Patent Publication Number: US-2023161364-A1

Title: Linear regulator

Description:
BACKGROUND 
     Field 
     This invention relates generally to regulated voltage supplies, and more specifically to gallium nitride (GaN) transistor-based regulated power supply circuits. 
     Related Art 
     Voltage regulators operate to receive electrical power at varying voltages, such as from an unregulated power source, and produce electrical power with a constant, defined, voltage. Voltage regulators often use a voltage reference source that produces a consistent and known voltage but generally with only a small amount of electrical current. Voltage regulators often include circuitry to produce an electrical power output at a determined voltage with an appreciable amount of electrical current to supply various circuits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
         FIG.  1    illustrates a high-level diagram a GaN transistor-based regulated voltage source, according to an example. 
         FIG.  2    illustrates an example GaN transistor-based regulated voltage source circuit, according to an example 
         FIGS.  3 - 5    illustrate alternative GaN transistor-based regulated voltage source circuits, according to various examples. 
         FIG.  6    illustrates a fourth alternative GaN transistor-based regulated voltage source circuit, according to an example. 
         FIG.  7    illustrates a fifth alternative GaN transistor-based regulated voltage source circuit, according to an example. 
         FIG.  8    illustrates a sixth alternative GaN transistor-based regulated voltage source circuit, in accordance with an example. 
         FIG.  9    illustrates input filtered alternative GaN transistor-based regulated voltage source circuits, according to an example. 
         FIG.  10    illustrates a GaN transistor-based voltage regulator with lower reference, according to an example. 
         FIGS.  11  and  12    illustrate first and second alternative GaN transistor-based voltage regulators with lower references, respectively, according to examples. 
         FIG.  13    illustrates a sixth alternative GaN transistor-based regulated voltage source circuit, according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     The below described voltage regulators receive a raw/unregulated input voltage from an input power supply and provides a regulated voltage output with appreciable electrical current. In an example, a voltage regulator receives unregulated electrical power and a voltage reference signal and then produces a regulated output voltage to a gallium nitride (GaN) gate driver. In various examples, voltage regulators are able to provide one output with a fixed, regulated, voltage, or have multiple outputs that each provide its own different fixed, regulated voltage. These voltage regulators are able to provide regulated voltages to any suitable load within the electrical current capacity of the regulator. 
       FIG.  1    illustrates a high-level diagram a GaN transistor-based regulated voltage source  100 , according to an example. The depicted example GaN transistor-based regulated voltage source  100  has a voltage input port  120  that receives an unregulated input voltage, a voltage output port  122  that provides a regulated output voltage, a threshold voltage compensation network  102 , a reference voltage generator  104 , a ramp rate controller circuit  106 , and a GaN regulation transistor  108 . The reference voltage generator  104  in an example receives a reference voltage input  130  and provides a reference voltage to a reference input  110  of the threshold voltage compensation network  102  to support deriving appropriate internal references to regulate its output voltages at the desired levels. In further examples, the reference voltage generator is able to have its own voltage reference and not use a reference voltage input  130 . 
     In an example, the GaN regulation transistor  108  is a GaN high electron mobility transistor (HEMT). Challenges of using GaN HEMT devices in an output stage of a voltage regulator include wide variations in the threshold voltage of GaN HEMT devices that are dependent upon its manufacturing process. The threshold voltages of different GaN HEMT devices are able to vary in some cases by, for example, 0.5 V to 2.2 V. However, the threshold voltage of all the GaN HEMTs on the same die is within a reasonable range. In order to address these variations, the threshold voltage compensation network  102  includes an intermediate GaN transistor that is formed on the same substrate as the GaN regulation transistor  108 . In some examples, these GaN transistors are devices, such as GaN HEMTs, resistors and capacitors, that are able to be fabricated by commercially available GaN processes. In some examples, MOS P-type transistors or equivalent devices are not used. 
     In some instances, such as during startup/power up and power down of circuits, the input voltages at the voltage input port  120  of the GaN transistor-based regulated voltage source  100  can rise up or fall down with a fast ramp rate. The loads or circuits receiving electrical power from the output of the regulator might be sensitive to high ramp rates. In order to protect the loads or circuits from such high ramp rates, the illustrated GaN transistor-based regulated voltage source  100  includes the ramp rate control circuit  106  to control its output voltage ramp rate to a slower or controlled ramp rate during times such as startup and power down. 
     The reference voltage generator  104  in various examples is able to have any suitable design. In some examples, the reference voltage generator  104  is able to have a reference voltage input  130  that is connected to a voltage reference source (not shown) that produces a reference voltage. In an example, the voltage reference source (not shown) is able to be connected between the reference voltage input  130  and ground  124 . Such a reference voltage is able to be generated by utilizing, for example, a Zener diode, a voltage reference IC, a DC voltage source, a silicon diode or transistor, a potential divider network, a BandGap voltage Reference (BGR), any other known methods, or combinations of these. In various examples, one or more devices producing the reference voltage are able to be integrated with other components of the GaN transistor-based regulated voltage source  100 ; are able to be implemented as separate non-GaN components or devices; or combinations of these. 
     In further examples, the reference voltage source (not shown), such as those described above, is able to be connected between the voltage input port  120  and the reference input  110 . In some examples, the reference voltage generator  104  itself is able to include a voltage reference source and not rely on a reference voltage input  130 . The voltage reference source (not shown) in various examples is able to be one or more current references, voltage references, or a combination of current and voltage references. In some examples, a current reference is able to be realized using a current source IC, a BGR, using any other suitable method, or combinations of these. In various examples, one or more devices producing the reference voltage are able to be integrated with other components of the GaN transistor-based regulated voltage source  100 ; are able to be implemented as separate non-GaN components or devices; or combinations of these. 
     In various examples, as is described in further detail below, the value or magnitude of a reference voltage used by the reference voltage generator  104 , whether internal or external to the reference voltage generator  104 , is able to be lower than, equal to, or higher than the output voltage produced at the voltage output port  122 . It is also to be understood that current source references (not shown) are able to be used to generate reference voltages that are able to be of any value. 
     The GaN transistor-based regulated voltage source  100  shows that the reference voltage generator  104  provides a reference voltage signal to a reference input  110  of the threshold voltage compensation network  102 . The threshold voltage compensation network  102  in an example adds a small voltage component to the reference voltage signal received from the reference voltage generator  104  through the reference input  110  to compensate for the threshold voltage that is present in the GaN regulation transistor  108 . Because the threshold voltage compensation network  102  in some examples includes an intermediate GaN transistor that is formed on the same die, and thus the same substrate, as the GaN regulation transistor  108 , the threshold voltage compensation network  102  is able to accurately compensate for the threshold voltage of the GaN regulation transistor  108 . 
     The compensated voltage reference produced by the threshold voltage compensation network  102  in the illustrated example is passed through the ramp rate control circuit  106 . The illustrated ramp rate control circuit  106  is an example of a ramp rate control block that couples the intermediate GaN transistor and the GaN regulation transistor and is configured to operate in an example so as to limit or slow down the ramp rate of the gate voltage that is provided to GaN regulation transistor  108 . That threshold voltage compensated and ramp rate controlled reference voltage is applied to the gate terminal of the GaN regulation transistor  108 . 
     In various embodiments, the reference voltage generator  104 , the threshold voltage compensation network  102 , and the ramp rate control circuit  106  are able to be connected in any order or sequence. For example, the ramp rate control circuit  106  can be connected before the threshold voltage compensation network  102 . In various examples, elements of one or more of these components are able to be incorporated into other of these components. For example, elements of the ramp rate control circuit  106  are able to be incorporated into the threshold voltage compensation network  102 . 
       FIG.  2    illustrates a GaN transistor-based regulated voltage source circuit  200 , according to an example. The GaN transistor-based regulated voltage source circuit  200  depicts electrical circuit components of an example that realizes the GaN transistor-based regulated voltage source  100  described above. In further examples, the GaN transistor-based regulated voltage source  100  is able to be implemented by any other suitable circuit design. 
     The GaN transistor-based regulated voltage source circuit  200  depicts the voltage input port  120 , GaN regulation transistor  108 , voltage output port  122  and ground  124  of the GaN transistor-based regulated voltage source  100 . Various circuit elements are shown in this example that make up the threshold voltage compensation network  102  and the ramp rate control circuit  106 . 
     The Zener diode  212  in the GaN transistor-based regulated voltage source circuit  200  operates as a reference voltage source  130  that, in this example, also operates as the reference voltage generator  104  that provides a reference voltage to the reference input  110  of the threshold voltage compensation network  102 . In this example, this reference voltage is provided relative to ground  124 . The GaN regulation transistor Q 2   108  is the main GaN HEMT and carries the load current ILoad that is provided to the voltage output port  122 . 
     Resistor R 1   220  and intermediate GaN transistor Q 1   214  are elements of the threshold voltage compensation network  102 . Resistor R 1   220  provides a bias current to Zener diode  212 . The intermediate GaN transistor Q 1   214  is an auxiliary GaN HEMT device that is configured to operate as a GaN diode with its gate and drain shorted together. In operating as a GaN diode, the voltage drop between the source and drain of the intermediate GaN transistor Q 1   214  is approximately equal to, and thus compensates for, the threshold voltage (VT) of the GaN regulation transistor  108 . 
     The ramp rate control circuit  106  in this example is formed by the second resistor R 2   222  and capacitor C 2   224 , which form a lowpass filter. This low pass filter controls the ramp rate of the gate voltage presented to the GaN regulation transistor Q 2   108 , and hence, the ramp rate of the output voltage. In some examples, capacitor C 2   224  is able to be realized by using GaN HEMT capacitance, by any other capacitor, or by combinations of these. 
     The operation of the GaN transistor-based regulated voltage source circuit  200  is as follows. As unregulated input voltage VIN is applied to the voltage input port  120 , a bias current to the Zener diode  212  flows through resister R 1   220  and the intermediate GaN transistor Q 1   214 . The voltage reference at the reference input  110  is then fixed at voltage VREF, which is the Zener voltage VZ in this example. 
     The voltage at the drain of the intermediate GaN transistor Q 1   214  is equal to the sum of the Zener voltage of the Zener diode  212  and the threshold voltage of the intermediate GaN transistor Q 1   214 , i.e., VREF+VT. That voltage is passed through the low pass filter formed by the second resistor R 2   222  and capacitor C 1   224  to the gate terminal of the GaN regulation transistor Q 2   108 . That filter controls the ramp rate of voltage presented to the gate terminal of the GaN regulation transistor Q 2   108 . After a delay set by the values of the second resister R 2   222  and capacitor C 1   224 , the voltage on the gate terminal of the GaN regulation transistor Q 2   108  equals the sum of the Zener voltage of the Zener diode  212  and the threshold voltage of the intermediate GaN transistor Q 2   214 . 
     The voltage at the source of the GaN regulation transistor Q 2   108  follows its gate voltage, VGQ2, but the voltage at the source of the GaN regulation transistor Q 2   108  is (VT+VON) volts lower than its gate voltage VGQ2, where VON is the conduction voltage drop of the GaN regulation transistor Q 2   108 . The conduction voltage drop VON is a function of the channel resistance, Rds_on, of the GaN HEMT transistor and of the load current passing through the transistor. 
     In the steady state, VOUT at the voltage output port = VREF + VT - VT - VON =VREF - VON. 
     The voltage drop VON is typically small, and can be controlled by adjusting the width of the GaN regulation Q 2   108  that is formed on a substrate to meet the desired output voltage tolerance specification. As is clear from the above, the voltage present at the voltage output port  122  is maintained at VOUT ≈ VREF. 
       FIGS.  3 - 5    illustrate alternative GaN transistor-based regulated voltage source circuits, according to various examples.  FIG.  3    illustrates a first alternative GaN transistor-based regulated voltage source circuit  300 ,  FIG.  4    illustrates a second alternative GaN transistor-based regulated voltage source circuit  400 , and  FIG.  5    illustrates a third alternative GaN transistor-based regulated voltage source circuit  500 . These example alternative GaN transistor-based regulated voltage source circuits  300 ,  400 , and  500  illustrate realizations of a GaN transistor-based regulated voltage source that use alternative circuit topologies. In further examples, one or more capacitors are able to be connected across any particular resistor or a number of resistors to speed up response times to variations in various voltages such as during startup. 
       FIG.  6    illustrates a fourth alternative GaN transistor-based regulated voltage source circuit  600 , according to an example. The fourth alternative GaN transistor-based regulated voltage source circuit  600  includes components of the ramp rate control circuit  106 , which includes resistor  608  and capacitor  604 , rearranged to be included in the reference voltage generator  104 , which in this example is Zener diode  602 . The fourth alternative GaN transistor-based regulated voltage source circuit  600  supplies bias current for the Zener diode  602  from the output of an RC filter formed by capacitor  604  and resistor R 1   606  and R 2   608 . 
       FIG.  7    illustrates a fifth alternative GaN transistor-based regulated voltage source circuit  700 , according to an example. The fifth alternative GaN transistor-based regulated voltage source circuit  700 , as is true for other circuits described herein, is able to produce a regulated voltage at its voltage output port  122  that is able to supply a regulated voltage to an electrical load or the voltage present at the voltage output port  122  is able to be uses as a reference voltage for a further stage of a voltage regulator circuit, or both. 
     The fifth alternative GaN transistor-based regulated voltage source circuit  700  connects the positive terminal of its voltage reference, i.e., Zener diode  702  in this example, to the positive terminal of the voltage input port  120 . The fifth alternative GaN transistor-based regulated voltage source circuit  700  operates to generate a voltage VREF+VT at the gate terminal of the GaN regulation transistor Q 5   712 . The source of the GaN regulation transistor Q 5   712  provides another voltage reference VREF2 at the voltage output port  122  with respect to ground  124 . The value of the voltage reference VREF2 at the voltage output port  122  in this example is equivalent to the voltage reference VREF that is the Zener voltage of the Zener diode  702 . The fifth alternative GaN transistor-based regulated voltage source circuit  700  advantageously allows a reference voltage generator, such as the Zener diode  702 , to be connected to the positive terminal of the regulator’s voltage input port  120 . 
     The fifth alternative GaN transistor-based regulated voltage source circuit  700  includes a current copier circuit using two GaN HEMTs, a first internal GaN transistor Q 3   706  and a second internal GaN transistor Q 4   708 . The electrical current I 1  that flows through the first resistor R 5   704  equals (VIN - VREF - VT)/R5. In the illustrated example, the value of the first resister R 5   704  is equal to the value of the second resistor R 6   710 , such that R 6  = R 5 . That electrical current is copied through the second internal GaN transistor Q 4   708  as I 2 . Therefore, I 2  = I 1  = (VIN - VREF - VT)/R5. 
     The voltage at drain of the second internal GaN transistor Q 4   708  is equal to VIN -I2*R5 = VREF + VT. The voltage at the source of the GaN regulation transistor Q 5   712 , i.e., VREF2, follows its gate voltage of the GaN regulation transistor Q 5   712  by an amount that is approximately less than one threshold voltage VT. Therefore, VREF2 ≈VREF. 
     In the fifth alternative GaN transistor-based regulated voltage source circuit  700 , the GaN regulation transistor Q 5   712  also acts as a buffer and can source higher current to the voltage output port  122  without disturbing the original reference voltage input. Further variations of the fifth alternative GaN transistor-based regulated voltage source circuit  700  are also able to be realized, such as using either VREF2 and/or VREF+VT as a reference voltage for other circuits. In further examples, the first resistor R 5   704  that is in series with the first internal GaN transistor Q 3   706  and the second resistor R 6   710  that is in series with the second internal GaN transistor Q 4   708  are able to be specified to have different values in order to achieve a desired voltage gain at the gate terminal of the GaN regulation transistor Q 5   712 . In further examples, a separate biasing resistor (not shown) can be connected from the anode of Zener diode  702  to ground  124 . In another example, the GaN regulator transistor Q 5   712  is able to be specified to supply electrical current to an electrical load and operate as a main series pass regulator GaN HEMT, with the load connected to the voltage output port. 
       FIG.  8    illustrates a sixth alternative GaN transistor-based regulated voltage source circuit  800 , in accordance with an example. The sixth alternative GaN transistor-based regulated voltage source circuit  800  integrates the ramp rate control circuit, implemented by a first resistor R 2   804  and a capacitor C 1   802  together with its internal voltage reference circuit  806 . 
       FIG.  9    illustrates input filtered alternative GaN transistor-based regulated voltage source circuits  900 , according to an example. The input filtered alternative GaN transistor-based regulated voltage source circuits  900  depicts a seventh alternative GaN transistor-based regulated voltage source circuit  902  that connects capacitor C 1   926  across a first internal GaN transistor Q 3   922  in order to affect the ramp rate of the voltage across the first internal GaN transistor Q 3   922 , and thus the voltage at the gate terminal of the GaN regulation transistor Q 5   930 . In some examples, the first resistor R 5   920  and the second resistor R 7   928  have equal values. In further examples, the first resistor R 5   920  and the second resistor R 7   928  are able to be given different values by designers to vary the relationship between the values of the Zener diode voltage VREF and the voltage present on the gate terminal of the GaN regulation transistor Q 5   930 , thereby allowing any value of VREF2 at the voltage output port  122  and not limiting that output voltage value to the value of VREF which is the Zener voltage of the Zener diode  910 . 
     The input filtered alternative GaN transistor-based regulated voltage source circuits  900  also depicts an eighth alternative GaN transistor-based regulated voltage source circuit  904  that connects capacitor C 1   946  across the series connected first internal GaN transistor Q 3   942  and first resistor R 5   940  in order to limit the ramp rate of voltages across the first internal GaN transistor Q 3   942  and thus the ramp rate of the voltage at the voltage output port  122  as discussed above with respect to the seventh alternative GaN transistor-based regulated voltage source circuit  902 . 
     As discussed above with regards to the first resistor R 5   920  and the second resistor R 7   928  of the seventh alternative GaN transistor-based regulated voltage source circuit  902 , the values of the first resistor R 5   940  and the second resistor R 7   948  of the eighth alternative GaN transistor-based regulated voltage source circuit  904  are able to similarly have equal values or different values. In various other examples the internal GaN transistors Q 3  and Q 4  of the of the seventh alternative GaN transistor-based regulated voltage source circuit  902 , the eighth alternative GaN transistor-based regulated voltage source circuit  904 , or both, are able to be the same type of transistor or are able to be different types of transistors. 
       FIG.  10    illustrates a GaN transistor-based voltage regulator with lower reference  1000 , according to an example. The GaN transistor-based voltage regulator with lower reference  1000  operates with an input reference voltage VREF that is less than the regulated output voltage VOUT provided at the voltage output port  122 . In the illustrated example, the GaN transistor-based voltage regulator with lower reference  1000  includes a first resistor R 9   1020  and a second resistor R 11   1050  that are configured as a potential divider. This potential divider is an example of an output voltage adjuster that is configured to, when operating, reduce the regulated output voltage to the reference voltage for comparison to a voltage on the reference voltage input. For a specified output voltage VOUT to be delivered to the voltage output port  122 , the values of the first resister R 9   1020  and the second resistor R 11   1050  in this voltage potential divider are selected so that the voltage at their mid-point voltage is reduced to a value that is set be VFB = VREF, which is the Zener voltage of the Zener diode  1002 . The incorporation of this voltage potential divider to reduce the voltage present at the voltage output port  122  advantageously allows that output voltage to be compared to a lower input references voltage and thus allows the GaN transistor-based voltage regulator with lower reference  1000  to provide a regulated output voltage that is greater than its reference input voltage. In general, voltages produced at any mid-point of a voltage potential divider structure are able to be compared to any suitable reference voltage that is lower than the voltage desired to be produced at the voltage output port  122 . 
     The GaN transistor-based voltage regulator with lower reference  1000  includes an auxiliary GaN HEMT Q 6   1024 . In an example, the value of a third resistor R 10   1022  is much greater than the value of the first resistor R 9   1020 , i.e., R 10  » R 9 , and the auxiliary GaN HEMT Q 6   1024  is biased through the high-valued third resistor R 10   1022 . In some examples, the third resistor R 10   1022 , auxiliary GaN HEMT Q 6   1024 , and first resistor R 9   1020 , can be part of the potential divider itself. 
     The voltage drop VT across the auxiliary GaN HEMT Q 6   1024  in the illustrated GaN transistor-based voltage regulator with lower reference  1000  compensates for the threshold voltage VT of a second auxiliary GaN HEMT Q 5   1026 . The auxiliary GaN HEMT Q 6   1024  is an example of an intermediate GaN transistor that couples an output of the output voltage adjuster to the gate terminal of the GaN regulation transistor to introduce a voltage increase between the output of the output voltage adjuster and the gate terminal of the GaN regulation transistor, where the voltage increase is based on the voltage drop introduced by the second intermediate GaN HEMT Q 5   1026 . 
     The conduction drop VON for both the second auxiliary GaN HEMT Q 5   1026  and the auxiliary GaN HEMT Q 6   1024  are able to be neglected in this example due to their low drain currents. A sixth resistor R 2   1030  and first capacitor C 1   1032  function as the ramp rate control circuit  106 . In further examples, the GaN transistor-based voltage regulator with lower reference  1000  is able to be implemented with a fourth resistor R 8   1040  being replaced with short. A second capacitor C 2   1042  speeds up the feedback signal and in some examples is able to not be present. 
     The operation of the GaN transistor-based voltage regulator with lower reference  1000  is as follows. An input voltage VIN is applied to the voltage input port  120 . In general, the input voltage will ramp up from zero. A fifth resistor R 1   1004  biases the Zener diode  1002 , thereby establishing VREF at the source of the second auxiliary GaN HEMT Q 5   1026 . The second auxiliary GaN HEMT Q 5   2016  is initially off. The input voltage VIN at the voltage input port  120 , after slewing through the sixth resistor R 2   1030  and the first capacitor C 1   1032 , is present at the gate terminal of the GaN regulation transistor Q 2   1006 . The output voltage present at the voltage output port  122  follows the gate voltage of GaN regulation transistor Q 2   1006  at a value that is one threshold voltage VT below its gate voltage. As the feedback signal VFB at the source of the first auxiliary GaN HEMT Q 6   1024  tries to exceed the Zener voltage VREF of the Zener diode  1002 , the second auxiliary GaN HEMT Q 5   1026  starts conducting and starts pulling the gate voltage of the GaN regulation transistor Q 2   1006  downward. The feedback signal VFB at the source of the first auxiliary GaN HEMT Q 6   1024  then settles around VREF, which is the Zener voltage of the Zener diode  1002 , and the output voltage settles around the desired voltage VOUT. 
     In some examples, circuits similar to the GaN transistor-based voltage regulator with lower reference  1000  are able to be realized by rearranging resistive elements, removing or rearranging capacitive elements, or combinations of these. For example, the second capacitor C 2   1042  is able to be removed. 
     In some examples, the GaN transistor-based voltage regulator with lower reference  1000  is able to be used as a reference level enhancer, where VOUT at the voltage output port  122  is able to be used as a derived voltage reference, which in turn is able to be at a higher level than the reference VREF used by the GaN transistor-based voltage regulator with lower reference  1000 . By adjusting the first resistor R 9   1020  and the second resistor R 11   1050 , the derived reference VOUT provided at the voltage output port  122  is able to be set to any desired level, including values that are above the VREF used by that circuit. 
       FIG.  11    illustrates a first alternative GaN transistor-based voltage regulator with lower reference  1100 , according to an example. The first alternative GaN transistor-based voltage regulator with lower references  1100  is similar to the above-described GaN transistor-based voltage regulator with lower references  1000  where the Zener biassing resistor R 1   1004  is replaced by a biasing network  1102  that includes depletion HEMTs Q 1  and Q 2  and resistors R 1  and R 2 . This configuration operates to improve Zener regulation and reduces circuit area. The first alternative GaN transistor-based voltage regulator with lower reference  1100  also includes an error amplifier  1104  that includes enhancement-HEMT Q 5  and resistor R 5  resistor. The fourth resistor R 8   1040  of the GaN transistor-based voltage regulator with lower reference  1000  is also replaced by a network including depletion HEMT Q 4   1106  and R 3   1108 . These alternative aspects allow achieving more impedance thus higher gain, also saves the area compared to using resistive networks. 
       FIG.  12    illustrates a second alternative GaN transistor-based voltage regulator with lower reference  1200 , according to an example. The second alternative GaN transistor-based voltage regulator with lower reference  1200  is similar to the above-described GaN transistor-based voltage regulator with lower references  1000  and the first alternative GaN transistor-based voltage regulator with lower reference  1100 . With regards to the GaN transistor-based voltage regulator with lower references  1000 , the second alternative GaN transistor-based voltage regulator with lower references  1200  replaces the Zener biassing resistor R 1   1004  by a biasing network  1204  that includes depletion HEMTs Q 1  and Q 2  and resistors R 1 , R 2 , and R 3 . An error amplifier  1202  for the linear regulator is provided by a cascode amplifier formed by Q 4 , Q 5  and R 8 . This structure improves the bandwidth of the error amplifier thus improving the transient performance of the regulator and enhances the resistance of the error amplifier  1202 . The enhancement-HEMT Q 5  is the input transistor of this error amplifier  1202  and Q 4  is the cascode transistor which gets it bias from the network formed by Q 1 , R 1  and R 2 . 
       FIG.  13    illustrates a sixth alternative GaN transistor-based regulated voltage source circuit  1300 , according to an example. The sixth alternative GaN transistor-based regulated voltage source circuit  1300  is similar to the fifth alternative GaN transistor-based regulated voltage source circuit  700 . In this example, an internal current mirror  1302  includes Q 1 , Q 2  and pass-FET Q 6  receives the regulated voltage Vx  1306 , which is regulated by depletion HEMT Q 3   1304 . In this example, depletion HEMT Q 3   1304  receives its bias from a bias network  1308  that is formed by Q 4 , R 3 , Q 5 , R 4 , D 1  and D 2 . In this example, Q 4  and Q 5  are depletion HEMTs. The gate voltage of depletion HEMT Q 3 , i.e., Vgate, is: 
     
       
         
           
             Vgate 
             = 
             Vout 
             + 
             Vd1 
             + 
             Vd2 
           
         
       
     
     
       
         
           
             = 
             Vref 
             + 
             Vd1 
             + 
             Vd2 
           
         
       
     
     Returning to the GaN transistor-based regulated voltage source  100 , the voltage reference, such as the reference voltage input  130  provided to the reference voltage generator  104  or equivalent structures in any of the above-described circuits, is in general able to be of any suitable design. For example, current sources are able to be used with a series resistor to produce a reference voltage across the series resistor. In an example, an external current reference and an associated resistor (not shown) that form a current loop to generate the reference voltage across the associated resistor are able to be connected either between the voltage input port  120  and the reference voltage input  130  (i.e., high-side), or between the reference voltage input  130  and ground  124  (i.e., low-side). In examples that incorporate a high-side current reference, the transformed voltage generated across the associated resistor is able to be set with respect to ground and can be readily used as the desired voltage reference. 
     In some examples, a multi-output GaN transistor-based regulated voltage source is able to be created by integrating any combination of any number of the above-described GaN transistor-based regulated voltages sources. In one such example, each output voltage of the different GaN transistor-based regulated voltage sources is able to be of the same or of different magnitudes from one another. Different reference generator circuits are able to be used to generate appropriate voltage references from a common reference input, which is able to be a voltage or a current reference, or a combination of both. Further, some output voltage levels could be of higher magnitude than the reference voltage, while other outputs could be either same or less than the reference voltage. In some examples, one or more of the above-described GaN transistor-based regulated voltage sources are able to be used as reference voltage sources for other GaN transistor-based regulated voltage sources. In some examples, a GaN transistor-based regulated voltage source is able to provide a regulated voltage output that is able to be used as a voltage reference for another GaN transistor-based regulated voltage source as well as provide electrical power at a regulated voltage for other loads. In some examples, the output of a GaN transistor-based regulated voltage source is able to incorporate a resistive voltage divider between the GaN regulation transistor and ground in order to reduce the delivered output voltage. 
     As is understood by practitioners of ordinary skill in the relevant arts in light of the present discussion, the above-described circuits are examples and variations are able to be utilized. Various additional elements are able to be incorporated into circuits to achieve desired results, such as placing capacitors across resistors to speed up or slow down the response of any particular circuit, or placing resistors across capacitors in the circuit to modify response times. As is further understood, any of the above-discussed GaN HEMT devices are able to be replaced with multiple GaN HEMT devices in series, parallel or a combination of both. 
     The above-described circuits used to realize regulated voltage sources provide advantages over existing regulator circuits by obviating the use of op-amp based, high-gain amplifiers to regulate the output voltage at the reference level. The above-described circuits provide regulated voltage sources with tighter tolerance. The above-described circuits also overcome difficulties in of realizing op-amps using only GaN HEMT devices, which are equivalent to NMOS devices, without using CMOS devices. The above-described circuits implement GaN transistor-based regulated voltage sources that do not use op-amps. 
     The above-described circuits overcome a challenge in making GaN HEMT based circuits insensitive to, or independent of, the threshold voltage variation of different GaN HEMT devices that are used in a given circuit. The threshold voltage variation between devices produced by typically available processes varies widely, such as by 0.5 V to 2.2 V. The above-described circuits are able to compensate for the GaN threshold voltage variation by constructing circuits that use a pair of GaN HEMT devices that are formed on the same substrate to cancel out their threshold voltages and produce an output at a desired level. 
     The above-described circuits also support alternatives that allow a Zener-diode/voltage-reference to be connected to the high-rail, i.e., positive voltage supply, or the low-rail, i.e., negative or ground power supply. These alternatives overcome a challenge of conventional series pass linear regulators that have external voltage reference, e.g., a Zener diode, only able to be connected to the low-rail, i.e., the negative or ground supply. In the above example, the Zener diodes are non-GaN devices. 
     The specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages or solutions to problems described herein with regard to specific embodiments are not intended to be construed as a critical, required or essential feature or element of any or all the claims. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. 
     Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. Note that the term “couple” has been used to denote that one or more additional elements may be interposed between two elements that are coupled such that the one or more additional elements are able to be one of directly coupled without intermediate elements or indirectly coupled in which case intermediate elements are able to be present within the coupling structure. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below.