Reference voltage generation circuit

An FDSOI reference voltage generation circuit, including a CTAT current generation circuit; a PTAT-type voltage generation circuit including a first branch including first and second series-connected transistors, the front surface gates of the first and second transistors being connected to the conduction node of the second transistor opposite to the first transistor; a third diode-assembled transistor having a conduction node connected to an output node of the PTAT voltage generation circuit and having its other conduction node forming a reference voltage supply node; and a current mirror; wherein the first and second transistors are of LVT type and the third transistor is of RVT type.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of French patent application number 15/61551, filed Nov. 30, 2015, which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

BACKGROUND

The present disclosure generally relates to the field of electronic systems, and more particularly aims at a reference voltage generation circuit.

DISCUSSION OF THE RELATED ART

Many electronic systems use a reference voltage generation circuit to generate, from a DC power supply voltage of the system, a DC reference voltage independent from fluctuations of the power supply voltage and independent from temperature variations. Such a circuit is generally integrated in a semiconductor chip, which may be an autonomous chip or which may comprise other circuits intended to implement other functions of the system.

Reference voltage generation circuits formed from bipolar transistors have already been provided. A disadvantage of such circuits is that, to obtain a good temperature stability, the reference voltage should be relatively high, typically in the order of 1.2 V.

In certain electronic systems, particularly in low power supply voltage systems (for example, systems intended to be powered under a voltage in the range from 1.2 V to 4 V), it is desired to have a lower reference voltage, typically smaller than 1 V, for example, a voltage in the order of 0.9 V. Circuits for generating a reference voltage smaller than 1 V formed based on MOS transistors have been provided. Examples of such circuits are in particular described in the following publications: [1] “A 300 nW, 15 ppm/C, 20 ppm/V CMOS Voltage Reference Circuit Consisting of Subthreshold MOSFETs”, Ken Ueno, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 44, No. 7, July 2009; [2] “173 nA-7.5 ppm/C-771 mV-0.03 mm2 CMOS Resistorless Voltage Reference”, A. Samir, 2011 Faible Tension Faible Consommation (FTFC); [3] “A 280 NA, 87 PPM/oC, HIGH PSRR FULL CMOS VOLTAGE REFERENCE AND ITS APPLICATION”, Song QIN, 978-1-4673-1717-7112/$31.00 ©2012 IEEE; [4] “A Sub-1-V, 10 ppm/C, Nanopower Voltage Reference generator”, Giuseppe De Vita, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, No. 7, July 2007; [5] “A Sub-1V 32 nA Process, Voltage and Temperature Invariant Voltage Reference Circuit”, Anvesha A, 2013 26th International Conference on VLSI Design; and [6] “1.2-V Supply, 100-nW, 1.09-V Bandgap and 0.7-V Supply, 52.5-nW, 0.55-V Subbandgap Reference Circuits for Nanowatt CMOS LSIs”, Yuji Osaki, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 48, No. 6, June 2013.

Such circuits however have various disadvantages. In particular, such circuits are relatively sensitive to manufacturing process variations, and accordingly have a relatively low intrinsic accuracy. In other words, two different circuits formed according to the same process may, due to process dispersions, generate different reference voltages. In the circuit described in above-mentioned article [1], the variability of the reference voltage according to manufacturing process variations is actually searched for and used to characterize and compensate for process dispersions.

The forming of a reference voltage generation circuit based on MOS transistors is here more particularly considered, this circuit having a better intrinsic accuracy than known circuits, that is, supplying a reference voltage which is less dependent on method dispersions than in known circuits.

It should be noted that to guarantee that different chips do supply the same reference voltage, post-manufacturing adjustment steps may be provided. However, such steps, as well as the possible provision of adjustment components on the chips, generate an excess cost which is all the greater as the intrinsic accuracy of the circuit is low.

It would be desirable to have a circuit for generating a reference voltage overcoming all or part of the disadvantages of known circuits, and in particular having a better intrinsic accuracy than known circuits.

SUMMARY

Thus, an embodiment provides a circuit for generating a reference voltage formed in FDSOI technology, comprising: a first circuit for generating a CTAT-type bias current; a second circuit for generating a PTAT-type voltage comprising a first branch comprising first and second series-connected transistors, the front surface gates of the first and second transistors being connected to the conduction node of the second transistor opposite to the first transistor; a third diode-assembled transistor having a conduction node connected to a node for supplying the output voltage of the second circuit and having its other conduction node forming a node for supplying the reference voltage; and a current mirror imposing, in the third transistor on the one hand, and in the first branch, on the other hand, currents proportional to the bias current, wherein the first and second transistors are of LVT type and the third transistor is of RVT type.

According to an embodiment, the first transistor has a first front surface gate oxide thickness and the second and third transistors have a second front surface gate oxide thickness greater than the first thickness.

According to an embodiment, the first, second and third transistors are NMOS transistors, the drain of the first transistor being connected to the source of the second transistor, the drain of the second transistor being connected to the gates of the first and second transistors, and the source of the third transistor being connected to a node for supplying the output voltage of the second circuit.

According to an embodiment, the second circuit further comprises a second branch comprising fourth and fifth series-connected transistors, the front surface gates of the fourth and fifth transistors being connected to the conduction node of the fifth transistor opposite to the fourth transistor, and the conduction node of the fourth transistor opposite to the fifth transistor being connected to the junction point of the first and second transistors.

According to an embodiment, the current mirror imposes in the second branch a current proportional to the bias current.

According to an embodiment, the junction point of the fourth and fifth transistors forms a node for supplying the output voltage of the second circuit.

According to an embodiment, the fourth and fifth transistors are NMOS transistors, the drain of the fourth transistor being connected to the source of the fifth transistor, and the drain of the fifth transistor being connected to the gates of the fourth and fifth transistors.

According to an embodiment, the fourth and fifth transistors are both of RVT type or both of LVT type.

According to an embodiment, the first circuit comprises sixth and seventh transistors assembled as a current mirror and an eighth transistor series-connected with the seventh transistor, the sixth and seventh transistors being of same LVT or RVT type and having the same front surface gate oxide thickness, and the sixth transistor having a channel-width-to-channel-length ratio greater than that of the seventh transistor.

According to an embodiment, the eighth transistor is of LVT type.

According to an embodiment, the sixth, seventh, and eighth transistors are of NMOS type.

According to an embodiment, the eighth transistor has its front surface gate coupled to the reference voltage supply node.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and detailed. In particular, the uses which may be made of the described reference voltage generation circuits are not detailed, the described embodiments being compatible with usual applications of a reference voltage generation circuit. Unless otherwise specified, expressions “approximately”, “about”, “substantially”, and “in the order of” mean to within 10%, preferably to within 5%. In the present description, term “connected” is used to designate a direct electric connection, with no intermediate electronic component, for example, by means of one or a plurality of conductive tracks, and term “coupled” or term “linked” is used to designate either a direct electric connection (then meaning “connected”) or a connection via one or a plurality of intermediate components (resistor, diode, capacitor, etc.).

FIG. 1is an electric diagram of an embodiment of a reference voltage generation circuit.

The circuit ofFIG. 1is formed from MOS transistors in FDSOI (“Fully Depleted Semiconductor On Insulator”) technology. More particularly, the MOS transistors of the circuit ofFIG. 1are formed inside and on top of a structure of semiconductor-on-insulator type comprising a stack of a semiconductor substrate coated with a layer of a dielectric material, the layer being itself coated with a semiconductor layer. Each transistor comprises an insulated conductive gate, called front surface gate, coating the surface of the semiconductor layer opposite to the dielectric layer. The channel-forming region of the transistor is located under the front surface gate, in the semiconductor layer. The source and drain regions of the transistor are for example implanted regions formed in the semiconductor layer, on either side of the channel-forming region. The source and drain regions are respectively P-type doped for a P-channel transistor (PMOS) and N-type doped for an N-channel transistor (NMOS). The substrate region located under the dielectric layer, opposite the channel-forming region of the transistor, is called rear surface gate, and may be biased to control the threshold voltage of the transistor.

A manufacturing process in FDSOI technology is here considered, where, for each conductivity type (NMOS and PMOS), two types of transistors, respectively called RVT (“Regular Voltage Threshold”) and LVT (“Low Voltage Threshold”) having, for identical front surface gate dimensions and for identical rear surface gate bias voltages, different threshold voltages, are available. More particularly, for identical front surface gate dimensions and identical rear surface gate bias voltages, RVT transistors have a greater threshold voltage than LVT transistors. In this example, to obtain transistors having different threshold voltages, the doping of the substrate region located in contact with the dielectric layer, opposite the channel-forming region of the transistor (corresponding to the rear surface gate of the transistor), is varied. More particularly, LVT transistors comprise a well of same conductivity type as the source and drain regions of the transistor, extending in the substrate, under the dielectric layer, opposite the channel-forming region of the transistor, and RVT transistors comprise a well of a conductivity type opposite to that of the source and drain regions, extending in the substrate, under the dielectric layer, opposite the channel-forming region of the transistor. As a variation, the LVT or RVT behavior of the transistors may be obtained by varying a parameter other than the doping of the substrate region located under the channel-forming region of the transistor, for example, by varying the doping of the front surface gate of the transistor.

Further, in this example, a manufacturing process in FDSOI technology where each of the fourth above-mentioned transistor types, that is, the NMOS LVT type, the NMOS RVT type, the PMOS LVT type, and the PMOS RVT type, may be obtained in two sub-types, respectively called SO and DO, corresponding to different front surface gate insulator or oxide thicknesses. More particularly, transistors of SO (simple oxide) type have a first front surface gate oxide thickness, and transistors of DO (double oxide) type have a second front surface gate oxide thickness greater than the first thickness, for example, twice greater than the first thickness.

The circuit ofFIG. 1comprises terminals or nodes VDD and VSS of application of a power supply voltage VSUPPLY, and a terminal or a node REF for supplying a reference voltage VREF. In the shown example, node VDD is intended to receive the high potential of power supply voltage VSUPPLY, and node VSS is intended to receive the low potential of power supply voltage VSUPPLY. Reference voltage VREF supplied on node REF is referenced to node VSS, which for example corresponds to the circuit ground.

The circuit ofFIG. 1comprises a circuit101for generating a bias current I of CTAT (“Complementary To Absolute Temperature”) type, that is, having an intensity which decreases as the temperature increases. In the shown example, current I is generated from a gate-source voltage difference between two transistors N1and N2of the same type having different dimensions. Such a gate-source voltage difference is applied across a transistor N3operating in linear state to generate current I. In this example, transistors N1, N2, and N3are NMOS transistors. Transistors N1and N2are for example both LVT transistors. As a variation, transistors N1and N2are both RVT transistors. Transistors N1and N2are for example both double oxide (DO) transistors. Transistor N3is for example an NMOS double oxide (DO) LVT transistor. Ratio KN1of channel width WN1to channel length LN1of transistor N1is different from ratio KN2of channel width WN2to channel length LN2of transistor N2. As an example, ratio KN1is smaller than ratio KN2so that, in operation, the gate-source voltage of transistor N1is greater than the gate-source voltage of transistor N2. Transistors N1and N2are current-mirror assembled. More particularly, transistor N1has its front surface gate connected to its drain and has its source coupled to node VSS. The front surface gate of transistor N2is connected to the front surface gate of transistor N1. The source of transistor N2is coupled to node VSS via transistor N3. More particularly, the drain of transistor N3is connected to the source of transistor N2, and the source of transistor N3is coupled to node VSS. In this example, the front surface gate of transistor N3is connected to output node REF of the circuit.

In addition to transistors N1, N2and N3, circuit101for generating bias current I comprises a PMOS transistor P1coupling the drain of transistor N1to node VDD, and a PMOS transistor P2coupling the drain of transistor N2to node VDD. Transistor P1has its drain connected to the drain of transistor N1and transistor P2has its drain connected to the drain of transistor N2. Transistor P1has its source coupled to node VDD and transistor P2has its source coupled to node VDD. Transistors P1and P2are current-mirror assembled. More particularly, transistor P1has its front surface gate connected to the front surface gate of transistor P2, and transistor P2has its front surface gate connected to its drain. Transistors P1and P2are for example both RVT transistors. As a variation, transistors P1and P2are both LVT transistors. Transistors P1and P2are for example both double oxide (DO) transistors.

The circuit ofFIG. 1further comprises a circuit103for generating a voltage V of PTAT (“Proportional To Absolute Temperature”) type, that is, having a value which increases as the temperature increases.

In this example, circuit103comprises a first branch comprising a transistor N4series-connected with a transistor N5, and a second branch comprising a transistor N6series-connected with a transistor N7. In this example, transistors N4, N5, N6, and N7are of NMOS type.

Transistors N4and N5are for example respectively of simple oxide (SO) LVT type and of double oxide (DO) LVT type. As a variation, transistors N4and N5of the first branch are respectively of simple oxide (SO) LVT type and of double oxide (DO) RVT type. As a variation, transistors N4and N5of the first branch are respectively of simple oxide (SO) RVT type and of double oxide (DO) LVT type. More generally, the first branch is a so-called mixed oxide thickness branch (that is, its transistor located on the side of node VSS, that is, its transistor N4, is a simple-oxide transistor, and its transistor opposite node VSS, that is, its transistor N5, is a double-oxide transistor), having at least two transistors N4and N5of LVT type.

Transistor N4has its source coupled to node VSS and its drain connected to the source of transistor N5. Transistor N5has its drain connected to its front surface gate. The front surface gate of transistor N5is further connected to the front surface gate of transistor N4. Transistor N6has its source connected to the junction point of transistors N4and N5, that is, to the source of transistor N5and to the drain of transistor N4. Transistor N6has its drain connected to the source of transistor N7. Transistor N7has its drain connected to its front surface gate. The front surface gate of transistor N7is further connected to the front surface gate of transistor N6. The junction point of transistors N6and N7, that is, the source node of transistor N7or drain node of transistor N6, forms the node for supplying output voltage V of circuit103(referenced to node VSS).

In this example, circuit103further comprises a PMOS transistor P3coupling the drain of transistor N5to node VDD, and a PMOS transistor P4coupling the drain of transistor N7to node VDD. Transistor P3has its drain connected to the drain of transistor N5, and transistor P4has its drain connected to the drain of transistor N7. Transistors P3and P4each have their source coupled to node VDD. Each of transistors P3and P4is assembled to form a current mirror with transistor P2. More particularly, transistor P3has its front surface gate connected to the front surface gate of transistor P2, and transistor P4has its front surface gate connected to the front surface gate of transistor P2. Transistors P3and P4are for example both RVT transistors. As a variation, transistors P3and P4are both LVT transistors. Transistors P3and P4are for example both double-oxide (DO) transistors.

The circuit ofFIG. 1further comprises a diode-assembled transistor N8, where CTAT-type bias current I is applied, and having a conduction node receiving PTAT-type output voltage V of circuit103. In this example, transistor N8is an NMOS transistor. Transistor N8for example is an RVT transistor, for example, a double-oxide transistor (DO). The source of transistor N8is connected to the node for supplying output voltage V of circuit103, that is, to the source node of transistor N7and to the drain node of transistor N6in this example. The drain of transistor N8is connected to its front surface gate and to output node REF of the circuit ofFIG. 1. In this example, the circuit ofFIG. 1further comprises a PMOS transistor P5coupling the drain of transistor N8to node VDD. Transistor P5has its drain connected to the drain of transistor N8and its source coupled to node VDD. Transistor P5is assembled to form a current mirror with transistor P2. More particularly, transistor P5has its front surface gate connected to the front surface gate of transistor P2. Transistor P5may be of RVT type or of LVT type. As an example, transistor P5is a double-oxide transistor (DO). Transistors P1, P2, P3, P4and P5are for example identical, that is, of the same type (RVT or LVT, of same oxide thickness DO or SO) and substantially have the same dimensions.

In operation, a same bias current I flows through the branch comprising transistors P1and N1, and through the branch comprising transistors P2, N2, and N3. Transistor N3, operating in linear state, sees between its terminals a PTAT voltage equal to the difference between the gate-source voltage of transistor N1and the gate-source voltage of transistor N2, which sets the value of current I. The internal resistance of transistor N3increases with temperature faster than the PTAT voltage seen by transistor N3, so that current I (which is the ratio of the voltage across transistor N3to the internal resistance of transistor N3) decreases with temperature.

Bias current I generated by circuit101is copied in the branch comprising transistors P3, N5, and N4, and in the branch comprising transistors P4, N7, and N6. Under the effect of this current a PTAT-type voltage v1is supplied onto the junction point of transistors N4and N5, and a voltage v2, also of PTAT type but having a level greater than v1, is supplied onto the junction point of transistors N6and N7. Voltages v1and v2are referenced to node VSS. In this example, output voltage V of circuit103is voltage v2.

The bias current I generated by circuit101is further copied in the branch comprising transistors P5and N8. Output voltage VREFof the circuit ofFIG. 1is equal to the sum of the gate-source voltage of transistor N8and of output voltage V of circuit103. When the temperature increases, current I tends to decrease, and the threshold voltage of transistor N8tends to decrease, which would tend to lower voltage VREF. However, output voltage V of circuit103increases with temperature, which enables to maintain a relative temperature stability for voltage VREF.

Power supply voltage VSUPPLYand the dimensions of the transistors of the circuit ofFIG. 1are preferably selected so that, in operation, transistors P1, P2, P3, P4, P5, N4, N5, and N8are in saturation state, transistors N1, N2, N6, and N7are in subthreshold conduction state, and transistor N3is in linear state.

where WNi, LNiand KNirespectively designate the channel width of transistor Ni, the channel length of transistor Ni, and the channel-width-to-channel-length ratio of transistor Ni, i being an integer in the range from 1 to 8. As an example, transistors of the SO (simple oxide) type are capable of withstanding with no degradation a maximum voltage in the order of 1 V, and DO-type transistors are capable of withstanding with no degradation a maximum voltage in the order of 1.8 V.

As an example, all the NMOS transistors of the circuit ofFIG. 1have their rear surface gates coupled to ground, that is, to node VSS, and all the PMOS transistors of the circuit have their rear surface gates coupled to node VDD of application of the high power supply potential of the circuit. The described embodiments are however not limited to this specific case. As a variation, all the transistors of the circuit ofFIG. 1may have, in operation, their rear surface gates biased to a same reference potential different from the potential of node VSS or VDD. As a variation, different transistors of the circuit ofFIG. 1may have, in operation, their rear surface gates biased to different potentials.

FIG. 2is a diagram illustrating the behavior of the circuit ofFIG. 1. More particularly,FIG. 2shows the variation according to temperature, within a temperature range from −40° C. to +125° C., of bias current I, in nanoamperes, of voltages v1and v2, in mV, and of output voltage VREF, in mV of the circuit ofFIG. 1. As shown inFIG. 2, current I substantially linearly decreases according to temperature from a high value in the order of 20.2 nA for a −40° C. temperature to a low value in the order of 16.5 nA for a 125° C. temperature, voltage v1substantially linearly increases according to temperature from a low value in the order of 172 mV for a −40° C. temperature to a high value in the order of 215 mV for a 125° C. temperature, and voltage v2substantially linearly increases according to temperature from a low value in the order of 280 mV for a −40° C. temperature to a high value in the order of 385 mV for a 125° C. temperature. Reference voltage VREF follows a bell shape between 928 mV and 934 mV within the temperature range from −40° C. to +125° C.

The tests which have been performed have shown that the circuit ofFIG. 1has a very good intrinsic accuracy as compared with existing circuits (that is, the output voltage being almost independent from process variations), as illustrated, in particular, inFIG. 3.

FIG. 3shows the variation of output voltage VREF of the circuit ofFIG. 1according to temperature, within the temperature range from −40° C. to +125° C., at the different limits of the variations of parameters of the manufacturing process, in the considered FDSOI manufacturing technology (here, the 28-nm FDSOI technology). More particularly,FIG. 3comprises a curve FSA corresponding to the case where the NMOS transistors are faster than usual and where the PMOS transistors are slower than usual, a curve FFA corresponding to the case where the NMOS and PMOS transistors are faster than usual, a curve SFA corresponding to the case where the NMOS transistors are slower than usual and the PMOS transistors are faster than usual, a curve SSA corresponding to the case where the NMOS and PMOS transistors are slower than usual, and a curve TYP corresponding to the case where the NMOS and PMOS transistors have an average speed.

As shown inFIG. 3, the inaccuracy of the circuit ofFIG. 1due to manufacturing dispersions is in the order of 5.5 mV at 25° C. for a typical reference voltage in the order of 934 mV, which corresponds to a 0.5% peak-to-peak inaccuracy. The measurements which have been performed show that at a given temperature, the ratio of the standard deviation of the distribution of the reference voltages supplied by the circuits of a batch representative of the manufacturing process variations to the average reference voltage of the distribution is in the order of +/−0.1%.

The inventors have determined that the good intrinsic accuracy of the circuit ofFIG. 1, that is, the fact for the reference voltage delivered by the circuit to be relatively little dependent on process variations, mainly results from the combination of a circuit103for generating a PTAT-type voltage V having a first branch (transistors N4and N5) of mixed oxide thickness and comprising at least one LVT-type transistor (N4or N5), and a double oxide RVT-type transistor N8to form the output stage of the circuit for generating reference voltage VREF. The selection of an LVT-type transistor N3in the second branch of circuit101for generating bias current I also contributes to increasing the intrinsic accuracy of the circuit.

In addition to its good intrinsic accuracy, an advantage of the circuit ofFIG. 1is that the level of the supplied reference voltage can be easily adjusted on design by varying bias current I and the channel-width-to-channel-length ratio of the different transistors. In particular, the reference voltage supplied by the circuit ofFIG. 1may, if need be, be set to a level close to power supply voltage VSUPPLY. Indeed, the minimum interval between power supply voltage VSUPPLY and output voltage VREF corresponds to the minimum drain-source voltage necessary to obtain a good copying of bias current I by transistor P5, which may be in the order of 200 mV.

Further, since the circuit ofFIG. 1only comprises MOS transistors, it requires but a small silicon surface area to be formed, and has a relatively low electric power consumption. As concerns the occupied surface area, a compromise can be chosen between the intrinsic accuracy and the silicon surface area according to the needs of the application. Indeed, the larger the surface areas W*L of the MOS transistors of the circuit, the better the intrinsic accuracy of the circuit. As concerns the power consumption, an advantage of the circuit ofFIG. 1is that, due to the fact that bias current I is of CTAT type, the circuit power consumption does not increase when the temperature increases.

Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, the described embodiments are not limited to the example of circuit101for generating a bias current I described in relation withFIG. 1. More generally, circuit101may be replaced with any other circuit capable of generating a CTAT-type bias current I.

As a variation, circuit101may be replaced with a circuit capable of generating a PTAT-type bias current I. In this case, the sizing of the transistors, and in particular the sizing of transistor N8, may be adjusted to preserve a good temperature stability of the output voltage. It should however be noted that the use of a circuit101capable of generating a CTAT-type bias current I is preferable since it enables to limit the general electric power consumption of the circuit.

Further, the described embodiments are not limited to the example of circuit103for generating a PTAT-type voltage V described in relation withFIG. 1.

As a variation, it may in particular be provided to suppress the branch comprising transistors P4, N7and N6, and to couple the source of transistor N8directly to the junction point of transistors N4and N5. In this case, the voltage V applied to the source of transistor N8is voltage v1.

In another variation, it may be provided to replace the second branch (transistors N6and N7) with a branch of mixed oxide thickness comprising at least one LVT transistor. In other words, the transistor N6located on the side of node VSS may be replaced with a simple oxide transistor, transistor N7remaining a double oxide transistor, and at least one of the two transistors N6and N7being an LVT-type transistor, while the other transistor may be of LVT or RVT type.

In another variation illustrated inFIG. 4, each of the first (transistors N4and N5) and second (transistors N6and N7) branches of circuit103is a branch of mixed oxide thickness comprising at least one LVT-type transistor (such as described in the previous paragraph), and circuit103further comprises a third branch comprising a transistor N9series-connected with a transistor N10. In this example, transistors N9and N10are of NMOS type. The third branch is a double oxide branch, that is, its two transistors N9and N10are double oxide transistors (DO). Transistors N9and N10are for example both RVT transistors or both LVT transistors.

Transistor N9has its source connected to the junction point of transistors N6and N7, that is, to the source of transistor N7and to the drain of transistor N6. Transistor N9has its drain connected to the source of transistor N10. Transistor N10has its drain connected to its front surface gate. The front surface gate of transistor N10is further connected to the front surface gate of transistor N9. The junction point of transistors N9and N10, that is, the source node of transistor N10and drain node of transistor N9, forms the node for supplying output voltage V of circuit103(referenced to node VSS).

In this example, circuit103further comprises a PMOS transistor P6coupling the drain of transistor N10to node VDD. Transistor P6has its drain connected to the drain of transistor N10, and its source coupled to node VDD. Transistor P6is assembled to form a current mirror with transistor P2. More particularly, transistor P6has its front surface gate connected to the front surface gate of transistor P2. Transistor P6is for example of RVT type. As a variation, transistor P6is of LVT type. Transistor P6for example is a double oxide (DO) transistor. Transistor P6is for example identical to transistors P1, P2, P3, P4and P5.

In the variation ofFIG. 4, diode-assembled transistor N8has its source connected, no longer to the midpoint of the second branch, that is, to the source node of transistor N7and drain node of transistor N6, but to the midpoint of the third branch, that is, to the source node of transistor N10and drain node of transistor N9.

In operation, the bias current I generated by circuit101is copied in the branch comprising transistors P3, N5, and N4, in the branch comprising transistors P4, N7, and N6, and in the branch comprising transistors P6, N10, and N9. Under the effect of this current, a PTAT-type voltage v1is supplied onto the junction point of transistors N4and N5, a voltage v2, also of PTAT type but having a level greater than v1, is supplied onto the junction point of transistors N6and N7, and a voltage v3also of PTAT type but having a level greater than v2is supplied onto the junction point of transistors N9and N10. In this example, output voltage V of circuit103is voltage v3.

Thus, the operation of the circuit ofFIG. 4is similar to that of the circuit ofFIG. 1, but for the fact that output voltage V of circuit103is greater than in the example ofFIG. 1.

An advantage of the circuit ofFIG. 4is that it has an intrinsic accuracy which is even better than that of the circuit ofFIG. 1, that is, a dependence of its output voltage VREF to process variations which is lower than in the example ofFIG. 1, especially due to the increase in the value of output voltage V of circuit103.

The described embodiments are not limited to the above examples where transistors N1, N2, N3, N4, N5, N6, N7, N8, and, possibly (FIG. 4), N9and N10are N-channel MOS transistors. As a variation, a similar (complementary) circuit can be obtained by inverting the conductivity types of all transistors.