Patent Publication Number: US-2023145151-A1

Title: Power supply circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number FR2111782, filed on Nov. 5, 2021, entitled “Circuit d&#39;alimentation”, which is hereby incorporated by reference to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     The present description generally concerns power supply circuits. 
     BACKGROUND 
     Many applications require a low-noise electric power supply. This is for example the case of sensors, particularly image sensors. In these applications, the effects of the noise injected by the power supply and of the electronic noise of the components are significant since the sensor pixels generally do not reject noise sufficiently to be able to obtain the desired image quality. 
     For such noise-sensitive applications, it is thus desirable for the power supply to have a very good power supply rejection ratio (PSRR), in the order of 50 dB, and over a wide frequency range, for example, up to 10 MHz for image sensors. 
     In existing solutions, either the PSRR is not adequate, or the performance is limited in terms of frequency and noise. 
     SUMMARY 
     An embodiment provides a power supply circuit comprising: a first transistor device comprising a first gate associated with a first transconductance and a second gate associated with a transconductance greater than the first transconductance; and a second transistor device comprising a third gate associated with a second transconductance and a fourth gate associated with a transconductance greater than the second transconductance. The first and the third gates are controlled by a closed regulation loop, the second and the fourth gate are controlled by a sampled reference voltage; and the second transistor device are capable of powering at least one load. 
     An embodiment provides a load powering method, comprising: controlling a first gate of a first transistor device with a closed regulation loop; controlling a second gate of the first device with a sampled reference voltage, the first gate being associated with a first transconductance and the second gate being associated with a transconductance greater than that associated with the first gate; controlling a third gate of a second transistor device with the closed regulation loop; and controlling a fourth gate of the second transistor device with the sampled reference voltage, the third gate being associated with a second transconductance and the fourth gate being associated with a transconductance greater than the second transconductance; the second transistor device being configured to power at least one load when the control steps are carried out. 
     According to an embodiment, the closed regulation loop is formed by an operational amplifier taking, as a non-inverting input, a voltage to be followed and, as an inverting input, a voltage present on a source of the first transistor device; and the first and the third gates are coupled to the output of the operational amplifier. 
     According to an embodiment, the first gate of the first transistor device and the third gate of the second transistor device are back gates. 
     According to an embodiment, the reference voltage is generated by another closed regulation loop comprising a transistor having its front gate coupled to the output of an operational amplifier; the transistors of the first and second transistor devices and the transistor of the other closed regulation loop being matched together. 
     According to an embodiment, the transistor of the other closed regulation comprises a back gate; and the voltage applied to the first gate of the first transistor device by the amplifier is equal to the voltage applied to said back gate of the transistor of the other closed regulation loop. 
     According to an embodiment, the first transistor device comprises a first transistor comprising the first gate and a second transistor comprising the second gate, a source of the first transistor being connected to a source of the second transistor and a drain of the first transistor being connected to a drain of the second transistor; and the second transistor device comprises a third transistor comprising the third gate and a fourth transistor comprising the fourth gate, a source of the third transistor being connected to a source of the fourth transistor, a drain of the third transistor being connected to a drain of the fourth transistor. 
     According to an embodiment, a width of the second transistor is greater than that of the first transistor and/or a width of the third transistor is greater than that of the fourth transistor. 
     According to an embodiment, the circuit comprises a plurality of other second transistor devices having their third gates connected to one another; each second device being configured to power a different load. 
     According to an embodiment, the second transistor devices each comprise a source, the sources being coupled together. 
     According to an embodiment, the width of the second transistor device is a multiple k of the width of the first transistor device, multiple k being equal to or greater than 5, for example equal to or greater than 150. 
     According to an embodiment, the width W2 of transistor MLOOP 2  is greater than the width W1 of transistor MLOOP 1  by a scale factor Mw=W2/W1 equal to or greater than 5, for example, in the range from 10 to 20. 
     According to an embodiment, the sampling of the reference voltage to control the second gate is implemented by a first capacitor having an electrode receiving the reference voltage via a first switch; and the sampling of reference voltage to control the fourth gate is implemented by a second capacitor having an electrode receiving the reference voltage via a second switch. 
     An embodiment provides an image sensor comprising: a power supply circuit; and one or a plurality of pixels powered by the power supply circuit. 
     An embodiment provides an image sensor comprising: a power supply circuit; and
         pixel columns, each powered by one of the second transistor devices.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG.  1    schematically shows an example of a closed loop power supply circuit powering a pixel array; 
         FIG.  2    schematically shows an example of an open loop power supply circuit powering a pixel array; 
         FIG.  3    schematically shows an image sensor comprising a power supply circuit powering a pixel array according to an embodiment of the present description; 
         FIG.  4    schematically shows a closed bias loop of  FIG.  3    according to another embodiment of the present description; 
         FIG.  5    schematically shows a power supply circuit powering a pixel array of an image sensor according to another embodiment of the present description; and 
         FIG.  6    schematically shows a power supply circuit powering a pixel array of an image sensor according to another embodiment of the present description. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties. 
     For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. 
     Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements. 
     In the following disclosure, unless otherwise specified, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “upper”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures. 
     Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%. 
       FIG.  1    schematically shows an example of a closed loop power supply circuit  10  powering a pixel array  20  (PIXEL ARRAY). Generally, in the figures of the present description, pixel array  20  is represented by a current source corresponding to the bias current used for the reading of the pixels. This current source is arranged in parallel with a capacitive element Cvrtsf, which represents the sum of the capacitive elements of the pixels. 
     According to an example of  FIG.  1   , power supply circuit  10  comprises a transistor  102  powering pixel array  20 . The gate of transistor  102  is coupled to the output of an operational amplifier  106 . The drain of transistor  102  is connected to a rail of supply of a voltage VDD. The source of transistor  102  is connected to pixel array  20 . The source is further coupled to the inverting input of operational amplifier  106  to form a closed loop. Operational amplifier  106  is configured to receive on its non-inverting input a reference voltage VREF delivered by a voltage generator  112 . Voltage VREF is a set point voltage and corresponds to the value desired for the power supply of the pixels. In particular, reference voltage VREF may be considered as non-noisy and may be decoupled from the power supply, for example by being sampled from a capacitive element. Reference voltage VREF is for example positive and in the range from 1.4 V to 2.8 V. 
     In operation, the closed loop formed by operational amplifier  106  imposes a voltage to the gate of transistor  102  so that the voltage VRTSF delivered to pixel  20  tends towards VREF. 
     Although this example of power supply circuit enables the active regulation of a power supply voltage VRTSF, it comprises certain disadvantages. For example, the active correction, like the PSRR, may be frequency-limited to ensure the stability due to the strong capacitance Cvrtsf of the pixels. Noise originating from operational amplifier  106  may further appear at its output, and thus on power supply voltage VRTSF. This may cause row noise in pixel array  20  in the case where the shuttering and/or the reading of the array are performed row by row. Such a power supply circuit may further require providing a decoupling capacitive element external to the power supply circuit. This external decoupling capacitive element increases the time constants and might imply a separation of the inner and outer grounds, which adversely affects the decoupling. It also causes inductive effects due to its implantation. 
       FIG.  2    schematically shows an example of the open loop power supply circuit  200  powering pixel array  20 . Power supply circuit  200  is passive in that closed loop correction is not enabled. 
     The power supply circuit  200  of  FIG.  2    comprises transistor  102  and voltage generator  112 , similar to power supply circuit  10 . In power supply circuit  200 , the gate of transistor  102  is coupled on the one hand to an electrode of a sampling capacitor Csmp referenced to ground and on the other hand to the output of an operational amplifier  203  via a switch  202  controlled by a switching signal SMP. The output of operational amplifier  203  is further coupled to the gate of a transistor  206 . The drain of transistor  206  is coupled to the rail of supply of voltage VDD. The source of transistor  206  is coupled both to a current source  212  and to the inverting input of operational amplifier  106  to form a closed loop. Operational amplifier  203  receives on its non-inverting input reference voltage VREF. 
     When switch  202  is set to the off state, the voltage present at the level of the gate of transistor  206  is sampled, and held by capacitor Csmp on the gate of transistor  102 . The source of transistor  102  follows the voltage present at the level of its gate to deliver power supply voltage VRTSF to pixel array  20 . 
     The advantage of using switch  202  is that it enables making the circuit passive on the side of the pixel power supply. As opposed to the example of  FIG.  1   , switch  202  further enables no longer including the high capacitance of pixels Cvrtsf in the closed loop, and it enables avoiding the frequency limitation due to this capacitance. Switch  202  further enables making the delivery of voltage VRTSF independent from the closed regulation loop  230  formed of amplifier  203 , of current source  212 , and of transistor  206 . However, power supply circuit  200  comprises certain disadvantages. For example, in certain cases where voltage VDD is relatively low, transistor  102  might be insufficiently saturated, which would limit the PSRR. To better saturate the transistor, its width should be increased. In this case, the capacitive divider formed by the gate-drain capacitive element of transistor  102  and sampling capacitive element Csmp might limit the attenuation of the power supply noise. The Early effect of transistor  102  also is a limitation of the example of  FIG.  2    since the source undergoes a modulation proportional to the drain modulation. To obtain a better saturation of transistor  102 , its voltage VDD could be increased by a charge pump, but this would result in an increase in the power consumption. Further, the voltage at the gate of transistor  102  being fixed when switch  202  is in the off state, the insulation of power supply VDD is passive and thus less efficient than a closed loop. Such a power supply circuit may further require providing a decoupling capacitive element external to the power supply circuit. 
       FIG.  3    schematically shows an image sensor  350  comprising a power supply circuit  300  according to an embodiment of the present description. Power supply circuit  300  for example powers the above-described pixel array  20 . 
     The power supply circuit  300  of  FIG.  3    comprises a closed feedback loop REGULATION LOOP associated with a closed bias loop BIAS LOOP. Loop BIAS LOOP enables defining the operating point of feedback loop REGULATION LOOP by for example delivering bias and set point voltages VGVRT and VREF 1 . Feedback loop REGULATION LOOP corrects low-amplitude disturbances which are in particular due to the power supply noise. 
     Closed feedback loop REGULATION LOOP for example acts on a transistor MVRT having its source coupled, preferably connected, to pixel array  20 . The drain of transistor MVRT is coupled, preferably connected, to power supply rail VDD. Transistor MVRT comprises a front gate  302  and a back gate  304 . 
     Generally, the terms “back gate” and “front gate” signify that the back and front gates may act separately, and with a different associated transconductance, on a same channel of the transistor. The back and front gates may for example correspond to the technology using a depleted, preferably fully depleted, semiconductor substrate on insulator (Fully Depleted Silicon on Insulator, FDSOI). 
     Feedback loop REGULATION LOOP comprises a transistor MLOOP having a front gate  308  coupled, preferably connected, to the front gate  302  of transistor MVRT and a back gate  310  coupled, preferably connected, to the back gate  304  of transistor MVRT and to the output of operational amplifier  106 , similar to that of  FIG.  1   .  FIG.  3    illustrates a specific embodiment of this operational amplifier  106 . 
     According to an example of embodiment, back gate  304  is completely arranged in a substrate of power supply circuit  300 , and the channel of transistor MVRT is formed in a silicon layer insulated from the substrate by an insulating layer. Back gate  304  is thus electrically insulated from the channel of transistor MVRT. Further, front gate  302  is for example arranged at the surface of the substrate and electrically insulated from the transistor channel with an insulating layer having a lower thickness than the insulator layer separating the back gates of the transistor channel. By matching, the back gate  310  and the front gate  308  of transistor MLOOP may be respectively arranged similarly to the back gate  304  and to the front gate  302  of transistor MVRT. 
     Due to the relative oxide thicknesses of the front and back gates, the capacitance associated with each back gate  304 ,  310  is smaller than the capacitance associated with each front  302 ,  308 . The transconductance associated with each back gate  304 ,  310  is also smaller than the transconductance associated with each front gate  302 ,  308 . 
     According to an embodiment, the substrate of power supply circuit  300  is a FDSOI-type substrate. In this case, back gates  304 ,  310  may be formed under the insulator layer. Further, front gate  302  and/or front gate  308  may be formed at the surface above the insulator. 
     In power supply circuit  300 , the front gate  302  of transistor MVRT is coupled, preferably connected, at the level of a node  324 , on the one hand to an electrode of a sampling capacitor Csmp 2  referenced to ground and on the other hand to the output of bias loop BIAS LOOP, via a switch  306  controlled by switch signal SMP. 
     According to an embodiment, transistor MLOOP has a size smaller, for example, one hundred times smaller, than that of transistor MVRT. Indeed, transistor MVRT is sized to power pixel array  20  while transistor MLOOP is used to generate the voltage of the back gate to the applied to transistor MVRT. Transistors MLOOP and MVRT, as well as the associated current sources, may in particular be matched with respect to each other, that is, the density of current flowing therethrough is the same or substantially the same. The matching between transistors may further signify that the voltages that they receive on their drains, sources, and gates are similar from one transistor to the other and/or that their structure is similar. 
     The drain of transistor MLOOP is connected to power supply rail VDD. The source of transistor MLOOP is coupled, preferably connected, both to a current source  312 , coupled to ground, and to the inverting input of operational amplifier  106  to form a closed loop. On the contrary, the source of the transistor MVRT is not coupled, or connected to the closed loop, and in particular to the operational amplifier. Otherwise said, the transistor MVRT source does not supply the closed loop. Furthermore, the source of the transistor MLOOP is not coupled, or connected, to the transistor MVRT, nor to the load. Thus, the capacitances at the sources of the transistors MLOOP and MVRT are independent from each other, and the capacitance at the source of the transistor MLOOP can be configured to be lower than the capacitance at the source of the transistor MVRT. 
     In an example, the width of transistor device MVRT is a multiple k of the width of the transistor device MLOOP, where the multiple k is for example equal to or greater than 1, and the source of the transistor device MLOOP is configured to have a capacitance that is at least 10.k times smaller than the capacitance of the load  20  present at the source of the transistor MVRT, and in some cases at least 100*k times smaller than the capacitance of the load  20  present at the source of the transistor MVRT. 
     An advantage associated with the fact that the transistor MVRT source is not supplying the closed loop, is that the product gain x bandwidth of the closed loop can be high enough to optimize the PSRR in the desired operating frequency range, which is for example of around 10 MHz. Moreover, in order to guarantee the stability of the closed loop, the other elements of the loop should not introduce a pole below the product gain x bandwidth of the closed loop. More specifically, the pole formed by the output impedance of amplifier  106  combined with the capacitance of the backgate of devices MLOOP and MVRT, and the pole formed by the output impedance of the device MLOOP combined with the capacitance at node VRTLOOP, should both have a frequency greater than a product gain x bandwidth of the closed loop. Including the source of the transistor MVRT in the closed loop would create a pole at a relatively low frequency due to the load capacitance of the load  20 . Indeed, the load capacitance of the load  20  would cause an instability of the circuit at the expected PSRR performance and the product gain x bandwidth would limit the PSRR performance to an insufficient level. 
     Another advantage of having the source of the transistor MLOOP not connected to the load  20  is that the transistor MLOOP can be configured to have a relatively low capacitance at its source, rather than the capacitance of the load, which for example corresponds to the capacitance of the pixel array. This allows a stability, particularly around the desired operating frequency range of 10 MHz. 
     The output voltage of the amplifier  106 , applied to the back gate of the device MVRT, is for example able to correct the PSRR at the output node VRTSF of the power supply circuit, as its bandwidth is higher than the voltage supply noise from the supply rail VDD due to the fact that the capacitance of the back gates is significantly less than (for example only 15% of) the capacitance of the front gates. 
     The actuation of switch  306  with signal SMP sampling, and storing voltage VGVRT on capacitor Csmp 2 . In particular, when switch  306  is set to the off state, voltage VGVRT is sampled and held by capacitor Csmp 2  at the level of front gates  302 ,  308 . The source of transistor MVRT follows the voltage present at the level of front gate  302  to deliver power supply voltage VRTSF, linked to voltage VGVRT, to pixel array  20 . The source of transistor MLOOP follows the voltage present at the level of gate  308  to deliver a voltage VRTLOOP, linked to voltage VGVRT, which is fed back the loop. 
     In operation, the voltage held on front gate  308  by capacitor Csmp 2  determines a voltage at the source of transistor MLOOP so that voltage VRTLOOP is, for example by matching, an image of voltage VREF 1 . Further, the loop formed by operational amplifier  106  imposes a voltage to the back gate  310  of transistor MLOOP, so that voltage VRTLOOP tends towards VREF 1 , independently from power supply VDD. This power supply noise can thus be corrected. 
     The output of operational amplifier  106  further imposes a voltage on the back gate  304  of transistor MVRT to correct the voltage induced on front gate  302 . According to an example, the feedback to the back gate  304  of the transistor MVRT and to the back gate  310  of transistor MLOOP is generated without any resistive divider, the source of the transistor MLOOP being connected, for example, directly to an input of the operational amplifier  106 . In this case, more gain is obtained in the closed loop, which enhances the gain times bandwidth product contrary to the case in which a resistive divider is present. According to an example where there exists a matching between transistors MLOOP and MVRT, the correction applied by the regulation loop to back gate  310  to correct the power supply noise at node VTRLOOP, is that same as that to be applied to the back gate of transistor MVRT, to correct the power supply noise at node VRTSF. According to an embodiment, the width of transistor MVRT is a multiple k of the width of transistor MLOOP. For example, multiple k is equal to or greater than 5, for example, equal to or greater than 150. The size, that is, the width, of transistor MVRT being greater than the size of transistor MLOOP, the current supplied at the level of the source of transistor MVRT may be higher than at the level of the source of transistor MLOOP. In particular, the density of the current within these transistors is the same or substantially the same. 
     The operational amplifier  106  of circuit  300  for example comprises a first differential stage  303 . The output of first differential stage  303  is coupled, for example connected, to the gate of a transistor M 2 STAGE to form a second amplification stage of operational amplifier  106 . The drain of transistor M 2 STAGE is charged by a current source  305  connected for example to power supply rail VDD, and also connected to the back gates of transistors MVRT and MLOOP. 
     In certain cases, a capacitor Cmiller, which is for example a so-called “Miller” compensation capacitive element, couples the drain and the gate of transistor M 2 STAGE. This helps ensure the stability of the amplifier. 
     Operational amplifier  106  receives on its non-inverting input reference voltage VREF 1 SMP, which in the example of  FIG.  3   , represents a reference voltage VREF 1  once sampled. According to an embodiment, to obtain reference voltage VREF 1 SMP, the source of transistor MBIAS is coupled to an electrode of sampling capacitor Csmp 1 , referenced to ground, via a switch  320 , at the level of a node  326 . Switch  320  is for example controlled by switching signal SMP. Node  326  is further coupled, preferably connected, to the non-inverting input of amplifier  106 . According to another embodiment, switch  320  is directly coupled to the output of potentiometer  315  instead of being coupled to the source of transistor MBIAS. 
     Bias loop BIAS LOOP comprises a current source  311  powering a potentiometer  315  coupled to ground. The output voltage of the potentiometer is reference voltage VREF. Voltage VREF 1  is equal to voltage VREF or is approximately equal to voltage VREF. For example, voltage VREF 1  is offset with respect to voltage VREF by the offset voltage between the inverting and non-inverting inputs of amplifier  203 . The output of the potentiometer is coupled, preferably connected, to the non-inverting input of an operational amplifier similar to the amplifier  203  of  FIG.  2    and which is thus referenced in the same way. The output of operational amplifier  203  is coupled, preferably connected, to the front gate of a transistor MBIAS. The inverting input of operational amplifier  203  is coupled, preferably connected, to the source of transistor MBIAS and to a current source  318 . Current source  318  is for example coupled to ground. The drain of transistor MBIAS is powered by power supply rail VDD. Transistors MBIAS, MLOOP, and MVRT are for example matched together, as well as with the current sources which are dedicated thereto, so that the average output potential, that is, its idle level, of operational amplifier  106  corresponds to the back gate potential of transistor MBIAS. In other words, due to the matching between transistors MBIAS and MLOOP, the voltage applied to the back gate  310  of transistor MLOOP by amplifier  106 , that is, its average output level, corresponds to the voltage applied to the back gate of transistor MBIAS in loop BIAS LOOP. According to an embodiment, the voltage applied to the back gate of transistor MBIAS is obtained by connecting the back gate of transistor MBIAS to its front gate. It is also possible to deliver the voltage on the back gate of transistor MBIAS in a different way. For example, according to an embodiment, not shown, the applied voltage originates from a source external to loop BIAS LOOP. 
     The value of the output voltage VREF of potentiometer  315  is selected to target the power supply voltage VRTSF of pixel array  20 . If power supply voltage VRTSF at the level of the source of transistor MVRT is desired to be obtained, then voltage VGVRT should take into account the gate-source voltage of transistor MVRT. The value of the voltage VGVRT obtained at the output of operational amplifier  203  is increased, with respect to the targeted voltage VRTSF, by the gate-source voltage of transistor MBIAS. 
     Both samplings with switches  306  and/or  320  are aimed to improve the noise, eliminating any contribution from the bias loop at the output. 
     In the example of  FIG.  3   , operational amplifier  106 , which drives the back gates of MLOOP and MVRT, has an operating portion centered on voltage VGVRT. It will be within the abilities of those skilled in the art to implement other embodiments to deliver a voltage to the back gate of transistor MBIAS, which will determine the operating point at the output of amplifier  106 . 
     By controlling the voltage at the level of back gates  304  and  310 , it is possible to finely and continuously regulate over time the voltage at the level of the source of transistor MVRT and this, over a wide frequency range. 
     By coupling the output of operational amplifier  106  to back gates  304 ,  310 , the noise generated during the voltage regulation is advantageously limited by a ratio, equal to gmb/(gm+gmb), which is similar for transistors MLOOP and MVRT when the latter are matched together. Transconductance gmb is specific to each of these transistors, and associated with the back gate. Transconductance gm is specific to each of these transistors, and associated with the front gate. For a given transistor, transconductance gmb is smaller than transconductance gm, the ratio is thus smaller than 1. Further, the capacitance of each of back gates  304 ,  310  being much smaller than the capacitance linked to each of front gates  302 ,  308 , it is possible to obtain a regulation of voltages VRTLOOP and VRTSF with the closed loop over a wide frequency band. 
     An additional advantage of the power supply circuit  300  of  FIG.  3    is that a decoupling capacitive element, external to the circuit, is not necessary. Indeed, the loop is internally stabilized by Miller capacitive element Cmiller, which is of low value, which enables the integration of Miller capacitive element Cmiller within the circuit, and allows regulation at high frequency. The absence of a decoupling capacitive element external to the power supply circuit particularly enables performance improvement, cost reduction, and compliance with the PSRR specification in the targeted frequency band, which is 10 MHz for the power supply of certain image sensors. 
     Although, in the example of  FIG.  3   , a single switch and a single capacitor are used to sample and store voltage VGVRT at the level of the two front gates  302 ,  308 , according to an embodiment, not shown in  FIG.  3   , switch  306  and capacitor Csmp 2  may be duplicated for each front gate  302 ,  308 . 
     Although, in the example of  FIG.  3   , bias voltages VGVRT and VREF originate from the same bias loop BIAS LOOP, it will be within the abilities of those skilled in the art to envisage a closed loop adapted to each of these voltages. Further, those skilled in the art may implement other means than a closed loop to generate voltages VGVRT and/or VREEF 1 . In view of the different possibilities accessible to those skilled in the art to implement voltages VGVRT and/or VREF 1 , in  FIGS.  5  and  6   , only the rails of supply of voltages VGVRT and VREF 1  will be shown. 
       FIG.  4    schematically shows the closed bias loop BIAS LOOP of  FIG.  3    according to another embodiment of the present description. In this example, the back gate of transistor MBIAS is coupled to the source of transistor MBIAS. 
       FIG.  5    schematically shows a power supply circuit  500  powering the pixel array  20  of an image sensor  550  according to another embodiment of the present description. 
     The power supply circuit  500  of  FIG.  5    is similar to power supply circuit  300 , except for the fact that transistor MLOOP is replaced in  FIG.  5    with transistors MLOOP 1  and MLOOP 2 , and transistor MVRT is replaced in  FIG.  5    with transistors MVRT 2  and MVRT 1 . 
     The drains of transistors MLOOP 1 , MLOOP 2  are coupled, preferably connected, together and the sources of transistors MLOOP 1  and MLOOP 2  are also coupled, preferably connected, together. Transistors MLOOP 1  and MLOOP 2  thus form a dual-gate transistor device. The gate of transistor MLOOP 1  is coupled, preferably connected, to the output of operational amplifier  106  and the gate of transistor MLOOP 2  is coupled, preferably connected, to node  324 . 
     Similarly, the drains of transistors MVRT 2  and MVRT 1  are coupled, preferably connected, together and the sources of transistors MVRT 2  and MVRT 1  are also coupled, preferably connected, together. Transistors MVRT 2  and MVRT 1  thus form a dual-gate transistor device. The gate of transistor MVRT 1  is coupled, preferably connected, to the output of operational amplifier  106  and the gate of transistor MVRT 2  is coupled, preferably connected, to node  324 . 
     Transistors MLOOP 1 , MLOOP 2 , MVRT 2 , and MVRT 1  are for example bulk transistors with their substrate node coupled, for example, to ground. 
     In certain cases where the lengths of transistors MLOOP 1  and MLOOP 2  are equal, the width W2 of transistor MLOOP 2  is greater than the width W1 of transistor MLOOP 1  by a scale factor (M w =W2/W1) equal to or greater than for example 5, or for example in the range from 10 to 20. The width of transistor MVRT 2  is greater than that of transistor MVRT 1  for example by the same scale factor. According to an example, the size of transistor MVRT 2  is a multiple N of the size of transistor MLOOP 2 . According to an example, the size of transistor MVRT 1  is a multiple, preferably the same multiple N, of the size of transistor MLOOP 1 . According to this example, transistor MVRT 1  has a width W1*N and transistor MVRT 2  has a width W2*N. Multiple N is for example equal to the ratio of the current supplied to pixel array  20  to the current supplied by current source  312 . Multiple N is for example proportional to the number of columns to be powered in pixel array  20 . In an example, multiple N is equal to or greater than 100, for example, equal to or greater than 150. 
     According to an example where the lengths of transistors MLOOP 1 , MLOOP 2 , MVRT 1 , or MVRT 2  are different, the scale factor takes into account the respective lengths of the transistors and can be expressed as M WL =(W2/L2)/(W1/L1) where L2 and L1 are respectively the lengths of transistors MLOOP 2  and MLOOP 1 . Scale factor M WL  is of the same order of magnitude as M w . 
     In particular, for an optimal implementation, transistors MLOOP 1 , MLOOP 2 , MVRT 2 , and MVRT 1 , as well as the associated current sources, are matched. 
     In this configuration, the transconductance associated with the gate of transistor MVRT 1  is thus smaller than the transconductance associated with the gate of transistor MVRT 2 . Similarly, the transconductance associated with the gate of transistor MLOOP 1  is smaller than the transconductance associated with the gate of transistor MLOOP 2 . The ratio of the transconductance associated with the gate of transistor MVRT 1  to the transconductance associated with the gate of transistor MVRT 2  may be, by matching, the same as the ratio of the transconductance associated with the gate of transistor MLOOP 1  to the transconductance associated with the gate of transistor MLOOP 2 . The gate of transistor MLOOP 1  may thus have the same effect as the back gate  310  of transistor MLOOP on the total current conducted by transistors MLOOP 1  and MLOOP 2 . Similarly, the gate of transistor MLOOP 2  has the same effect as the front gate  308  of transistor MLOOP on this total current. The gate of transistor MVRT 1  thus has the same effect as the back gate  304  of transistor MVRT on the total current conducted by transistors MVRT 2  and MVRT 1  and the gate of transistor MVRT 2  has the same effect as the front gate  308  of transistor MVRT on this total current. 
       FIG.  6    schematically shows a power supply circuit  600  powering a pixel array  20  of an image sensor  650  according to another embodiment of the present description. 
     Power supply circuit  600  comprises operational amplifier  106 , voltage generator  112 , transistor MLOOP and current source  312  arranged similarly to the left-hand portion of the power supply circuit  10  of  FIG.  1   . The pixel array  20  of  FIG.  6    is divided into five columns  610 ,  612 ,  614 ,  616 , and  618  of pixels in parallel. The number of columns is however not limited and may reach several hundreds or even several thousands. 
     In the power supply circuit of  FIG.  6   , the transistor MVRT of  FIG.  3    is distributed into a plurality of transistors to have a transistor powering each column or group of columns. In this case, the combined width of all the transistors MVRT obtained in  FIG.  6    is equivalent, for example, to the width of the transistor MVRT of the example of  FIG.  3   . Further, switch  306  and capacitor Csmp are for example repeated to sample and store voltage VGVRT on the gate of each transistor MVRT. 
     The distribution of the transistors MVRT of  FIG.  6    enables decreasing the current level of each source of the transistors MVRT of power supply circuit  600 . This enables homogenization of the power supply between each column or group of columns so that all columns see the same voltage while avoiding voltage drops between the columns. 
     Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants may be combined, and other variants will occur to those skilled in the art. 
     Finally, the practical implementation of the described embodiments and variations is within the abilities of those skilled in the art based on the functional indications given hereabove.