Patent Publication Number: US-2010109435-A1

Title: Linear Voltage Regulator with Multiple Outputs

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/100,565 filed Sep. 26, 2008, the entire contents of which disclosure is specifically incorporated herein by reference without disclaimer. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to electronic circuitry, and in particular, electronic circuits for voltage regulation. 
     BACKGROUND 
     A variety of electronic circuits, e.g., analog, digital, and/or radio frequency (RF) circuits, can use linear voltage regulators to regulate the voltage level supplied to the circuitry. In some cases, the variety of electronic circuits can be included on a single printed circuit board (PCB). Many of the PCBs can be included in devices where a battery provides the voltage source to operate the device. Therefore, the linear voltage regulators can be included in battery-operated devices on PCBs along with a variety of different types of electronic circuits. These regulators can operate under low voltage, mixed signal conditions. For example, wireless handheld communications and remote-control devices can include mixed analog, digital, and RF circuitry, all in one device. Examples of these devices can include, but are not limited to, cellular phones, wireless devices implanted in living beings, television remote-controls, etc. 
     Linear voltage regulators can be used in many different types of electronic devices to convert an unregulated—and sometimes noisy—direct current (DC) power supply voltage to a regulated, clean DC power supply voltage. 
     Many battery-powered portable electronic devices, such as laptop computers, cell phones, and the like, may have at least one linear voltage regulator for regulating the output voltage of the battery into a regulated and clean DC power supply voltage. Additionally, many portable devices that can be powered by other kinds of remote power, such as inductive coupling or electromagnetic radiation, may use a linear voltage regulator to generate a clean DC power supply voltage required for the operation of that electronic device. 
     In addition, many non-portable electronic devices (e.g., desktop computers, TVs, etc.) may use an alternating current (AC) to DC converter and a linear voltage regulator to convert the AC power supply voltage from, for example, municipal power lines into a clean DC supply voltage required for the operation of the electronic device. 
     With the growth in circuit integration capabilities, many circuit blocks of an electronic device can be integrated into a single integrated circuit (IC) chip. This can be referred to as a System-On-Chip (SOC). 
     SUMMARY OF THE INVENTION 
     In general, this document describes various systems, methods, and apparatuses that can be used to convert an unregulated, and in some cases, noisy power supply voltage to a clean, e.g., exhibiting a reduced noise spectrum, stable power supply voltage. 
     In a first aspect, a system includes a circuit configured to supply a plurality of isolated, regulated supply voltages. The circuit further includes a voltage regulator. The voltage regulator further includes a first transistor configured to supply a first regulated supply voltage and a second transistor, operably coupled to the first transistor, configured to supply a second regulated supply voltage, where the first regulated supply voltage and the second regulated supply voltage are electrically isolated from one another. 
     Implementations can include any, all or none of the following features. The voltage regulator can include an operational amplifier whose output is operably coupled to the first transistor. The voltage regulator can further include at least two resistors operably coupled between an input to the voltage regulator and the first transistor. The first transistor can be a pass transistor. The at least two resistors and the pass transistor can form a feedback loop for the operational amplifier. The pass transistor can be a p-channel metal-oxide-semiconductor (PMOS) transistor. The pass transistor can be an n-channel metal-oxide-semiconductor (NMOS) transistor. The second transistor can be an n-channel metal-oxide-semiconductor (NMOS) transistor. The second transistor can be an n-channel metal-oxide-semiconductor (NMOS) transistor. The first regulated supply voltage can be configured to supply power to analog circuitry. The second regulated supply voltage can be configured to supply power to digital circuitry. Switching noise from the digital circuitry coupled to the second regulated supply voltage may not be coupled onto the first regulated supply voltage. A gate of the first transistor can be operably coupled to an output of the operational amplifier and a gate of the second transistor. A gate of the first transistor can be operably coupled to an output of the operational amplifier and a gate of the second transistor. A gate of the first transistor can be operably coupled to an output of the operational amplifier and a drain of the first transistor can be operably coupled to a gate of the second transistor. A gate of the first transistor can be operably coupled to an output of the operational amplifier and a drain of the first transistor can be operably coupled to a gate of the second transistor. A gate of a third transistor can be operably coupled to said gate of said first transistor. The third transistor can be configured to supply a third regulated supply voltage. The first regulated supply voltage, the second regulated supply voltage, and the third regulated supply voltage can be electrically isolated from one another. An additional resistor can be operably coupled between a drain of the first transistor, and a gate of the second transistor. The additional resistor can be operably coupled to the at least two resistors coupled between the input to the voltage regulator and the first transistor. An additional resistor can be configured to control a value of the second regulated supply voltage. A value for the first regulated supply voltage can substantially match a value for the second regulated supply voltage. Alternatively, a value for the first regulated supply voltage can be different from a value for the second regulated supply voltage. The voltage regulator can include one or more diode-connected transistors operably coupled in series to one another, the first transistor operably coupled to an end of the plurality of diode-connected transistors operably coupled in series to one another, and a resistor operably coupled to the first transistor and the plurality of diode-connected transistors operably coupled in series to one another. A gate of the first transistor can be operably coupled to the plurality of diode-connected transistors operably coupled to one another and a gate of the second transistor. A gate of the first transistor can be operably coupled to the plurality of diode-connected transistors operably coupled to one another, and a gate of the second transistor can be operably coupled to a subset of the plurality of diode-connected transistors operably coupled to one another. The subset of the plurality of diode-connected transistors operably coupled to one another can control a value of the second regulated voltage supply. 
     In a second aspect, a multi-output voltage regulator, having a first, second, and third input power supply rail includes a single-output linear voltage regulator generating a first output of the multi-output voltage regulator, where the first output can be a first regulated supply voltage. The multi-output voltage regulator, having a first, second, and third input power supply rail further includes at least one pass transistor, operably coupled to the single-output linear voltage regulator, where the pass transistor can be configured to supply at least one additional regulated supply voltage. 
     Implementations can include any, all or none of the following features. The first and the second power supply rails can be coupled to ground. The second and the third power supply rails are coupled to ground. The single-output linear voltage regulator can be a series-type linear voltage regulator. The single-output linear voltage regulator can be a shunt-type linear voltage regulator. The pass transistors can be field effect transistors (FET). Gate terminals of the field effect transistors can be coupled to the single-output linear voltage regulator, where drain terminals of the field effect transistors can coupled to the first power supply rail and source terminals of the field effect transistors can generate the at least one additional regulated supply voltage. Gate terminals of the field effect transistors are coupled to the single-output linear voltage regulator, where drain terminals of the field effect transistors can be coupled to an output of the single-output linear voltage regulator and source terminals of the field effect transistors can generate the at least one additional regulated supply voltage. The pass transistors can be bipolar transistors. Base terminals of the bipolar transistors can be coupled to the single-output linear voltage regulator, collector terminals of the bipolar transistors can be coupled to the first power supply rail, and emitter terminals of the bipolar transistors can generate the at least one additional regulated supply voltage. Base terminals of the bipolar transistors can be coupled to the single-output linear voltage regulator, collector terminals of the bipolar transistors can be coupled to an output of the single-output linear voltage regulator, and emitter terminals of the bipolar transistors can generate the at least one additional regulated supply voltage. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary as well as the following detailed descriptions of various implementations will be better understood when read in conjunction with the appended drawings. It should be understood, however, that preferred implementations are not limited to the precise arrangements and instrumentalities shown herein. The components in the drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of various implementations. 
         FIG. 1A  is an exemplary portable monitoring device that can include a low drop-out linear voltage regulator with multiple outputs, according to one embodiment. 
         FIG. 1B  is an exemplary block diagram of a system on a chip design, according to one embodiment. 
         FIG. 2A  is an additional exemplary multi-output linear voltage regulator according to one embodiment. 
         FIG. 2B  is an additional exemplary multi-output linear voltage regulator according to one embodiment. 
         FIG. 2C  is an additional exemplary multi-output linear voltage regulator, according to one embodiment. 
         FIG. 3  is an additional exemplary multi-output linear voltage regulator, according to one embodiment. 
         FIG. 4  is an additional exemplary multi-output linear voltage regulator, according to one embodiment. 
         FIG. 5A  is an additional exemplary multi-output linear voltage regulator, according to one embodiment. 
         FIG. 5B  is an additional exemplary multi-output linear voltage regulator, according to one embodiment. 
         FIG. 6A  is a graph of an exemplary waveform of an analog voltage signal, according to one embodiment. 
         FIG. 6B  is a graph of an alternate exemplary waveform of an analog voltage signal, according to one embodiment. 
         FIG. 7  is a graph of exemplary time-domain measurement results of output, regulated voltages, according to one embodiment. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     A portable wireless device can use a linear voltage regulator with multiple outputs to regulate the DC voltage to a variety of circuits included in the device. In some implementations, the circuits may be included together on a single integrated chip, which can be referred to as a system on a chip (SOC). For example, a portable wireless device can be a portable, implantable device that can monitor the concentration of biological species (e.g., oxygen, glucose or cholesterol) in human blood. The device can be implanted in a patient and may wirelessly transmit a value representative of the concentration to a receiving device. The receiving device may include a visual indicator that can display the value representative of the concentration level. The patient can view the value of the concentration level to determine if they require any immediate medication. In some implementations, the portable monitoring device may include integrated circuits, which may advantageously function with low power requirements. Therefore, a battery may be used to supply power to the integrated circuits in the device. 
     Referring now to  FIG. 1A , a portable monitoring device  102  can, in some embodiments, include a system on a chip (SOC)  106  that can include a voltage regulator  104  with multiple outputs  104   a ,  104   b . In the embodiment of  FIG. 1 , the SOC  106  can also include a transponder  112 . A power source  114 , included in the portable monitoring device  102 , can provide power to the SOC  106 . The SOC can be fabricated on a single integrated circuit and can include mixed signal designs (e.g., analog, digital, and RF). 
     In some implementations, the SOC  106  can be used in a variety of applications that include, but are not limited to, environmental monitoring, food preparation, including industrial food preparation, and biomedical applications. For example, the portable monitoring device  102  can be a wireless implantable device dedicated for blood glucose monitoring that can be implanted into a human body to continuously measure the blood glucose level in the body. In some implementations, an electrochemical cell  116  can be included in an electrochemical sensor circuit  108 . The electrochemical cell  116  can include a working electrode, a counter electrode, and a reference electrode. The sensor can be an electrochemical hydrogen peroxide electrode-based glucose biosensor. A current flow through the sensor (e.g., from the working electrode to the counter electrode) can be the result of the oxidation of hydrogen peroxide at the surface of the working electrode of the electrochemical cell  116 . 
     In some implementations, a potentiostat circuit associated with the electrochemical sensor circuit  108  can determine the value of the sensor current. The measured sensor current value can be proportional the amount of hydrogen peroxide that diffuses to the working electrode, which can be proportional to the amount of glucose in the bloodstream. For example, the sensor circuit  108  can convert the measured sensor current value to a voltage. In some implementations, an analog to digital converter can convert the voltage to a digital value (e.g., a numeric value proportional to the voltage). The digital value can be representative of the measured sensor current and therefore representative of the amount of glucose in the bloodstream. 
     In some implementations, the digital value can be input to the transponder  112 . The transponder  112  can transmit a radio frequency signal wirelessly to a receiving device. The transponder  112  can include both digital and analog circuitry that converts a digital value (e.g., the digital value representative of the amount of glucose in the bloodstream) received from the sensor circuit  108  to an analog value. The transponder  112  can modulate the analog value with a radio frequency signal and transmit the RF signal to a receiving device  118 . In some implementations, the receiving device  118  can also include a transponder that can receive the RF signal and determine the digital value. The digital value can be translated into a glucose level value that can be output to a display  120 . 
     In some implementations, the SOC  106  can be fabricated using complementary metal oxide semiconductor (CMOS) processes. CMOS circuits can use reduced power while working with low power supply voltages, making them beneficial for use in battery-operated devices. In some implementations, the power source  114  can be a battery. In some implementations, the power source  114  can be an inductive power transfer link. The continuous blood glucose monitoring device can have low power consumption because the power provided to the device (either by a battery or by an inductive power transfer link) is limited. 
     Referring back to  FIG. 1A , the voltage regulator  104  can provide two outputs,  104   a ,  104   b , to the transponder  112  that can be individual, isolated, stable, noise-free voltages. In some implementations, the voltage regulator  104  may provide more than two isolated, regulated output voltages. The number of output voltages supplied by a voltage regulator can be dependent on the number of isolated, regulated voltages used by the circuits included on a SOC. For example, the voltage output  104   a  can provide power to analog circuitry in the transponder  112  and the voltage output  104   b  can provide power to digital circuitry in the transponder  112 . The isolated outputs  104   a ,  104   b  can decrease the likelihood that the switching noise from the digital circuits will be coupled to the analog circuits. This coupling can occur through the power supply rails of the SOC  106 . Switching noise coupled to the analog circuitry can degrade the functionality of the analog circuitry. 
     In some implementations, the voltage regulator  104  can be a linear voltage regulator that can convert a noisy, unstable DC supply voltage (e.g., provided by power source  114 ) to a noise-free, stable DC supply voltage. In some implementations, the power source  114  can be an alternating current (AC) power source. In this case, the voltage regulator  104  can additionally include circuitry to convert the AC power signal to a DC power signal. In some implementations, the power source  114  can be one or more batteries that can supply the DC power needed to operate the circuitry included in the SOC  106 . In this case, the voltage regulator  104  can be a linear voltage regulator that can provide stable DC supply voltages to the circuitry on the SOC  106 . 
     In some implementations, linear voltage regulators can be classified into shunt-type and series-type regulators. In a shunt-type regulator, the regulating device (e.g., a zener diode) can be connected in parallel with a load resistance. In a series-type regulator, the regulating device (e.g., a transistor) can be connected in series with the load resistance. In this implementation, the regulating device can also be referred to as a pass device. In some implementations, a device that uses a shunt-type regulator may consume more power from a power source to drive the same load as a device that uses a series-type regulator. For example, the communication range of a wireless device can be inversely proportional to its power consumption (the less power the device uses, the broader the range). Therefore, a device that includes a shunt-type regulator can degrade the communication range of its wireless link due to its increased power consumption. 
     In some implementations, in order to prevent noise coupling between voltage regulator outputs, a separate linear voltage regulator can be used for each supply voltage. For example, a first linear voltage regulator can supply an isolated, regulated voltage to analog circuitry, and a second linear voltage regulator can supply an isolated, regulated voltage to digital circuitry. Each regulator may receive its voltage supply from the same power source. The separate regulators can isolate the power to each circuit reducing the likelihood of noise coupling from the digital circuitry to the analog circuitry. In some implementations, the separate voltage regulators can be included with the analog and digital circuitry in a SOC. In some implementations, the voltage regulators may be external to a SOC that includes the analog and digital circuitry. In some implementations, the voltage regulators, the analog circuitry, and the digital circuitry may be implemented using a plurality of integrated circuits. 
     In some implementations, a voltage regulator may provide a plurality of outputs that can be isolated from one another using an isolation circuit. The isolation circuit can prevent noise coupling from the digital circuitry to the analog circuitry by isolating the analog supply rail in the voltage regulator from the digital supply rail. 
     The use of separate voltage regulators and/or isolation circuits in a voltage regulator, in all likelihood, can increase the power consumption of a device. The die area and the complexity of the chip may also increase when the separate voltage regulators and/or isolation circuits, and digital and analog circuitry are included together in a SOC. In cases where supply power is limited and may need to be conserved (e.g., battery supplied power), the use of separate voltage regulators may not be desirable. 
     In some implementations, the power consumption of a SOC can be decreased by using a single linear voltage regulator with multiple, isolated, regulated outputs. For example, a linear voltage regulator (e.g., a series-type) can generate an analog supply voltage to power analog circuitry. One of the node voltages of the linear voltage regulator generating the analog supply voltage can control the gate (or base) of one or more pass devices, where each pass device can generate a digital supply voltage. The node voltages in the linear voltage regulator can supply a constant voltage that can be used for gate (or base) control of the pass devices. This can result in a single linear voltage regulator generating a plurality of isolated, regulated output voltages. 
     Referring now to  FIG. 1B , an exemplary block diagram of a system on a chip design (SOC)  152  can include analog circuit block  156 , radio frequency (RF) circuit block  158 , digital circuit block  160 , memory circuit block  162 , and power management circuit block  164 . In some implementations, other circuit blocks may be included. 
     Analog circuit block  156  and RF circuit block  158  can include circuits that may be sensitive to electric noise. In some implementations, the electric noise can come from the elements included in the circuit. In some implementations, the electric noise can be injected from other circuits through bias rails, power supply rails, or the substrate of the chip. In some implementations, the electric noise may come from the external environment of the SOC. 
     In contrast to the analog and RF blocks, digital and memory blocks can be less sensitive to noise. Instead, they may generate substantial noise due to the voltage and/or current switching happening in these blocks. In some implementations, switching noise can penetrate into analog and RF blocks through the power supply rails and/or substrate of the SOC and degrade the performance of those blocks. 
     In some implementations, a power management block  104  can receive power from an external power source  164  (e.g. a battery or an AC-DC converter) and can generate different supply voltages for the different circuit blocks in the SOC  152 . In some implementations, the generation of different power supply voltages can be to prevent the switching noise of noisy circuits, such as the digital  160  and memory  162  blocks, from injecting into sensitive analog circuits such as the analog  156  and RF  158  blocks. In some implementations, the generation of different power supply voltages can be that different circuit blocks need different supply voltages. 
     The power management block  154  can include several voltage regulators, each one generating a single supply voltage for one or more circuit blocks. This approach can consume a relatively high amount of power because each supply voltage requires a separate voltage regulator. As a result, there may be a need for a voltage regulator that can generate multiple and different output voltages with low power consumption. 
     Referring to  FIG. 2A , a first input  268  to the amplifier  256  can be a voltage, V P , which can be a percentage of the first output voltage, V out1 , where the percentage is determined by the resistor ratio of resistor  258  (R F1 ) and resistor  260  (R F2 ) (e.g., V P =V out1 *(R F1 /R F1 +R F2 )). The first input  268  can monitor the voltage, V P . A second input  270  (V REF ) to the amplifier  256  can be a stable voltage reference (e.g., a bandgap reference). If the first output voltage, V out1  increases to a voltage that is greater than the second input  270  (V REF ), which is the reference voltage, the drive to the transistor  252  can change to maintain a constant voltage for the first output voltage, V out1 . Therefore, the feedback loop can stabilize the regulated first output voltage, V out1 , of the series-type voltage regulator  254  to a pre-defined DC voltage. 
     The series-type voltage regulator  254  can include the NMOS pass transistor  252 . A feedback loop to the amplifier  206  can include pass transistor  252  and resistors  258 ,  260 . The feedback loop can be implemented in a source follower configuration and can act as a control loop to control the gate of the pass transistor  252 . In some implementations, the use of a control loop in a source follower configuration in a series-type voltage regulator (as shown in  FIG. 2A ), in all likelihood, can increase the stability of the voltage regulator as compared to the use of a control loop in a common source configuration (as shown in  FIG. 2A ). Referring again to  FIG. 2A , connecting an external capacitor across a load  262  (load 1 ) is no longer required due to the inherently low output impedance of the source follower configuration. The first output voltage, V out1 , of the series-type voltage regulator  254  can be at least one gain-to-source voltage drop (V GS ) below the input supply voltage, V in . In some implementations, this additional voltage drop may be a disadvantage. 
     In some implementations, the series-type voltage regulators of  FIG. 2A  regulator  254  can be used in low drop out voltage (LDO) regulators as each regulator uses a single pass device (e.g., pass transistor  252 ). In some implementations, where a power supply (e.g., power supply  114  in  FIG. 1 ) can include one or more batteries, a voltage regulator (e.g., regulator  254 ) can be implemented as a low drop out voltage regulator. For example, a LDO voltage regulator can be a linear voltage regulator, which can operate with a small input-output differential voltage. In some implementations, a charge pump may be used to provide a supply voltage that is slightly higher that an input reference voltage to a series-type voltage regulator, such as those shown in  FIG. 2A  in order to allow the regulator  254  to function as a low drop-out voltage regulator. 
     As previously described, a linear voltage regulator may provide a regulated voltage to mixed signal circuitry (e.g., analog, digital, and RF) included in a SOC. In a system where a single regulator supplies voltage to multiple circuit types, unwanted noise can be coupled from one circuit type to another. For example, switching noise from the digital circuitry can be coupled to the analog circuitry. This can result in undesired effects in the analog circuitry. Various circuits and methods have been described herein to provide separate, isolated voltages to the various circuit types without any negative effects. 
     A series-type linear voltage regulator  254  can provide a first output voltage, V out1 , generated by the pass transistor  252  to a circuit (e.g., analog circuitry). The pass transistor  252  can generate the first voltage, V out1 , in the closed loop feedback of the series-type voltage regulator  254 . An output  264  of the operational amplifier  256  can be a well-defined, stable voltage. The output  264  can control the gate of a transistor  256  (M 2 ). The transistor  256  can generate a second output voltage, V out2 , in an open loop circuit. The second output voltage, V out2 , can be isolated from the first output voltage, V out1 . The pass transistor in the series-type regulator can effectively be divided into two transistors, the pass transistor  252  and a second transistor  266 . In some implementations, the multi-output linear voltage regulator  250  can use the pass transistor  252  and the second transistor  266  to generate separate, isolated voltages (V out1  and V out2 , respectively) that can be supplied to analog circuitry and digital circuitry, respectively. 
     In some implementations, the second transistor  266  can consume little to no additional power from the input supply voltage, V in . The second transistor  266  may also add little to no additional die area or complexity to the SOC that includes the multi-output linear voltage regulator  250 . The multi-output linear voltage regulator  250  can divide the pass device for the series-type regulator into two transistors, the pass transistor  252  and the second transistor  266 . The two transistors, transistor  252  and transistor  266 , can each generate a separate, isolated, regulated supply voltage, (V out1  and V out2 , respectively). In some implementations, a pass transistor can be divided into more than two transistors, each of which can generate a separate, isolated, regulated supply voltage, which can then be connected to various types of circuitry. 
     In some implementations, the feedback loop for the series-type voltage regulator  254  can maintain the first output voltage, V out1 , at a desired pre-defined voltage. The second output voltage, V out2 , in some cases, may not be as well-defined a voltage as the first output voltage, V out1 , because the control circuit for the second output voltage, V out2 , is open loop. In some implementations, the second voltage, V out2 , can be a supply voltage for digital circuits because the digital circuits may not require a tightly regulated supply voltage. The isolation of the first supply voltage, V out1 , from the second supply voltage, V out2 , and vice versa can be limited by the coupling due to the gate-to-source capacitances of the divided pass transistors  252  and  266 . 
     In some implementations, a pass device may be an alternate type of transistor (e.g., a bipolar transistor). In some implementations, a pass device may be a combination of bipolar transistors coupled to form, for example, a Darlington transistor pair. In some implementations, the feedback resistors may be replaced by diodes or diode connected transistors. 
     In one embodiment, the first output voltage, V out1 , can be essentially equal to the second output voltage, V out2 , as the gates of both transistors  252  and  266  are coupled to the output of the operational amplifier output  264 . In some implementations, the base of the second transistor may be coupled to the gate of a second transistor may be coupled to the drain or source of a pass transistor. In these implementations, the output voltage of the second transistor will vary from the regulated output voltage of the pass transistor. 
     In a linear voltage regulator, the transistor that is in the path of the electric current flow from the input terminal to the output terminal of the regulator can be referred to as a pass transistor. The pass transistor of a linear regulator can be a field effect transistor (FET), bipolar or other type of transistor. It can also be a combination of a number of transistors, for example, a Darlington transistor pair. 
     In a multi-output voltage regulator used in this embodiment, a conventional single-output linear voltage regulator, either a series-type or a shunt-type, can be used to generate one of the outputs of the multi-output regulator, and one or more additional pass transistors can be used to generate additional output voltages. The gate (or base) terminals of the additional pass elements can be coupled to nodes, with relatively constant potentials, of the single-output regulator. 
     Referring back to  FIG. 2A , transistor M 1  can act as the pass transistor of the conventional single-output regulator  254 . In operation, the output V out1  of regulator  254  can be a constant and stable voltage. In operation, the output  264  of amplifier A 1  equals V out1 +V GS1 , where V GS1  is the gate-to-source voltage (V GS ) drop of transistor M 1 . In operation, transistor M 1  can operate in a saturation region and therefore its V GS  voltage can be relatively constant. Therefore, the output voltage  264  of amplifier A 1  can also be relatively constant. In regulator  250 , the output  264  of amplifier A 1  can control the gate terminal of a second pass transistor M 2  that generates a second output V out2  of regulator  250 . V out2  is given by V out2 =V out1 +V GS1 −V GS2 . In operation, both transistors M 1  and M 2  can work in saturation, thus both V GS1  and V GS2  are relatively constant. By adjusting the aspect ratio (W/L) of transistors M 1  and M 2 , it may be possible to make V out2  greater than, lower than, or almost equal to V out1 . 
     The regulator  250  can generate two regulated outputs; however, it may be possible to add more pass transistors in order to generate more output voltages. The output V out1  of regulator  250  can be better controlled than the output V out2  because V out1  is controlled with the closed-loop negative feedback loop generated by amplifier A 1 , transistor M 1 , resistor R F1  and resistor R F2 ; the resistor divider, consisting of resistor R F1  and resistor R F2 , samples the output V out1  and feeds back the sample to the negative input of amplifier A 1 . The output of amplifier A 1  moves according to the sampled voltage of V out1  such that transistor M 1  keeps V out1  at a desired voltage level. V out2  can be generated in an open-loop circuit topology, meaning that V out2  changes depending on the current in load 2  and there is no feedback mechanism to re-adjust V out2 . Therefore, V out1  can be used for circuits that need precise supply voltage, such as analog and RF circuits, while output V out2  can be used for digital and memory blocks. 
     Compared to two conventional regulators generating two different output voltages, the regulator  250  shown in  FIG. 2A , can be advantageous in terms of power consumption, because it can generate two different output voltages while consuming the power of only one regulator. In addition, regulator  250  can occupy a smaller integrated circuit chip area and can be less complex to design. 
     Compared to one conventional regulator (similar to the regulator  254 ) generating a single supply voltage for all the circuit blocks in an SOC, the regulator  250  can be advantageous because it consumes the same amount of power and consumes the same integrated circuit chip area, while it can isolate the supply voltage of sensitive-to-noise blocks from noisy supply voltages. 
     Referring now to  FIG. 2B , an exemplary multi-output linear voltage regulator  290  with multiple, isolated, regulated output voltages, V out1  and V out2 , can include a second transistor  294  (M 2 ) whose gate is coupled to the source of a pass transistor  292  (M 1 ), according to one embodiment. Referring to  FIG. 2B , the pass transistor  292  (M 1 ) can be an NMOS transistor. The voltage regulator  290  can function in a similar manner as the voltage regulator  250  in  FIG. 2A . 
     Regulator  291  can be a conventional single-output linear voltage regulator similar to regulator  254 . The combination of amplifier A 1 , transistor M 1 , resistor RF 1  and resistor RF 2  can generate a negative feedback loop which can keep the output V out1  of regulator  291  at a constant voltage given by V out1 =V REF *(R F1 +R F2 )/R F2 . 
     In regulator  291 , transistor M 1  can act as the pass transistor. Regulator  290  can be implemented by adding a second pass transistor M 2  to regulator  291 . The pass transistor M 2  can generate a second output V out2  of the regulator  290 . In operation, the output V out1  can be a constant and stable voltage. In regulator  290 , the gate terminal of the pass transistor M 2  can be coupled to the output V out1  which can have a relative constant and well-defined value. V out2  is given by V out2 =V out1 −V GS2 . In operation, transistor M 2  can operate in a saturation mode, thus V GS2  can be relatively constant, making V out2  a relatively constant and stable supply voltage. 
     In regulator  290 , V out2  can be less than V out1 . By adjusting the aspect ratio (W/L) of transistor M 2 , it may be possible to adjust the voltage difference between V out1  and V out2 . It may also be possible to use a native or low threshold voltage NMOS transistor to realize transistors M 1  and M 2 . In that case, it may be possible to make V out2  relatively equal to V out1 . 
     Regulator  290  can have similar advantages to regulator  250  when compared to multiple regulators generating multiple supply voltage for different circuit blocks of a SOC, or compared to a single regulator generating a single supply voltage for all circuit blocks of the SOC. 
     Referring now to  FIG. 2C , an exemplary multi-output linear voltage regulator  280  with multiple, isolated, regulated output voltages, V out1  and V out2 , can include a second transistor  284  (M 2 ) whose gate is coupled to the drain of a pass transistor  282  (M 1 ), according to one embodiment. Referring to  FIG. 2C , the pass transistor  282  (M 1 ) can be a PMOS transistor. 
     Regulator  281  can be a conventional voltage regulator similar to regulator  291 , but instead of using an NMOS transistor as the pass element, a PMOS transistor M 1  can be used as the pass element in regulator  281 . This can reduce the voltage drop from V in  to V out1 . The combination of amplifier A 1 , transistor M 1 , resistor R F1  and resistor R F2  can generate a negative feedback loop which can keep the output V out1  of regulator  281  at a constant voltage given by V out1 =V REF *(R F1 +R F2 )/R F2 . 
     Regulator  280  can be implemented by adding a second pass transistor M 2  to the single-output voltage regulator  281 . The pass transistor M 2  can generate a second output V out2  of the regulator  280 . In operation, the output V out1  can be a constant and stable voltage. In regulator  280 , the gate terminal of the pass transistor M 2  can be coupled to the output V out1 . V out2  is given by V out2 =V out1 −V GS2 . In operation, transistor M 2  can operation in a saturation mode, thus V GS2  can be relatively constant, making V out2  a relatively constant and stable supply voltage. 
     In regulator  280 , V out2  can be less than V out1 . By adjusting the aspect ratio (W/L) of transistor M 2 , it may be possible to adjust the voltage difference between V out1  and V out2 . It may also be possible to use a native or low threshold voltage NMOS transistor to realize transistor M 2 . In that case, it may be possible to make V out2  relatively equal to V out1 . 
     Regulator  280  can have similar advantages to regulator  250  when compared to multiple regulators generating multiple supply voltage for different circuit blocks of a SOC, or compared to a single regulator generating a single supply voltage for all circuit blocks of the SOC. 
     Referring now to  FIG. 3 , an exemplary multi-output linear voltage regulator  300  with multiple, isolated, regulated output voltages, V out1 , and V out2  can include a plurality of feedback resistors (resistor  302  (R′ F1 ), resistor  304  (R″ F1 ), and resistor  306  (R F2 )), according to one embodiment. The voltage regulator  300  can function in a similar manner as the voltage regulator  280  in  FIG. 2D . 
     Referring to  FIG. 3 , a first input  208  to an operational amplifier  310  can be a voltage, V P , which can be a percentage of the first output voltage, V out1 , where the percentage is determined by the resistor ratio of resistor  208  (R′ F1 ), resistor  304  (R″ F1 ) and resistor  306  (R F2 ) (e.g., V P =V out1 *((R′ F1 +R″ F1 )/R′ F1 +R″ F1 +R F2 )). The first input  218  can monitor the voltage, V P . A second input  312  (V REF ) to the amplifier  310  can be a stable voltage reference (e.g., a bandgap reference). A PMOS pass transistor  314  (M 1 ) can be coupled to an output  318  of the operational amplifier  310 . If the first output voltage, V out1 , increases to a voltage that is greater than the second input  312  (V REF ), which is the reference voltage, the drive to a pass transistor  314  can change to maintain a constant voltage for the first output voltage, V out1 . Therefore, the feedback loop can stabilize the regulated first output voltage, V out1 , of the series-type voltage regulator  316  to a pre-defined DC voltage. 
     Referring to  FIG. 3 , the second output voltage, V out2 , can be equal to the first output voltage, V out1 , minus the sum of the gate-to-source voltage drop across the second transistor  304  (M 2 ) and the voltage drop across the resistor  302  (R′ F1 ). The value of resistor  302  (R′ F1 ) can be selected to control the value of the second output voltage, V out2 . 
     Regulator  316  can be a conventional single-output voltage regulator similar to regulator  281  with the only difference being that resistor R F1  in regulator  281  is realized with the series connection of two resistors R′ F1  and R″ F1  as shown in  FIG. 3 . The combination of amplifier A 1 , transistor M 1 , resistor R′ F1 , resistor R″ F1  and resistor R F2  generate a negative feedback loop by which the output V out1  of regulator  316  is kept at a constant voltage given by V out1 =V REF *(R′ F1 +R″ F1 +R F2 )/R F2 . 
     Regulator  300  can be implemented by adding a second pass transistor M 2  to the conventional regulator  316 . The pass transistor M 2  can generate a second output V out2  of the regulator  300 . In operation, the output V out1  can be a constant and stable voltage. In regulator  300 , the gate terminal of the pass transistor M 2  can be coupled to the node which couples R′ F1  to R″ F1 . V out2  is given by V out2 =(R″ F1 +R F2 )/(R′ F1 +R″ F1 +R F2 )V out1 −V GS2 . In operation, transistor M 2  can operate in a saturation mode, thus V GS2  can be relatively constant, making V out2  a relatively constant and stable supply voltage. 
     In regulator  300 , V out2  can be less than V out1 . By adjusting the aspect ratio (W/L) of transistor M 2  and the ratios among resistors R′ F1 , R″ F1 , and R F2 , it may be possible to adjust the voltage difference between V out1  and V out2 . 
     Regulator  300  can have similar advantages to regulator  250  when compared to multiple regulators generating multiple supply voltage for different circuit blocks of a SOC, or compared to a single regulator generating a single supply voltage for all circuit blocks of the SOC. 
     Referring now to  FIG. 4 , an exemplary multi-output linear voltage regulator  400  with multiple, isolated, regulated output voltages, V out1 , V out2 , and V out3 , can include a gate of a second transistor  404  (M 2 ) coupled to a source of a PMOS pass transistor  402  (M 1 ), and a gate of a third transistor  406  (M 3 ) coupled to an output  410  of an operational amplifier  412 , according to one embodiment. The PMOS pass transistor  402  (M 1 ) can also be coupled to the output  410  of the operational amplifier  412 . A voltage regulator  408  can function in a similar manner as the voltage regulator  290  in  FIG. 2D . The voltage regulator  400  additionally includes the third transistor  406  (M 3 ) that generates the third isolated regulated output voltage, V out3 . Referring to  FIG. 4 , the second output voltage, V out2 , is equal to the first output voltage, V out1 , minus the gate-to-source voltage drop across the second transistor  404  (M 2 ). The third transistor  406  (M 3 ) can generate the third output voltage, V out3 , which is essentially equal to the first output voltage, V out1 . 
     The exemplary multi-output voltage regulators  250 ,  280 ,  290  and  300  shown in  FIGS. 2 and 3  can generate two output voltages: V out1  and V out2 ; however, these regulators can be modified to generate more than two output voltages. For example,  FIG. 4  shows an exemplary multi-output voltage regulator  400 , which can generate three output voltages. The NMOS pass transistors M 2  and M 3  can be coupled to a conventional single-output voltage regulator  408  to generate two additional voltage outputs: V out2  and V out3 . In operation, nodes in the regulator  408 , which have relatively constant potentials, can control the gates of transistors M 2  and M 3 . Regulator  400  can operate similarly to regulators  250  and  290 . Similar to the voltage output V out2  of regulator  250 , the output V out3  of regulator  400  can be related to V out1  by: V out3 =V out1 +V GS1 −V GS3 . In operation, both transistors M 1  and M 3  can operate in a saturation mode, thus both the voltages V GS1  and V GS2  can be relatively constant. 
     By adjusting the aspect ratio (W/L) of transistors M 1  and M 3 , it may be possible to make V out2  greater than, lower than, or almost equal to V out1 . 
     Similar to V out2  of regulator  290 , the output V out2  of regulator  4300  can be related to voltage V out1  by: V out2 =V out1 −V GS2 . In operation, transistor M 2  can operate in a saturation mode, thus V GS2  can be relatively constant, making V out2  a relatively constant and stable supply voltage. In regulator  400 , V out2  can be less than V out1 . By adjusting the aspect ratio (W/L) of transistor M 2 , it may be possible to adjust the voltage difference between V out1  and V out2 . 
     The regulator  400  can generate three regulated voltage outputs: V out1 , V out2  and V out3 ; however, it may be possible to add more pass elements in order to generate more than three output voltages. For example, it may be possible to add a fourth NMOS pass transistor to regulator  400  such that the gate terminal of the fourth pass transistor is coupled to the node, which is connected to the negative input of amplifier A 1 , and its drain terminal is coupled to V in . The source of the fourth pass transistor can generate a fourth output of the regulator. The voltage output V out1  of regulator  400  can be better controlled than the voltage outputs V out2  and V out3  because voltage V out1  can be controlled in the closed-loop negative feedback loop generated by amplifier A 1 , transistor M 1 , resistor R F1  and resistor R F2 ; the resistor divider, which includes R F1  and R F2 , can sample the voltage output V out1  and can feed back the voltage sample to the negative input of amplifier A 1 . The output of amplifier A 1  can move according to the sampled voltage of V out1  such that transistor M 1  can keep voltage V out1  at a desired voltage level. Voltages V out2  and V out3  can be generated in open-loop circuit topologies, meaning that voltages V out2  and V out3  can change depending on the currents in load 2  and load 3  and there is no feedback mechanism to re-adjust voltages V out2  and V out3 . Therefore, voltage V out1  can be used for circuits that need precise supply voltage, such as analog and RF circuits, while voltage outputs V out2  and V out3  can be used for digital and memory blocks. 
     Referring to  FIGS. 5A and 5B , exemplary multi-output linear voltage regulators  500  and  550  with multiple, isolated, regulated output voltages, V out1 , and V out2  can include a plurality of diode-connected transistors, according to one embodiment. The implementations in  FIGS. 5A and 5B  do not use a feedback loop to control and stabilize the first output voltage, V out1 . Alternatively, the implementations in  FIGS. 5A and 5B  can control the first output voltage, V out1 , using an open loop configuration. Transistor  502  and transistor  552  are representative of a diode-connected transistor in  FIG. 5A  and  FIG. 5B , respectively. 
     All the output voltages of the multi-output regulators,  500  and  550 , shown in  FIGS. 5A and 5B  can be controlled with open-loop circuits. In  FIGS. 5A and 5B , a number of diode-connected MOS transistors can be connected in series. Transistor  502  and transistor  552  are representative of a diode-connected transistor in  FIG. 5A  and  FIG. 5B , respectively. In operation, there can be a current flowing in these diode-connected transistors, thus the V GS  of these diode-connected transistors can be relatively constant. Thus, the sum of the V GS  of these diode-connected transistors can be relatively constant and can be used for controlling the gate (or base) of one or more pass devices of a linear voltage regulator. 
     Referring to  FIG. 5A , the multi-output linear voltage regulator  500  can include an open loop controlled series-type voltage regulator  510 . The value of resistor  508  (R 1 ) can be selected to control the amount of current flowing through the diode-connected transistors such that the transistors operate in their saturation region providing a constant drain-to-source voltage. The number of diode-connected transistors connected in series can determine the voltage at the gate of pass transistor  504  (M 1 ). The pass transistor  504  (M 1 ) can act as the regulating device in the voltage regulator  500 . The pass transistor  504  (M 1 ) can be placed in series with the load resistance  506 . The gate of the pass transistor  504  (M 1 ) can be coupled to the resistor  508  (R 1 ) and the top of the series connection of diode-connected transistors. 
     Referring again to  FIG. 5A , the first output voltage, V out1 , generated by the pass transistor  504  can be coupled to a circuit (e.g., analog circuitry). A second transistor  512  (M 2 ) can be coupled to the gate of the pass transistor  504  (M 1 ) which is coupled to the resistor  508  (R 1 ) and the top of the series connection of diode-connected transistors. The second transistor  512  (M 2 ) can generate a second output voltage, V out2 , in an open loop circuit. The second output voltage, V out2 , can be isolated from the first output voltage, V out1 . The pass transistor in the series-type regulator can effectively be divided into two transistors, the pass transistor  504  (M 1 ) and the second transistor  512  (M 2 ). In some implementations, the multi-output linear voltage regulator  500  can use the pass transistor  504  (M 1 ) and the second transistor  512  (M 2 ) to generate separate, isolated voltages (V out1  and V out2 , respectively) that can be supplied to analog circuitry and digital circuitry, respectively. 
     Referring to  FIG. 5B , the multi-output linear voltage regulator  550  can include an open loop controlled series-type voltage regulator  560 . The value of resistor  558  (R 1 ) can be selected to control the amount of current flowing through the diode-connected transistors such that the transistors operate in their saturation region providing a constant drain-to-source voltage. The number of diode-connected transistors connected in series can determine the voltage at the gate of pass transistor  554  (M 1 ). The pass transistor  554  (M 1 ) can act as the regulating device in the voltage regulator  550 . The pass transistor  554  (M 1 ) can be placed in series with the load resistance  556 . The gate of the pass transistor  554  (M 1 ) can be coupled to the resistor  558  (R 1 ) and the top of the series connection of diode-connected transistors. 
     Referring again to  FIG. 5B , the first output voltage, V out1 , generated by the pass transistor  554  can be coupled to a circuit (e.g., analog circuitry). A second transistor  562  (M 2 ) can be coupled to the series connection of the diode-connected transistors at a connection point below the top of the series connection. The connection point can be selected based on the desired regulated voltage value for the second output voltage, V out2 . The second transistor  562  (M 2 ) can generate a second output voltage, V out2 , in an open loop circuit. The second output voltage, V out2 , can be isolated from the first output voltage, V out1 . The pass transistor in the series-type regulator can effectively be divided into two transistors, the pass transistor  554  (M 1 ) and the second transistor  562  (M 2 ). In some implementations, the multi-output linear voltage regulator  550  can use the pass transistor  554  (M 1 ) and the second transistor  562  (M 2 ) to generate separate, isolated voltages (V out1  and V out2 , respectively) that can be supplied to analog circuitry and digital circuitry, respectively. 
     Referring to  FIG. 5A , the value of resistor  508  (R 1 ) can be selected to control the amount of current flowing through the diode-connected transistors. The number of diode-connected transistors can determine the reference voltage (V REF ) at the gate of pass transistors  504  (M 1 ) and  512  (M 2 ). The pass transistor M 1  can generate a first output V out1  and the pass transistor M 2  can generate a second output V out2 . Voltage V out1  can be given by V out1 =V REF −V GS1 , and voltage V out2  can be given by: V out2 =V REF −V GS2 . 
     Regulator  550  shown in  FIG. 5B  can operate in a manner similar to the regulator  500  shown in  FIG. 5A , but different reference voltages can be used to control the gates of pass transistors M 1  and M 2 . The pass transistor M 1  can generate a first voltage output V out1  and the pass transistor M 2  can generate a second voltage output V out2 . Voltage V out1  can be given by V out1 =V REF1 −V GS1 , and voltage V out2  can be given by: V out2 =V REF2 −V GS2 . 
     The exemplary regulator circuit  250  shown in  FIG. 2B  can be implemented in a transponder chip designed for a wireless implantable microsystem dedicated for blood glucose monitoring. The transponder chip, which was a SOC, was fabricated using a Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm complementary metal-oxide-semiconductor (CMOS) process. The desired output voltages, due to system design criteria, were 1.9 volts for the first output voltage, Voutl, and 1.8 volts for the second output voltage. Voltage Vout 1  was used as the supply voltage of analog circuits in the transponder chip and Vout 2  was used as the supply voltage for the digital circuits. 
     Amplifier  256  (A 1 ) was realized using a simple single-stage single-output differential pair amplifier. In order to achieve low-drop-out voltage regulation, both transistors M 1  and M 2  are realized by native NMOS transistors, which are available in almost all modern CMOS technologies. 
     Referring to  FIG. 6A , a graph  600  of an exemplary waveform  602  can be of a voltage signal supplied to analog circuitry in an SOC, according to one embodiment. Referring to  FIG. 6B , a graph  650  of an alternate exemplary waveform  652  can be of a voltage signal supplied to analog circuitry, according to one embodiment. For example, referring to  FIG. 2B , the voltage regulator  250  can be used, now referring to  FIG. 1 , as the voltage regulator  104  in the SOC  106  that includes the transponder  112 . The SOC  116  can be fabricated using a Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm complementary metal-oxide-semiconductor (CMOS) process. The operational amplifier  256  can be a single-stage fully differential amplifier. The pass transistor  252  (M 1 ) and the second transistor (M 2 ) can be native (zero-threshold) NMOS transistors to achieve low drop out voltage regulation. The desired output voltages, due to system design criteria, are the first output voltage, V out1 , equal to 1.9 volts and the second output voltage, V out2 , equal to 1.8 volts. In the example implementation, the first output voltage, V out1 , can supply voltage to analog circuitry and the second output voltage, V out2 , can provide supply voltage to digital circuitry. 
     Referring back to  FIG. 6A , the waveform  602  can represent the voltage value of a supply voltage coupled to analog circuitry, where the voltage regulator can supply regulated, non-isolated voltages to mixed signal circuitry. The waveform  602  shows fluctuations in the voltage level due to the coupling of switching noise from digital circuitry onto the analog voltage supply. Referring to  FIG. 6B , the waveform  652  can represent the voltage value of the first output voltage, V out1 , in the voltage regulator  250  in  FIG. 2B . The first output voltage, V out1 , can be coupled to analog circuitry. The isolated, second output voltage, V out2 , can supply voltage to the digital circuitry. Since the first supply voltage, V out1 , and the second supply voltage, V out2 , can be isolated from one another, the switching noise from the digital circuitry, in all likelihood, may not be coupled onto the analog circuitry. The waveform  652  shows a reduction of approximately 26 dB in the fluctuations in the supply voltage. 
     In some implementations,  FIG. 6A  can show the simulated output voltage V out1  when V out1  is coupled to V out2  and it is used as the supply voltage for both analog and digital circuits in the transponder chip.  FIG. 6A  shows fluctuations in V out1  due to voltage and/or current switching in the digital circuits when V out1  and V out2  are tied together.  FIG. 6B  shows the simulated V out1  when it is only used as the supply voltage for analog circuits in the chip and V out2  is used as the supply voltage for the digital circuits in the transponder chip.  FIG. 6B  shows fluctuations in V out1  due to switching in the digital circuits when V out1  and V out2  are not tied together. 
     Comparing  FIGS. 6A and 6B  reveals that the fluctuations in V out1  due to the switching in the digital circuit are reduced by about 26 dB, when the multi-output regulator  250  is used to generate two isolated supply voltages for analog and digital circuits. 
     Referring to  FIG. 7 , a graph  700  can be of exemplary time-domain measurement results of a first output voltage waveform  702  and a second output voltage waveform  704 , according to one embodiment. An oscilloscope can obtain the time-domain measurement results of the output voltages, resulting in the waveforms in the graph  700 . Referring to  FIG. 2B , the first output voltage, V out1 , (whose signal can be represented by the waveform  702 ) can supply voltage to analog circuitry and the second output voltage, V out2 , (whose signal can be represented by the waveform  704 ) can supply power to digital circuitry. In order to highlight the fluctuation in the first output voltage, V out1 , and the second output voltage  704 , V out2 , due to the switching in the digital circuitry, the oscilloscope coupling is set to AC. The graph  700  shows that while there are sharp edges in the waveform  704  of the second output voltage, V out2 , due to switching noise in the digital circuitry, little to no fluctuation is visible in the waveform  702  of the first output voltage, V out1 . 
     Referring to  FIG. 2B , for example, voltage regulator  250  can be optimized to deliver 100 μA of current to the first output voltage, V out1 , and 10 μA to the second output voltage, V out2 . The current consumption of the voltage regulator  250  can be 22 μA, of which 12 μA can be consumed in operational amplifier  256 , and the reminder in the feedback resistors, resistor  258  (R F1 ) and resistor  260  (R F2 ). The circuit can exhibit a line regulation of 30 mV/V, a ripple rejection of 28 dB (at 13.56 MHz), and a dropout voltage of 120 mV for the first output voltage, V out1 , and the second output voltage, V out2 . The load regulation for the first output voltage, V out1 , can be 18 mV/mA, and the load regulation for the second output voltage, V out2 , can be 450 mV/mA. 
       FIG. 7  shows the time-domain measurement results of V out1  and V out2 . In order to highlight the fluctuation in of V out1  and V out2  due to the switching in the digital circuits, the oscilloscope coupling is set to AC.  FIG. 7  shows that while there are sharp edges in V out2 , very small fluctuations happen in V out1 . 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the disclosed implementations. Accordingly, other implementations are within the scope of the following claims.