Abstract:
Some embodiments regard a circuit comprising: a high voltage transistor providing a resistance; an amplifier configured to receive a current and to convert the current to a first voltage that is used in a loop creating the current; and an automatic level control circuit that, based on an AC amplitude of the first voltage, adjusts a second voltage at a gate of the high voltage transistor and thereby adjusts the resistance and the first voltage; wherein the automatic level control circuit is configured to adjust the first voltage toward the first reference voltage if the first voltage differs from a first reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional of and claims the priority of U.S. application Ser. No. 12/704,719, filed on Feb. 12, 2010, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure is generally related to automatic level control. In various embodiments, an automatic level control circuit is used in a microelectromechanical systems or structures (MEMS) gyroscope drive loop to cover a wide current range of the gyroscope. 
     BACKGROUND 
     In a MEMS gyroscope drive loop, an electrical current (e.g., current Igyro) is generally created from mechanical displacement of the gyroscope and converted to electrical voltage for a phase-lock loop (PLL) input signal. This current Igyro, however, can have a wide range covering many orders of magnitude. For example, in an approach, the range expands four orders of magnitude, such as from 0.2 nA to 2 μA. To convert this wide current range to a reasonable voltage level, a corresponding wide range of variable resistance (e.g., a resistor Rimp) is used so that the large resistance value at the upper range can correspond to the low current value amplification. Because of the large resistance requirement and thus large die area, resistor Rimp is typically implemented “off-chip,” i.e., outside of the die/semiconductor chip embodying the MEMS structure, and on a board level. Further, resistor Rimp is generally required to be adjustable to maintain sufficient electrical voltage level for various currents Igyro to lock the PLL, to limit the voltage level, and to prevent the device from being overstressed. Adjusting resistor Rimp for appropriate amplification is commonly done manually by human beings, which is not suitable for mass production. 
     In another approach, to achieve automatic control, an automatic level control (e.g., ALC) unit is designed utilizing an NFET with proper control at the gate to provide resistance in place of the gyro resistor Rimp. The unit, however, uses two stage of amplification to cover the four orders of magnitude difference of the Igyro. The unit also uses sophisticated circuitry with amplifiers and diodes, all of which is prone to errors and large power consumption in the μA range, making the unit inefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description, drawings, and claims. 
         FIG. 1  shows an exemplary circuit upon which embodiment of the invention may be implemented. 
         FIG. 2  shows an embodiment of the automatic level control of the circuit in  FIG. 1 . 
         FIGS. 3-9  show the relationship between various currents and voltages in accordance with the illustrative embodiment of  FIG. 1 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Embodiments, or examples, of the invention illustrated in the drawings are now being described using specific language. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and modifications in the described embodiments, and any further applications of principles of the invention described in this document are contemplated as would normally occur to one skilled in the art to which the invention relates. Reference numbers may be repeated throughout the embodiments, but this does not necessarily require that feature(s) of one embodiment apply to another embodiment, even if they share the same reference number. 
     Exemplary Application 
     Gyroscope Drive Loop 
       FIG. 1  shows a circuit  100  upon which embodiments of the invention may be implemented. 
     High voltage NMOS transistor (HV NMOS transistor) Mres provides a large resistance (e.g., resistance Res, not shown) when the voltage at its gate (e.g., voltage Vres) is properly adjusted. Generally, because of the inverse relationship between the gate voltage Vres of transistor Mres and its resistance Res, some embodiments of the invention increase voltage Vres to decrease resistance Res and decrease voltage Vres to increase resistance Res. Further, as transistor Mres can provide a wide resistance range, amplifier  105  can appropriately amplify current Igyro and convert it to voltage Vpllin. As a result, embodiments of the invention do not require more than one amplification stage (e.g., two amplification stages like another approach). Further, because transistor Mres does not take much die space as compared to a resistor, transistor Mres can be implemented “on-chip,” e.g., on the same die/semiconductor chip embodying circuit  100 . Depending on applications, an HV NMOS transistor (e.g., HV NMOS transistor Mres) is generated based on special doping as compared to a standard NMOS counterpart. In an embodiment, HV NMOS transistor Mres is configured to have an operation voltage (e.g., voltage Vdd) at about 1.8V to about 30V while voltage Vdd for other circuitry in circuit  100  remains at about 1.8V. Using an HV NMOS transistor (versus a standard NMOS transistor) is advantageous over other approach because the HV NMOS can provide a wider range of resistance than that of the standard NMOS transistor. 
     Generally, when MEMS gyro  130  is moved and/or rotated, current Igyro is created and includes an alternating current (AC) component. In various embodiments current Igyro oscillates in the range of 15 KHz, and embodiments of the invention can respond to a wide range of this current Igyro, which could expand from 0.2 nA to 2 μA in amplitude. For example, embodiments of the invention can detect the lower end of current Igyro at 0.2 nA, but are not subject to saturating or overstressing the circuit when current Igyro reaches its high end of 2 μA. 
     Amplifier  105  amplifies the I-to-V conversion, e.g., converting current Igyro to voltage Vpllin, which comprises a direct current (DC) voltage (e.g., from the common mode voltage) plus some AC amplitude. If current Igyro does not include an AC component, voltage Vpllin functions in the DC level of the common mode voltage. Typically voltage Vpllin oscillates at the same frequency as current Igyro, which, in various embodiments, is at about 15 KHz. Voltage Vpllin also serves as an input to phase-lock loop  110  and to automatic level control (ALC) circuit  134 . Generally, the amplitude of voltage Vpllin results from Igyro*Res, or, in another word, voltage Vpllin equals the common mode voltage of amplifier  105  (e.g., Vcm) plus some AC component resulting from the AC component of current Igyro. Embodiments of the invention detect this AC component, or in fact, the amplitude of voltage Vpllin. For example, if Vcm=1.5 Volts (V), the amplitude of Vpllin is 1 V, and α is the amplifying factor, then Vpllin=1.5±1*(α). In a particular embodiment, because amplifier  105  functions as a source follower, α is less than (“&lt;”) 1. If voltage Vpllin including its common mode and the amplitude is higher than the threshold of transistor M 1  ( FIG. 2 ), voltage Vpllin turns on this transistor M 1 , and turns it off otherwise. As will be explained in details below, if voltage Vpllin is undesirably high, embodiments of the invention pull it down, and if it is undesirably low, embodiments pull it up. Typically, the low of voltage Vpllin, when appropriate, should be high enough to be recognized by the circuitry environment in which voltage Vpllin operates, and the high of voltage Vpllin should not be too high with reference to voltage Vdd (the voltage supply operation of the circuitry) that can damage related circuitry. Depending on applications, various embodiments of the invention may be used in the CMOS environment. In an embodiment, voltage Vpllin is used to drive the next stage including an inverter, and the low of voltage Vpllin is configured to be high enough to toggle that inverter, and the high of voltage Vpllin is configured to be around Vdd or lower to not stress the circuitry. 
     Voltage Vcm provides a bias point for amplifier  105 . Based on the relationship between Igyro, resistance Res, and voltage Vpllin, ALC  134  controls resistance Res and thus controls the amplification of amplifier  105  or the amplitude of voltage Vpllin. 
     Phase-lock loop (PLL)  110  locks in the clock or frequency of voltage Vpllin. Driver  120  provides voltage Vdr to control MEMS gyro  130 . Generally, the higher the voltage Vdr, the higher oscillation results from MEMS gyro  130  and the higher current Igyro is created. 
     Automatic level control (ALC)  134  takes voltage Vpllin as an input, provides a corresponding voltage (e.g., voltage Vamp,  FIG. 2 ), and, based on the proportionality of the AC amplitude of voltage Vpllin and voltage Vamp with reference to a threshold voltage, adjusts voltage Vres to adjust resistor Res in a desired direction. For example, if Igyro*Res is larger than a certain threshold that can affect performance of other circuitry, ALC  134  increases voltage Vres to decrease Rres and thus decrease Igyro*Res. In effect, embodiments of the invention control amplification of amplifier  105  by controlling voltage Vres, thereby control resistance Res and voltage Vpllin. 
     Automatic Level Control 
     Exemplary Embodiment 
       FIG. 2  shows an automatic level control (ALC)  200  illustrating an embodiment of ALC  134 . ALC  200  may be considered as having an amplitude detector  210  and a level control  220 , but embodiments of the invention are not limited to such a characterization, but are applicable to various implementations of ALC  134  consistent with the disclosure in this document. 
     Transistor M 1  functions like a switch to toggle voltage Vamp based on the amplitude of voltage Vpllin. In operation, when there is no AC amplitude transistor M 1  settles in the DC operation point. In various embodiments of the invention, transistor M 1  is biased in the DC bias point, and the AC amplitude of voltage Vpllin turns on/off transistor M 1 . That is when Vcm (the common mode voltage) of amplifier  105  or the DC level of voltage Vpllin plus its amplitude is higher than the threshold of transistor M 1 , transistor M 1  is turned on causing a charge at capacitor Camp. When Vcm minus its amplitude is smaller than the threshold of transistor M 1 , transistor M 1  is off and there is no charge for capacitor Camp. 
     Capacitor Camp stores charges for voltage Vamp. That is, when transistor M 1  is on, voltage Vamp charges this capacitor Camp; and if transistor M 1  is off, there is no charge. 
     Voltage Vamp is proportional to voltage Vpllin. Mathematically, Vamp=Vpllin−Vthm1 (the threshold voltage of transistor M 1 ). 
     Programmable voltages Vhi and Vlo serve as the reference voltages for comparators Cup and Cdn. When appropriate, if voltage Vamp is lower than voltage Vlo, ALC  200  causes voltage Vamp to increase to voltage Vlo. Similarly, when Vamp is higher than Vhi, ALC  200  causes voltage Vamp to decrease to voltage Vhi. In effect, voltage Vlo and Vhi together control voltage Vamp in a range higher than voltage Vlo and lower than voltage Vhi. Depending on applications (e.g., if no range is desired), voltage Vlo may be equal to voltage Vhi. Alternatively expressing, voltage Vlo and Vhi may be set in a range for voltage Vamp to fit in. In various embodiments, circuit  100  (and thus  200 ) operates in the CMOS level (e.g., 0V to Vdd at 1.8V), voltage Vlo and Vhi may be set at about 1.5V and 1.8V (or even 2.0V, depending on situations) respectively. The 1.5V lower limit is sufficient for the CMOS low level operation while the 1.8V higher limit sets the limit to not overstress relevant circuitry. 
     Based on the proportionality between voltages Vamp and Vpllin as compared to voltages Vhi and Ylo, amplitude detector  210  creates appropriate signals (e.g., signals Up and Dn) to have voltage Vres and resistance Res adjusted accordingly. Amplitude detector  210  provides mechanisms to maintain a desirable amplitude of voltage Vamp. For example, if this amplitude of voltage Vamp is lower than desirable, amplitude detector  210  activates signal Dn to increase resistance Res, which, going through a loop, causes this amplitude of voltage Vamp to increase. In contrast, if this amplitude is higher than desirable, amplitude detector  210  activates signal Up to decrease resistance Res, which, via the same loop, causes this amplitude to decrease. Embodiments of the invention set (e.g., predefine) voltages Vhi and Vlo based on the desired amplitude of voltage Vpllin and/or voltage Vamp. 
     In effect, voltages Vhi and Vlo, via comparators Cup and Cdn, control (e.g., charge or discharge) voltage Vres based on the amplitude of voltage Vamp with respect to voltages Vhi and Ylo. If voltage Vamp is low, e.g., lower than voltage Vlo, comparator Cdn activates signal Dn, e.g., causing it to be high, while comparator Cup deactivates signal Up, e.g., causing it to be low. An activated signal Dn activates transistor Ndn and a de-activated signal Up de-activates transistor Nup, which causes capacitor Cres to be discharged, or voltage Vres to decrease. As voltage Vres decreases, resistance Res increases causing voltage Vpllin to increase because current Igyro remains the same. Voltage Vpllin increases causing voltage Vamp to increase until voltage Vamp=Vlo. 
     If voltage Vamp is large, e.g., higher than voltage Vhi, comparator Cup activates signal Up, e.g., causing it to be high, while comparator Cdn de-activates signal Dn, e.g., causing it to be low. An activated signal Up activates transistor Nup and a de-activated signal Dn de-activates transistor Ndn, which causes capacitor Cres to be charged up to voltage Vdd through transistor Up, which causes voltage Vres to increase. As voltage Vres increases, resistance Res decreases causing voltage Vpllin to decrease because current Igyro remains the same. Voltage Vpllin decreases causing voltage Vamp to decrease until Vamp=Vhi. In the above illustration, when transistor Nd is on, transistor Nup is off and vice versa. Further, capacitor Cres, transistors Nup and Ndn all together function as a charge pump. 
     Transistor Mleak provides a large resistance to compensate for current leakage of transistor M 1  and thus protect related circuitry. For example, based on the amplitude of voltage Vpllin and thus of voltage Vamp, the leakage of transistor M 1  can bring voltage Vamp up to voltage Vdd and thus damage related circuitry. In an embodiment, transistor Mleak is biased in the sub-threshold region (or weak-inversion region) where the gate-to-source voltage is below its threshold voltage. In an embodiment, voltage Vb that biases transistor Mleak is set to about the threshold voltage of transistor Mleak. Since transistor Mleak is in the sub-threshold region, its leakage, if arises, is small. This leakage also corresponds to the leakage of transistor M 1 . In effect, adding transistor Mleak prevents current from the drain to the source of transistor M 1  from reaching to a large but undesirable value, and voltage Vamp from being charged up to Vdd. Any current leakage from transistor M 1  would leak through transistor Mleak. As a result, voltage Vamp remains in the desired amplitude proportional to voltage Vpllin. In an embodiment transistor Mleak is also an HV NMOS transistor for a wider range of resistance. In some embodiments, the sizes of transistor M 1  and Mleak are about one to one for the corresponding leakage compensation. 
     Amplitude detector  210  is distinguished from a peak detector used in other approaches because amplitude detector  220  is much simpler without complicated circuitry such as amplifiers, diodes, etc., in those approaches. Using an amplitude detector  135  (versus a peak detector), embodiments of the invention do not require an exact measurement for voltage Vamp, but can rely on the proportionality between the amplitude of voltage Vamp and of voltage Vpllin. In an embodiment, amplitude detector  210  consumes less than 100 nA during detection, and is advantageous over other approaches that use amplifiers and complicated circuit and that consume power in the μA range. 
       FIGS. 3 ,  4 ,  5 , and  6  show waveforms  300 ,  400 ,  500 , and  600  illustrating the relationship between current Igyro and various voltages of circuit  100 , in accordance with an embodiment, Waveform  300  shows that the amplitude of current Igyro varies from a small value of 0.2 nA to a large value of 2 μA from time t 1  (e.g., at 0 S) to time t 2  (e.g., at about 1.0 S). Waveforms  400 ,  500 , and  600  show voltage Vpllin, voltage Vres, and voltage Vamp corresponding to current Igyro during the same time t 1  to t 2 . Waveforms  300  and  400  are sinusoidal, but for amplitude illustration, the sinusoidal details are not shown. 
     During time t 1  to t 2  when the amplitude of current Igyro in  FIG. 3  changes from 0.2 nA to 2 μA, voltage Vpllin in  FIG. 4  remains a steady level between 1.7 and 1.3 V, which is an amplitude of plus and minus 200 mv around the common voltage of 1.5 V. At the same time, voltage Vres in  FIG. 5  changes from about 2.5 V to about 3.05 V. These waveforms  300 ,  400 , and  500  illustrate that embodiments of the invention automatically adjust resistance Res as current Igyro continuously changes from 0.2 nA to 2 μA. This is because Vpllin (its amplitude)=Igyro*Res, and Igyro increases but voltage Vpllin (almost) remains the same, resistance Res must be decreasing, which corresponds to the increase of voltage Vres from about 2.V to 3.05V. Waveform  600  shows voltage Vamp remains constant at about 1.0V, which is consistent with the fact as explained above that voltage Vamp is proportional to voltage Vpllin. 
       FIG. 7  shows waveforms  700  illustrating the relationship between voltage Vres and current Igyro during a period of about 300 mS, in accordance with an embodiment. Waveforms  710 ,  720 ,  730 ,  740 , and  750  correspond to voltages Vres that corresponds to current Igyro at 0.2 nA, 2 nA, 20 nA, 200 nA, and 2 μA, respectively. Waveforms  700  show that, after some settling time t02 nA, t2 nA, t20 nA, t200 nA, and t2 μA, voltage Vres stays constant, but the constant level is proportional to current Igyro. For example, the levels of voltage Vres are at about 2.24 V, 2.44 V, 2.61 V, 2.8 V and 3.16 V corresponding to current 0.2 nA, 2 nA, 20 nA, 200 nA, and 2 μA, respectively. As a result, based on these waveforms, a desired level of voltage Vres may be selected based on a corresponding value of current Igyro. For example, some embodiments can set current Igyro at 0.2 nA, 2 nA, 20 nA, 200 nA, and 2 μA, etc., to get a desired amplitude of 2.24 V, 2.44 V, 2.61 V, 2.8 V and 3.16 V, etc., respectively. 
       FIGS. 8 and 9  show waveforms  800  and  900  illustrating the relationship between voltage Vpllin and current Igyro at 0.2 nA and 2 μA respectively. Waveforms  800  and  900  are sinusoidal, but for amplitude illustration, the sinusoidal details are not shown. In  FIGS. 8 and 9 , the common mode value for voltage Vpllin is 1.5 V. Waveforms  800  and  900  show that voltage Vpllin starts at the common mode voltage of 1.5 V, and, regardless of current Igyro, is settled in the DC constant voltage of the common mode voltage plus or minus the amplitude of 200 mV, after the settling time (e.g., tt02 nA and tt2 μA). For example, in  FIG. 8 , voltage Vpllin starts at 1.5 V, current Igyro is small (e.g., 0.2 nA) and does not affect voltage Vpllin, which remains at 1.5V. Current Igyro is then changed in accordance with techniques of embodiments of the invention, and voltage Vpllin in response to that change also changes, and eventually settles between 1.3 V and 1.7 V, which is 1.5 V plus or minus the amplitude of 200 mV. In  FIG. 9 , voltage Vpllin also starts at 1.5 V, but current Igyro at 2 μA is large, causing voltage Vpllin to swing between the plus and minus amplitude of 500 mV. Eventually, however, as current Igyro changes in accordance with techniques of embodiments of the invention, voltage Vpllin responds and eventually settles also at the plus and minus amplitude of 200 mV, or between 1.3 V and 1.7 V as in  FIG. 8 . 
     A number of embodiments of the invention have been described. It will nevertheless be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, in the illustrative circuits, when a capacitor is used, a capacitive circuit, component, device or network (e.g., combination of circuit, device, component, etc.) may be used in replace of that capacitor. Some transistors are shown to be of a particular type (e.g., N-type for transistors Nup and Ndn, etc.), but the invention is not limited to such a configuration because selecting a transistor type (e.g., N-type, P-type) is a matter of design choice based on need, convenience, etc. Embodiments of the invention are applicable in variations and/or combinations of transistor types. Additionally, some signals are illustrated with a particular logic level to operate some transistors (e.g., activated high, deactivated low, etc.), but selecting such levels and transistors are also a matter of design choice, and embodiments of the invention are applicable in various design choices. 
     Each claim of this document constitutes a separate embodiment, and embodiments that combine different claims and/or different embodiments are within scope of the invention and will be apparent to those skilled in the art after reviewing this disclosure. Accordingly, the scope of the invention should be determined with reference to the following claims, along with the full scope of equivalences to which such claims are entitled.