FIELD OF DISCLOSURE 
       [0001]    The present disclosure relates to electronic circuits. More specifically, the present disclosure relates to over-voltage clamping circuitry. 
       BACKGROUND 
       [0002]    In wireless data networking, there is a trend toward data converters with higher operating speeds. One design challenge in creating higher speed data converters is to create amplifiers that can rapidly charge, discharge, and amplify signals onto large capacitors. The charging, discharging, and amplifying generally consumes a significant portion of the power in data converters that include such amplifiers. 
         [0003]    Parasitic capacitances of the components of amplifiers tend to be a limiting factor in the speed of these amplifiers. Parasitic capacitance effectively puts a limit on the speed of amplifying voltages and getting the voltages to settle accurately. Parasitic capacitance also puts a limit on the clock rates of data converters. Nevertheless, wireless data networking standards continue to look for broadband data and require high sample rates in data converters. 
         [0004]    Many designers tend to choose higher voltage amplifiers in order to increase Signal to Noise Ratios (SNRs), since increased SNRs can often facilitate higher speed data processing. Thus, one traditional approach for designing data converters is to simply burn more power in an amplifier by making the amplifier use a higher supply voltage. However, higher voltage transistors (e.g., thick-oxide devices), used to make higher voltage amplifiers, tend to have larger parasitic capacitance than do lower voltage transistors (e.g., thin-oxide devices). The SNR gained from increasing power usage is offset by reduced speed due to increased parasitic capacitance. 
         [0005]    Another traditional design approach is to use folded cascode amplifiers. Folded cascode amplifiers provide greater speed and smaller parasitic capacitance than unfolded cascodes, but they also tend to use at least twice as much current. A third traditional approach is to accept the limitations and go with lower-speed data converters. 
       SUMMARY 
       [0006]    Various embodiments of the invention are directed to a clamp that prevents large voltages from being seen across a thin-oxide device. In one example, the thin-oxide device is a one-volt device that is used in an otherwise 2.5 volt amplifier. The clamp regulates the voltage to the thin-oxide device, thereby protecting the device from over-voltage situations. The effective voltage regulation of the clamp can help to ensure the reliability of the thin-oxide device. The end result, in many embodiments, is that a faster amplifier can be built around thin-oxide transistors, since thin-oxide transistors tend to be faster and subject to less parasitic capacitance than their thick-oxide counterparts. 
         [0007]    Other embodiments are directed to methods of providing voltage regulation. An example method includes producing a first current that corresponds to an input voltage (e.g., an input voltage of a thin-oxide device). A second current is also produced, which corresponds to a target level of the input voltage. The second current is subtracted from the first current, and any positive difference current is input to a clamping circuit. The clamping circuit uses the difference current to produce a fourth current. The fourth current is applied to one or more nodes to regulate the input voltage. 
         [0008]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a block diagram showing an exemplary wireless communication system in which an embodiment of the invention may be advantageously employed. 
           [0011]      FIGS. 2-2E  are an illustration of an exemplary amplifier adapted according to one embodiment of the invention; 
           [0012]      FIGS. 3-3F  are an illustration of a biasing circuit for use in the amplifier of  FIG. 2 ; 
           [0013]      FIG. 4  is an illustration of two voltage/current relationships, wherein the signal I diode  minus I replica  can be used by one or more embodiments to operate the clamping circuit of  FIG. 5 ; 
           [0014]      FIGS. 5-5C  are an illustration of an exemplary embodiment of a clamp, adapted for use in the amplifier of  FIG. 2 ; and 
           [0015]      FIG. 6  is an illustration of an exemplary method adapted according to one embodiment of the invention. 
       
    
    
       [0016]      FIGS. 2-5  generally use the following notational format: Using M 2  in the upper right of  FIG. 2D  as an example, TP indicates a transistor, PMOS, 2.5V (thick oxide), while TN would indicate a transistor, NMOS, core 1.3V (thin oxide); “50/0.4” indicates a width/length/width of a transistor gate in microns; m=8 indicates a multiplier factor of 8 (8 identical transistors in parallel); mp=25 is the number of fingers (thus 25 fingers of width=2 for total width=50). Furthermore, looking at  FIG. 2B , pdnB is an active low, inverted version of the pdn signal, and pdnD is a slightly delayed version of pdn, active high, after passing through two inverters for isolation purposes. “pdnD&lt;3:0&gt;” refers to an array of 4 signals: pdnD&lt;0&gt;, pdnD&lt;1&gt;, pdnD&lt;2&gt;, pdnD&lt;3&gt;. Similarly, “M 71 &lt;1:0&gt;” refers to an array of two transistors, M 71 &lt;1&gt; and M 71 &lt;0&gt;. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows an exemplary wireless communication system  100  in which an embodiment of the invention may be advantageously employed. For purposes of illustration,  FIG. 1  shows three remote units  120 ,  130 , and  140  and two base stations  150 ,  160 . It will be recognized that typical wireless communication systems may have many more remote units and base stations. Remote units  120 ,  130 , and  140  can include any of a variety of memory units, including, e.g., improved full-swing memory arrays. Remote units  120 ,  130 , and  140  can also include any of a variety of other components, such as Analog to Digital Converters (ADCs), Digital to Analog Converters (DACs), processors, delta sigma data converters, and the like. Embodiments of the invention can find use in various components, and especially in data converters, such as ADCs and DACs.  FIG. 1  shows forward link signals  180  from the base stations  150 ,  160  to the remote units  120 ,  130 , and  140  and reverse link signals  190  from the remote units  120 ,  130 , and  140  to base stations  150 ,  160 . 
         [0018]    Generally, remote units may include cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, fixed location data units such as meter reading equipment, and/or the like. In  FIG. 1 , remote unit  120  is shown as a mobile telephone, remote unit  130  is shown as a portable computer, and remote unit  140  is shown as a fixed location remote unit in a wireless local loop system. Base stations  150 ,  160  can be any of a variety of wireless base stations, including, e.g., cellular telephone base stations, wireless network access points (e.g., IEEE 802.11 compliant access points), and the like. Although  FIG. 1  illustrates remote units according to the teachings of the invention, the invention is not limited to these exemplary illustrated units. 
         [0019]      FIG. 2  is an illustration of exemplary amplifier  200  adapted according to one embodiment of the invention, and  FIG. 2  is accompanied by  FIGS. 2A-E , which show various portions of exemplary amplifier  200  in greater detail. In the exemplary embodiment shown, clamp  230  regulates voltages for thin-oxide transistors M 0 , M 1 , M 14 , and M 15 . The operation of amplifier  200  is described in detail below. 
         [0020]    Amplifier  200  may be employed in any of a variety of devices, including, but not limited to, data conversion devices. In this example, amplifier  200  is an Operational Transconductive Amplifier (OTA), which is a type of operational amplifier. Core amplifier portion  201  includes two boost amplifiers  210 ,  220 , the outputs of which are labeled oM and oP. Core amplifier portion  201  also includes input ports iM and iP that feed into the gates of transistors M 12  and M 13 . The differential input voltage at ports iM and iP is converted into a current by transistors M 12  and M 13 , which have a constant current bias as well as common mode control. A constant current flowing into the sources of transistors M 12  and M 13  will cause transistors M 12  and M 13  to amplify the differential input voltage and produce a differential current onto nodes nsm and nsp. 
         [0021]    Amplifier  200  also includes cascode transistors M 14  and M 15 , which increase the output impedance of core amplifier  201 . Further enhancing the effect of cascode transistors M 14  and M 15  is boost amplifier  220 , which adjusts the gate voltages (on nodes ngp and ngm) of cascode transistors M 14  and M 15  such that their sources are almost completely insensitive to the differential output voltage at oM and oP. Thus, the current coming out of transistors M 12  and M 13  is almost unchanged by a change in the output voltage of core amplifier  201 . The operation of boost amplifier  210  and transistors M 0  and M 1  is substantially the same as that described above with respect to boost amplifier  220  and transistors M 14  and M 15  (note that “TN” in  FIG. 2  indicates an n-type transistor, whereas as “TP” indicates a p-type transistor). Transistors M 2  and M 3  act as constant current sources to provide high impedance loads for the current from input transistors M 14  and M 15 . 
         [0022]    The high output impedance of core amplifier portion  201  allows amplifier  200  to add and subtract voltages onto capacitors (not shown), as well as to amplify voltages, all in a highly linear fashion enabled by the very high gain of amplifier  200 . In the particular embodiment shown, the circuitry of boost amplifiers  210  and  220 , as well as the surrounding transistors, operates in the range of 2.5V. However, transistors M 14  and M 15 , as well as M 0  and M 1 , are implemented as thin-oxide devices that operate in the 1.3V range. Various embodiments of the invention operate to regulate voltage at one or more terminals of transistors M 0 , M 1 , M 14 , and M 15  to ensure reliability and operability of those lower-voltage, thin-oxide transistors. In normal operation, transistors M 0 , M 1 , M 14 , and M 15  do not typically see high voltages because there is a constant current flowing through them. Generally, protection is provided to transistors M 0 , M 1 , M 14 , and M 15  only when very large voltages are driven into amplifier  200  during power up and power down. 
         [0023]    Amplifier  200  also includes clamp  230  and bias circuit  220 . Clamp  230  has nsm, nsp, psm and psp nodes, among others, as inputs. The outputs of core amplifier  210  (oM and oP) are used by clamp  230  as inputs and outputs (which is explained in more detail below). Clamp  230  also has bias inputs, the most relevant to this discussion being the nAdjThr signal, which is described in more detail below. In short, clamp  230  clamps the voltage at oM and oP, thereby protecting transistors M 0 , M 1 , M 14 , and M 15  from over-voltage conditions. The nAdjThr signal is one signal that is used to control the clamping procedure, ensuring no degradation during normal operation, but effective and accurate clamping when voltage exceeds normal operating voltages due to overly large input voltage to amplifier  200 . 
         [0024]    Bias circuit  220  provides various bias voltages for amplifier  200 , including the nAdjThr signal. The transistors between bias circuit  220  and clamp  230  (e.g., M 71  and M 54 ) are used for generating power down signals with various delays and shorting various nodes of the amplifier together so that the power down and power up process is sequenced in a way that keeps the transistors protected. 
         [0025]      FIGS. 3A-F , together, are an illustration of an exemplary embodiment of bias circuit  220  for use in exemplary amplifier  200 , and  FIG. 3  shows how various portions of exemplary bias circuit  220  are arranged in  FIGS. 3A-F . Bias circuit  220  includes a clamp sensor replica configured using transistors M 77  (an NMOS transistor) and M 58  (a PMOS transistor) ( FIG. 3C ). 
         [0026]    In this example, when the voltage on the gate of M 77  is more than an NMOS V th  and a PMOS V th  above the gate of M 58 , transistors M 77  and M 58  turn on, and have a current-to-voltage profile that roughly approximates square law. The gates of M 77  and M 58  are connected to a constant voltage difference generated by passing a constant current through series resistors R 2 -R 9  and R 12  ( FIG. 3B ). Transistors M 77  and M 58  drive this constant current through resistors R 2 -R 9  and R 12  to generate the constant voltage difference, which corresponds to the nominal trigger point of the clamp sensor  501  ( FIG. 5 ). In this particular example, the constant voltage is 1.28V across the gates of transistors M 77  and M 58 . 
         [0027]    The voltage across the gates of transistors M 77  and M 58  causes some current to flow into cascoded and diode-connected transistors M 116  and M 76 . Transistors M 116  and M 76  produce the nAdjThr signal, which corresponds to a nominal current that flows through the clamp sensor  501  ( FIG. 5 ) when the clamp sensor senses 1.28 volts. The nAdjThr signal should be substantially constant in this embodiment, but does vary over process and temperature and voltage to allow the clamp  230  to subtract the nominal current that flows through the clamp sensor  501  when the clamp sensor  501  senses 1.28 volts from the actual clamp sensor  501  current. Thus, the circuit including the transistors M 77  and M 58 , resistors R 2 -R 9  and R 12 , and transistors M 116  and M 76  ( FIG. 3E ) allows for the correction of process and temperature variations in clamp  230  by varying the nAdjThr signal so that variations in the nominal current through the clamp sensor are cancelled. 
         [0028]    Various embodiments of the invention use the nAdjThr signal to recreate the expected current that corresponds to sensing 1.28V. The expected current is then subtracted from current from an actual sensor (shown in  FIG. 5 ). When the difference current is positive, it indicates that the voltage at oP and oM is above 1.28V, its target voltage. 
         [0029]    Returning to  FIG. 2 , various embodiments of the invention use clamp  230  to protect thin-oxide transistors M 0 , M 1 , M 14 , and M 15  ( FIGS. 2D-E ). Clamp  230  keeps the voltage at oM and oP within a safe range and also releases the voltage very quickly once the over-voltage condition goes away. 
         [0030]    The output of example amplifier  200  is a substantially constant 1.2 mA of current, with output voltage nominally at 1.28V. Clamp  230  regulates the voltage at oM and oP by providing or counteracting up to the full 1.2 mA of current at the output nodes when needed. Further, in this example, transistors M 0 , M 1 , M 14 , and M 15  are nominally rated at about 1.3V and due to longer-than-minimum gate lengths have a safe maximum operating source-drain voltage V sd  voltage of about 1.46V. 
         [0031]    In an over-voltage condition, clamp  230  keeps Vsd at less than 1.46 volts by counteracting current at oP and oM. Thus, as the differential voltage at oP and oM nears 1.46V, the current produced by clamp  230  nears 1.2 mA, thereby bringing the voltage back down. When the differential voltage at oP and oM nears 1.28V, current produced by clamp  230  nears zero. The relationship of the voltage at oP and oM to the current produced by clamp  230  is shown in  FIG. 4 . In  FIG. 4 , I diode  is an approximate square law current signal produced by the clamp sensors shown in  FIG. 5 . Various embodiments of the invention use a difference current to activate clamp circuitry, and the difference current is shown in  FIG. 4  as I diode  minus I replica . I replica  is the current that corresponds to the nAdjThr signal. As can be seen from  FIG. 4 , the difference current has a steeper slope than does I diode , thereby providing quicker clamp circuit operation. 
         [0032]      FIGS. 5-5C  are an illustration of an exemplary embodiment of clamp  230 , adapted for use in amplifier  200 . Clamp  230 , in this embodiment, includes, among other things, sensor  501 , current mirror  502 , and clamp circuit  503 . 
         [0033]    Sensor  501  ( FIG. 5C ) includes four detectors, each detector including one NMOS and one PMOS transistor. The left-most detector includes M 87  and M 69  and it is connected to oM, the output of core amplifier  201  (which is a drain) and psm, which is at the source of M 0 . The next detector includes transistors M 86  and M 68 , and it is connected to oP, another output of core amplifier  201 , and psp, which is at a source of M 1 . The third detector includes M 85  and M 67 , and it is connected to oM and nsm, which is at a source of M 15 . The right-most detector includes M 78  and M 61 , and it is connected to oP and to nsp, which is at a source of M 14 . Sensor  501  produces a current corresponding to voltages in core amplifier  201 . There is also a constant trickle current from transistor M 130  that keeps current mirror  502  on. 
         [0034]    Current produced by sensor  501  is mirrored by current mirror  502  ( FIG. 5A ), which is a 4× current mirror in this example. The mirrored current goes to node oClamp by clamp circuit  503 . Transistors M 75  and M 133  produce a current that is subtracted from the current from current mirror  502 . The nAdjThr signal is supplied to the gates of transistors M 75  and M 133 , so that the nAdjThr signal is used to produce the replica current (I replica ), which is subtracted from the actual sensor current to produce a difference current. The difference current flows into clamp circuit  503 . 
         [0035]    It was mentioned above that transistor M 130  produces a trickle current. Current mirror  502  mirrors the sum of the trickle current and the sensor current. At node oClamp, six times the trickle current is subtracted from the 4× mirrored current. Also at node oClamp, six times the current that flows into transistor M 76  in  FIG. 3A  (to produce voltage nAdjThr) is subtracted from the 4× mirrored current. Thus, node oClamp will remain low and the clamp completely off until the current mirror  502  produces a current larger than six times the nominal current produced by the replica clamp sensor (M 58  and M 77  in  FIG. 3A ) plus twice the trickle current from transistor M 130 . Since the current mirror produces 4 times the current of the sensor  501  plus the trickle current from transistor M 130 , the clamp remains completely off until the sensor  501  produces a current larger than 150% of the nominal current plus half the trickle current. 
         [0036]    As long as the source/drain voltages (Vsd) of the detectors in sensor  501  are less than the 1.28 volt target voltage, the oClamp node will be pulled down because the entire current is being sunk by M 75  and M 133 , and node oCa will be very close to ground. M 88  is a diode-connected transistor, so there will be one gate/source voltage (Vgs) of about 0.6V. The oClamp node will then be at about one Vgs above ground, thereby guaranteeing that clamp circuit  503  is off when sensor  501  detects a voltage of 1.28 or less. 
         [0037]    Sensor  501  produces more current as the voltage as oM and oP gets higher. When the voltage at oM and oP is higher than 1.28V (an over-voltage condition), more current is being produced by 4× mirror  502  than is being sunk by M 75  and M 133 . In such a scenario, current flows into clamp circuit  503 . 
         [0038]    Clamp circuit  503  ( FIG. 5B ) includes three transistors—M 74 , M 73 , and Mclamp. Mclamp shorts the two outputs (oM and oP) of core amplifier  201  together. Clamp circuit  503  includes a 20:1 differential current mirror. Thus, any current coming from node oClamp into clamp circuit  503  results in twenty times as much current flowing, which is shorted across the outputs oM and oP. Clamp circuit  503  is a symmetrical device that produces current in one direction or the other depending on the polarity of the voltage at oM or oP. The current produced by the 20:1 current mirror is shorted to oM and oP, thereby lowering the voltage at oM and oP during an over-voltage condition. 
         [0039]    During normal operation, node oClamp is a Vgs above ground, and oM and oP are stable enough to stay around the target voltage of 1.28V. Thus, in normal operation, current will not flow into clamp circuit  503 . During an over-voltage condition, the differential voltage at oM and oP gets large, and some or all of the detectors gradually increase the current that flows into current mirror  502 . The mirrored current is sent to the oClamp node. Current flowing into clamp circuit  503  is mirrored and shorted to oM and oP up to a maximum of 1.2 mA, which is the constant current of core amplifier  201 . In a typical worst-case scenario, a very large voltage is applied to the inputs iP and iM of core amplifier  201 , thereby causing core amplifier  201  to completely drive its current to one side (oM or oP). When the differential voltage across oM and oP gets above 1.28V, clamp circuit  503  starts applying current up to a maximum of 1.2 mA, thereby stabilizing the voltage at oM and oP. Therefore, clamp circuit  503  protects thin-oxide transistors M 0 , M 1 , M 14 , and M 15  by regulating their input voltages to a safe operating range. 
         [0040]      FIG. 6  is an illustration of exemplary method  600  adapted according to one embodiment of the invention. Method  600  may be performed, in many embodiments, by circuitry operating according to the principles and concepts described above. 
         [0041]    In step  601 , a first current is produced, the first current corresponding to an input voltage. In some embodiments, the input voltage is an input voltage of one or more devices that should be kept within a safe operating range. For example, thin-oxide devices tend to have lower operating voltage ranges than do thick-oxide devices. However, various embodiments of the invention are not limited thereto, as the input voltage can be for a thick-oxide device, a thin-oxide device, or some other device. In the example embodiment of  FIGS. 2-5 , there are four input voltages, each being associated with one of transistors M 0 , M 1 , M 14 , and M 15 . In that example, the current corresponding to an input voltage is a mirrored current resulting from a square-law current produced from circuitry sensing the input voltage. 
         [0042]    In step  602 , a second current corresponding to a target input voltage is produced. The target input voltage in many embodiments is a voltage that falls within an operating range of one or more devices. For instance, in the example above, the target voltage corresponds to a safe operating voltage for four thin-oxide transistors. In that example, a signal (nAdjThr) is used to produce a constant current that draws down a node that also receives the first current. 
         [0043]    In step  603 , a third current is produced by subtracting the second current from the first current. Thus, when the first current is larger than the second current, the third current is positive. In the examples above, production of the third current is indicative of an over-voltage condition at one or more of transistors M 0 , M 1 , M 14 , and M 15 . 
         [0044]    In step  604 , the third current is input to a differential clamp. The differential clamp uses the third current to stabilize the input voltage by producing a fourth current and applying the fourth current to nodes connected to the input voltage. In the example above, the clamp is in a feedback loop during an over-voltage condition, acting as a constant current source so that the current applied from the clamp circuit displaces current produced by an amplifier, thereby regulating (limiting) the voltage at the nodes. 
         [0045]    Method  600  is shown as a series of discrete steps. However, the invention is not so limited. For instance, in many embodiments, steps  601 - 604  are performed continuously and are perceived to occur simultaneously and in real time. Further, various embodiments may add, delete, modify, or rearrange some steps. 
         [0046]    Embodiments of the invention may include one or more advantages over prior art solutions. For instance, clamping techniques, such as those described above, can be used to regulate voltages quickly and effectively. Effective voltage regulation can facilitate the use of thin-oxide devices in circuits that have normal operating voltages above the operating voltages of the thin-oxide devices. For instance, in the example embodiment of  FIG. 2 , transistors M 0 , M 1 , M 14 , and M 15  are thin-oxide devices that operate in the 1.3V range, whereas other circuitry of core amplifier  201  operates in the 2.5V range. 
         [0047]    Further, triggering a clamp circuit from a difference current, rather than from a current that directly models an input voltage, allows for sharper response. As shown in  FIG. 4 , the current I diode  minus I replica  shows a steeper slope that does I diode . The steeper slope provides faster response in the range of operation. Additionally, the current I diode  minus I replica  shows zero current when the input voltage is at or below 1.28V, so that the clamp is off when the voltage is in an optimal range. 
         [0048]    Although specific circuitry has been set forth, it will be appreciated by those skilled in the art that not all of the disclosed circuitry is required to practice the invention. Moreover, certain well known circuits have not been described, to maintain focus on the invention. 
         [0049]    It should also be further noted that while specific voltage and current ranges, transistor types (e.g., NMOS), and configurations have been shown, various embodiments of the invention are not limited thereto. In fact, voltage and current ranges, as well as sizes and types of transistors and circuit architecture may be adapted to a variety of systems, each system suggesting one or more changes to the embodiments shown in  FIGS. 1-6 . 
         [0050]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, although a read operation has been used in the discussion, it is envisioned that the invention equally applies to write operations. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
An active over-voltage clamp system includes at least one over-voltage detector that is responsive to an input voltage and provides a first current. The system also includes a replica over-voltage circuit that provides a second current, and circuitry subtracting the second current from the first current to produce a difference current. The system further includes a differential clamp activated in response to the difference current. The differential clamp prevents the input voltage from increasing beyond a target voltage.