transistor Q1, of type NPN, having its emitter on the side of ground GND;
        a MOS transistor M2 in series with a bipolar transistor Q2 (of type NPN, having its emitter on the side of ground GND) and with a resistor R1, the emitter of transistor Q2 defining an output terminal of the circuit providing a reference voltage VOUT, transistors M1 and M2 being assembled as a current mirror; and
        the power supply terminals of a follower assembly 3.
    
    


The input of the follower assembly is connected to the collector of transistor Q1 and its output is connected by an optional resistor R2 to the base of transistor Q2. A resistive dividing bridge formed of resistors R3 and R4 in series is connected between the output terminal of follower assembly 3 and ground GND. The midpoint of this dividing bridge is connected to the base of transistor Q1. Resistor R4 is connected between the base of transistor Q1 and ground GND.
Due to the current mirror formed of MOS transistors M1 and M2, transistors Q1 and Q2 receive the same collector current.
As indicated by the above-mentioned French patent application, reference voltage VOUT can be written as follows, neglecting base current ib2 of transistor Q2:

VOUT=VBE1 *(R4/R3)+(kT/q)*In(p2|1),  (1)

where VBE1 designates the base-emitter voltage of transistor Q1, k designates Boltzmann's constant, q designate the electron charge, T designates the temperature in Kelvin, and In(p2|1) designates the natural logarithm of surface ratio p2|1 between transistors Q1 and Q2 (p2|1 being greater than 1).

Follower assembly 3 is formed of a current source 4 and of a MOS transistor M3. The gate of transistor M3 corresponds to the input of follower assembly 3 and the source of MOS transistor M3 corresponds to the output of follower assembly 3. The follower assembly has the voltage present on its input follow on its output and delivers the current necessary to drive the bases of transistors Q1 and Q2 and for resistor R4. This circuit has an infinite input impedance, and no current flows through the gate of MOS transistor M3.
The base currents of transistors Q1 and Q2 are equal (due to transistors Ml and M2 assembled as a current mirror). Resistor R2 is added to cancel the effect of the base currents on the reference voltage. The compensation will be optimal if the values of resistances R2 and R3 are equal.
Resistor R1 sets the current in the two branches of the assembly. Power supply voltage VDD can be written as:

VDD=VOUT+VBE2+R2*ib2+V4,   (2)

where VOUT is the reference voltage generated by circuit, VBE2 is the base-emitter voltage of transistor Q2, and V4 is the voltage drop across current source 4.

In practice, in current integrated circuit technologies, the base-emitter voltage of a bipolar transistor is in the order of 0.8 V and the drain-source voltage of a MOS transistor at saturation is in the order of 0.1 V. If a reference voltage VOUT of 0.1 V is desired to be generated, formula (2) thus provides VDD=0.1+0.8+0.1=1 V, neglecting term R2*ib2, which is much smaller than 0.1 V.
FIG. 2 hereof corresponds to FIG. 2 of U.S. Pat. No. 7,408,400. This drawing shows an example of a circuit generating a reference voltage in the order of 0.1 V. This circuit comprises, between two terminals of application of a power supply voltage VDD and ground GND:

    
    
        a current source 11 generating a current I1 in series with a bipolar transistor Q3, of type NPN;
        a current source 13 generating a current I2 in series with a bipolar transistor Q4, of type NPN;
        a current source 15 generating the same current I1 as current source 11 in series with a bipolar transistor Q5, of type NPN, and with a resistor R7, the base of transistor Q5 being connected to the collector of transistor Q4; and
        a bipolar transistor Q6, of type NPN, in series with a current source 17, the base of transistor Q6 being connected to the collector of transistor Q5 and the emitter of transistor Q6 being connected to the base of transistor Q4.
    
    


Resistor R5 is connected between the base of transistor Q3 and ground GND. A resistor R6 is connected between the collector of transistor Q4 and the base of transistor Q3. A bipolar transistor Q7 is connected between terminal VDD and the emitter of transistor Q5. The base of transistor Q7 is connected to the collector of transistor Q3. The junction point of the emitters of transistors Q5 and Q7 forms output VOUT of the circuit.
Transistors Q3 and Q5 receive a same collector current Ii. As indicated by the above-mentioned US patent, reference voltage VOUT can be written as follows:

VOUT=VBE3*(R6/R5)+(kT/q)*In(p5|3),   (3)

where VBE3 designates the base-emitter voltage of transistor Q3, k, q, and T have been previously defined, and p5|3 designates the surface ratio between transistors Q3 and Q5 (p5|3 being greater than 1).

Power supply voltage VDD can be written as:

VDD=VOUT+VBE7+V11,   (4)

where VOUT is the reference voltage generated by circuit, VBE7 is the base-emitter voltage of transistor Q7, and V11 is the voltage drop across current source 11.

In practice, in current integrated circuit technologies, the base-emitter voltage of a bipolar transistor is in the order of 0.8 V and the drain-source voltage of a MOS transistor at saturation is in the order of 0.1 V. If a reference voltage VOUT of 0.1 V is desired to be generated, formula (4) thus provides VDD=0.1+0.8+0.1=1 V.
The power supply voltages of the circuits of FIGS. 1 and 2 are greater than or equal to 1 V.
Further, in the circuits of FIGS. 1 and 2, if voltage VOUT is desired to be increased by 1 V, the power supply voltage should increase by 1 V.
Recent circuits in CMOS technology operate under power supply voltages smaller than or equal to 1 V. The circuits of FIGS. 1 and 2 can thus not be used since they require a power supply voltage greater than 1 V.
BRIEF SUMMARY
It would be desirable to provide a reference voltage generation circuit having a power supply voltage smaller than 1 V.
It would also be desirable to provide such a circuit capable of generating a reference voltage greater than 0.1 V.
Thus, an embodiment provides a circuit for generating a reference voltage, comprising, between first and second terminals of application of a power supply voltage: a first current source in series with a first bipolar transistor; a second current source in series with a first resistive element, the junction point between the second current source and the first resistive element being connected to the base of the first bipolar transistor; a third current source in series with a second bipolar transistor, the third current source being assembled as a current mirror with the first current source; a second resistive element between the base of the second bipolar transistor and the junction point of the current source and of the first resistive element; and a fourth current source in series with a third resistive element, the junction point of the fourth current source and of the third resistive element defining a third terminal providing the reference voltage, the fourth current source forming a current mirror with the second current source.
According to an embodiment, a fifth current source is connected between the first terminal and the third terminal, and a fourth resistive element is series-connected with the second bipolar transistor, the fifth current source forming a current mirror with the first current source.
According to an embodiment, the current sources are formed of MOS transistors.
According to an embodiment, the surface area of the collector of the second bipolar transistor is larger than the surface area of the collector of the first bipolar transistor.
The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIGS. 1 and 2, previously described, illustrate two examples of circuits for generating a 0.1-V reference voltage; and
FIGS. 3 and 4 illustrate two embodiments of a circuit for generating a 0.1-V reference voltage.



DETAILED DESCRIPTION
The present description corresponds to the case of transistors in CMOS technology. It may however be applied to any other transistor technology or to a combination of different technologies. In the following, “PMOS transistor” will designate P-channel MOS transistors.
FIG. 3 illustrates an embodiment of a reference voltage generation circuit. This circuit comprises, between two supply terminals respectively providing a power supply voltage VDD and of ground GND:

    
    
        a PMOS transistor M4 in series with a bipolar transistor Q8, of type NPN, having its emitter on the side of ground GND;
        a PMOS transistor M5 in series with a resistor R8, the base of transistor Q8 being connected to the drain of transistor M5;
        a PMOS transistor M6 in series with a bipolar transistor Q9, of type NPN, the emitter being on the side of ground GND and transistors M4 and M6 being assembled as a current mirror; and
        a PMOS transistor M7 in series with a resistor R10, the gate of transistor M7 being connected to the collector of transistor Q9 and to the gate of transistor M5, transistors M5 and M7 thus forming a current mirror, the drain of transistor M7 forming a reference voltage terminal VOUT.
    
    


A resistor R9 is connected between the base of transistor Q9 and the drain of transistor M5.
The current mirror formed by transistors M4 and M6 results in that transistors Q8 and Q9 receive equal collector currents Ic8 and Ic9. The circuit is designed so that transistor M5 is in saturation state.
Power supply voltage VDD can be written as:

VDD=VBE8+VM5,   (5)

where VBE8 is the base-emitter voltage of transistor Q8, and VM5 is the drain-source voltage of transistor M5.

In practice, in current integrated circuit technologies, the base-emitter voltage of a bipolar transistor is in the order of 0.8 V and the drain-source voltage of a
MOS transistor at saturation is in the order of 0.1 V. Formula (5) thus provides VDD=0.8+0.1=0.9 V.
There appears from formula (5) that voltage VDD is smaller than 1 V and that it is independent from value VOUT, conversely to the cases of circuits of FIGS. 1 and 2 and of formulas (2) and (4).
Further, transistor M7 operates in linear state when reference voltage VOUT is smaller than voltage VBE8 (0.8 V). For a 0.9V power supply voltage, it is thus possible to set reference voltage VOUT in a range from 0.1 V to 0.8 V.
Reference voltage VOUT can be written as:

VOUT=R10*IM7,   (6)

where IM7 is the current in resistor R10. Transistors M5 and M7 being assembled as a current mirror, current IM7 is the copy of current IM5.

Current IM7 can be written as:

IM7=IM5=(VBE8/R8)+ib8+ib9,   (7)

where ib8 and ib9 are the base currents of transistors Q8 and Q9. The collector currents of transistors Q8 and Q9 being equal, currents ib8 and ib9 are equal.

Current ib9 can be written as:

ib9=ΔVBE/R9,

where ΔVBE=VBE8−VBE9=(kT/q)*ln(p9|8), VBE8 and VBE9 designate the base-emitter voltages of transistor Q8 and Q9 and In(p9|8) designates the natural logarithm of surface area ratio p98 between transistors Q8 and Q9 (p9|8 being greater than 1).

Reference voltage VOUT can be written as:

VOUT=R10*[(VBE8/R8)+(2*kT/q*R9)*ln(p9|8)],   (8)

An advantage of such a circuit is that power supply voltage VDD is 0.9 V only. This circuit may be used in recent circuits in CMOS technology operating under power supply voltages smaller than 1 V.
Another advantage is that for a power supply voltage of VDD of 0.9 V, the circuit can generate a reference voltage VOUT in the range from 0.1 V to 0.8 V.
However, as shown by formulas (6) and (7), reference voltage VOUT depends on base current ib9 of transistor Q9. Current collector ic9 of transistor Q9 is determined by relation ic9=β*ib9, β being the gain of transistor Q9. Gain β varies along with temperature and manufacturing dispersions. Currents ic8 and ic9 vary accordingly. Voltage VBE8 varies according to current Ic8. According to formula (8), voltage VOUT depends on VBE8. The variation of gain β of transistor Q9 thus degrades the accuracy of the generated reference voltage VOUT. As an example, for a variation of gain β of transistor Q9 by a factor 2, voltage VOUT varies by approximately 2%.
A reference voltage VOUT independent from the variation of current gain β would be desired.
FIG. 4 illustrates another embodiment of a reference voltage generation circuit having the advantages of the embodiment of FIG. 3 while avoiding the possible variation of VOUT with gain β.
This circuit comprises the elements of the circuit of FIG. 3 designated with the same reference numerals. Further, a resistor R11 is placed between the emitter of transistor Q9 and ground GND and a PMOS transistor M10 is connected between power supply voltage VDD and the drain of transistor M7. The source of transistor M10 is connected to voltage VDD. Transistor M10 forms a current mirror with transistors M4 and M6.
Power supply voltage VDD remains equal to:

VDD=VBE8+VM5,   (5)

Reference voltage VOUT can be written as:

VOUT=R10*IR10=R10*(IM7+IM10)   (9)

where IR10 is the current in resistor R10 and IM10 is the drain current of transistor M10. Transistors M4, M6, and M10 being assembled as a current mirror, currents ic8, ic9, and IM10 are equal. Transistors M5 and M7 being assembled as a current mirror, currents IM5 and IM7 are equal.

Current ic9 can be written as:

ic9=VE/R11−ib9,   (10)

where VE is the voltage across resistor R11.

Voltage VE can be written as:

VE=ΔVBE−R9*ib9,

where ΔVBE=VBE8−VBE9=(kT/q)*ln(p9|8).

Current ic9 can be written as:

ic9=ΔVBE/R11−ib9*(1+R9/R11).

Current IR10 can thus be written as:

IR10=VBE8/R8+2*ib9+ΔVBE/R11−ib9*(1+R9/R11).

If resistors R9 and R11 are equal, current IR10 no longer depends on current ib9, IR10 can be written as:

IR10=VBE8/R8+ΔVBE/R11

Reference voltage VOUT can thus be written as:

VOUT=R10*[(VBE8/R8)+(kT/q*R9)*In(p9|8)]  (11)

As shown by formula (11), current ic9 no longer depends on gain β, conversely to the case of the circuit of FIG. 3. Voltage VBE8 is no longer affected by the variation of gain β and, since voltage VOUT depends on VBE8, the accuracy of voltage VOUT is no longer affected by gain β.
An advantage of such a circuit is that a possible variation of gain β of transistor Q9 does not affect the accuracy of reference voltage VOUT.
Although term resistor has here been used to designate elements R1 to R11, it should be noted that these elements may be formed of any resistive element such as a resistor-connected MOS transistor.
The resistance values may be in the range from 1 to 100 kΩ, for example, 50 kΩ.
Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting.
The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

## Claims

1. A circuit for generating a reference voltage, comprising:
first and second supply terminals configured to provide a power supply voltage;
a first MOS transistor and a first bipolar transistor electrically coupled in series between the first and second supply terminals;
a second MOS transistor and a first resistive element electrically coupled between the first and second supply terminals, the second MOS transistor and the first resistive element being directly electrically coupled to each other by a first junction point that is electrically coupled to a base of the first bipolar transistor;
a third MOS transistor and a second bipolar transistor electrically coupled in series between the first and second supply terminals, the third MOS transistor forming a current mirror with the first MOS transistor;
a second resistive element electrically coupled between a base of the second bipolar transistor and the first junction point; and
a fourth MOS transistor and a third resistive element electrically coupled between the first and second supply terminals, the fourth MOS transistor and the third resistive element being electrically coupled to each other at a second junction point that defines an output terminal configured to provide the reference voltage, the fourth MOS transistor forming a current mirror with the second MOS transistor, a gate of the fourth MOS transistor and a gate of the second MOS transistor being electrically coupled to each other at a third junction point, the third MOS transistor being electrically coupled between the first supply terminal and the third junction point.

2. The device of claim 1, comprising:
a fifth MOS transistor electrically coupled between the first supply terminal and the output terminal, and
a fourth resistive element series-connected with the second bipolar transistor, the fifth MOS transistor forming a current mirror with the first MOS transistor.

3. The device of claim 1, wherein a surface area of a collector of the second bipolar transistor is greater than a surface area of a collector of the first bipolar transistor.

4. A circuit for generating a reference voltage, comprising:
first and second supply terminals configured to provide a power supply voltage;
a first transistor and a second transistor electrically coupled in series between the first and second supply terminals;
a third transistor electrically coupled between the first and second supply terminals, the third transistor being directly electrically coupled to the second transistor;
a fourth transistor and a fifth transistor electrically coupled in series between the first and second supply terminals, the fourth transistor forming a current mirror with the first transistor, the second and fifth transistors having respective control terminals electrically coupled to each other at a first junction point and the third transistor is electrically coupled between the first supply terminal and the first junction point; and
a sixth transistor and a first resistive element electrically coupled between the first and second supply terminals, the sixth transistor and the first resistive element being electrically coupled to each other at a second junction point that defines an output terminal configured to provide the reference voltage, the sixth transistor forming a current mirror with the third transistor, a control terminal of the sixth transistor and a control terminal of the third transistor being electrically coupled to each other by a third junction point, the fourth transistor being electrically coupled between the first supply terminal and the third junction point.

5. The device of claim 4, comprising:
a second resistive element electrically coupled to the first transistor by the first junction point.

6. The device of claim 4, comprising:
a second resistive element electrically coupled between the control terminal of the fifth transistor and the first junction point.

7. The device of claim 4, wherein the second and fifth transistors are bipolar transistors.

8. The device of claim 7, wherein a surface area of a collector of the fifth transistor is greater than a surface area of a collector of the second transistor.

9. The device of claim 4, comprising:
a seventh transistor electrically coupled between the first supply terminal and the output terminal, the seventh transistor forming a current mirror with the first transistor.

10. The device of claim 4, comprising:
a second resistive element electrically coupled in series with the fifth transistor.

11. The device of claim 4, wherein the first, third, fourth, and sixth transistors are MOS transistors.

12. A circuit for generating a reference voltage, comprising:
first and second supply terminals configured to provide a power supply voltage;
a first transistor and a second transistor electrically coupled in series between the first and second supply terminals;
a third transistor electrically coupled between the first and second supply terminals, the third transistor being directly electrically coupled to the second transistor;
a fourth transistor and a fifth transistor electrically coupled in series between the first and second supply terminals, the fourth transistor forming a current mirror with the first transistor, the second and fifth transistors having respective control terminals electrically coupled to each other at a first junction point and the third transistor is electrically coupled between the first supply terminal and the first junction point;
a sixth transistor and a first resistive element electrically coupled between the first and second supply terminals, the sixth transistor and the first resistive element being electrically coupled to each other at a second junction point that defines an output terminal configured to provide the reference voltage, the sixth transistor forming a current mirror with the third transistor, a control terminal of the sixth transistor and a control terminal of the third transistor being electrically coupled to each other at a third junction point, the fourth transistor being electrically coupled between the first supply terminal and the third junction point;
a second resistive element electrically coupled to the first transistor by the first junction point; and
a third resistive element electrically coupled between the first and second supply terminals, the second transistor and the third resistive element being electrically coupled to each other by the first junction point.

13. The device of claim 12, wherein the second resistive element is electrically coupled between the control terminal of the fifth transistor and the first junction point.

14. The device of claim 12, wherein the second and fifth transistors are bipolar transistors.

15. The device of claim 14, wherein a surface area of a collector of the fifth transistor is greater than a surface area of a collector of the second transistor.

16. The device of claim 12, comprising:
a seventh transistor electrically coupled between the first supply terminal and the output terminal, the seventh transistor forming a current mirror with the first transistor.

17. The device of claim 12, comprising:
a fourth resistive element electrically coupled in series with the fifth transistor.

18. The device of claim 12, wherein the first, third, fourth, and sixth transistors are MOS transistors.