# Patent

## Title
Current-limiting in an amplifier system

## Abstract

One example includes an amplifier system. The system includes a gain stage configured to conduct a gain current in response to an input voltage. The system also includes a current limit stage coupled to the gain stage and being configured to one of source and sink the gain current and to define a limit amplitude of the gain current during a current limit condition. The system further includes an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the gain current, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude.

## Background
TECHNICAL FIELD
This disclosure relates generally to electronic circuit systems, and more specifically to current-limiting in an amplifier system.
BACKGROUND
Amplifier systems can be implemented for a variety of electronic circuit applications to increase an amplitude of an input signal in generating an output signal. As an example, current amplifiers can generate an output current having an amplitude that is greater than the amplitude of a respective input current. Similarly, voltage amplifiers can generate an output voltage having an amplitude that is greater than the amplitude of a respective input voltage. One example application for an amplifier system is for servo control for the motion of mechanical components, such as in a dual-stage actuator in a hard-disk drive (HDD). As an example, the input signal can be provided for positioning control of a head for reading and writing data, such that the amplifier can provide the output signal to move the head.
SUMMARY
One example includes an amplifier system. The system includes a gain stage configured to conduct a gain current in response to an input voltage. The system also includes a current limit stage coupled to the gain stage and being configured to one of source and sink the gain current and to define a limit amplitude of the gain current during a current limit condition. The system further includes an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the gain current, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude.
Another embodiment includes a gain stage comprising a plurality of transistors configured to conduct at least one of a sinking current and a sourcing current in response to an input voltage. The system also includes at least one current limit stage comprising a current mirror configured to provide the at least one of the sinking current and the sourcing current and to define a limit amplitude of the at least one of the sinking current and the sourcing current during a current limit condition. The system further includes an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the at least one of the sinking current and the sourcing current to provide an output voltage at the output node, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude.
Another embodiment includes a hard-disk drive (HDD) system. The system includes a magnetic disk configured to store data and a spindle motor configured to control rotation of the magnetic disk. The system also includes a head configured to write data to and read data from the magnetic disk. The system further includes a dual-stage actuator configured to cooperate with a voice coil motor to position the head with respect to the magnetic disk in response to an output voltage generated by an amplifier system. The amplifier system includes a gain stage configured to conduct a gain current in response to an input voltage. The amplifier system also includes a current limit stage coupled to the gain stage and which is configured to one of source and sink the gain current and to define a limit amplitude of the gain current during a current limit condition. The amplifier system further includes an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the gain current to provide an output voltage at the output node, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example of an amplifier system.
FIG. 2 illustrates an example of an amplifier circuit.
FIG. 3 illustrates an example of a hard-disk drive (HDD) system.



DETAILED DESCRIPTION
This disclosure relates generally to electronic circuit systems, and more specifically to current-limiting in an amplifier system. The amplifier system can be, for example, a voltage amplifier configured to generate an output voltage that is an amplified version of an input voltage. As an example, the input voltage can be a differential voltage that is provided via feedback. The amplifier system can include a gain stage, a current limit stage, and an output stage. The gain stage can be configured to conduct a gain current, such as a sinking current or a sourcing current, based on the input voltage. As an example, the input voltage can activate one or more transistors in the gain stage to conduct the gain current. The output stage is coupled to the gain stage, and is configured to generate an output current that is based on the gain current, and to provide an output voltage based on the output current.
The current limit stage can be configured to define a limit amplitude of the gain current associated with a current limit condition, with the current limit condition corresponding to a current amplitude limit associated with the output current. As an example, the current limit stage can include a current limit stage configured to sink the gain current or source the gain current based on a difference between the two voltages that constitute the differential input voltage. For example, the gain stage can include sets of cross-coupled transistors that, during a current limit condition, are mutually exclusively activated to conduct the sinking gain current or the sourcing gain current, respectively. The current limit stage is configured to generate the gain current (e.g., sinking and/or sourcing current), and can define a limit amplitude associated with the gain current via a current mirror, such that the output current has an amplitude limit that is proportional to the defined limit of the gain current.
FIG. 1 illustrates an example of an amplifier system 10. The amplifier system 10 can correspond to a voltage amplifier, such as can be implemented to provide an output voltage VOUT that has an increased amplitude relative to an input voltage VIN. As an example, the amplifier system 10 can be implemented in a variety of electronics applications, such as for a dual-stage actuator driver in a hard-disk drive (HDD) system.
The amplifier system 10 includes a reference stage 12 that is configured to set at least one predetermined reference voltage based on a high-voltage rail and a low-voltage rail, demonstrated in the example of FIG. 1 as a voltage VDD and ground, respectively. As an example, the voltage VDD can be approximately 5 volts. The amplifier system 10 also includes at least one gain stage 14 that includes a plurality of transistors 16. The gain stage(s) 14 are configured to conduct a gain current that is generated based on the input voltage VIN. As an example, the transistors 16 can be activated based on the input voltage and the reference voltage(s) to conduct the gain current. For example, the transistors 16 can include two pairs of cross-coupled transistors as part of a first of the gain stage(s) 14, with a first transistor of each pair being controlled by one of the one or more reference voltages and a second transistor of each pair being controlled based on the input voltage VIN. The input voltage VIN can be, for example, a differential input voltage VIN having a first voltage and a second voltage that are provided via feedback.
As an example, a first of the gain stage(s) 14 can be configured to conduct the gain current as a sinking gain current or a sourcing gain current based on the input voltage VIN. For example, the first gain stage(s) 14 can provide the gain current as a sourcing gain current based on a positive difference between the differential voltages of the input voltage VIN or as a sinking gain current based on a negative difference between the differential voltages of the input voltage VIN. As an example, the first of the gain stage(s) 14 can be configured to mutually exclusively activate a given one pair of the cross-coupled transistors of the transistors 16 in response to a difference between the first and second voltages of the differential input voltage VIN exceeding a predetermined threshold.
The amplifier system 10 also includes an output stage 18 that is configured to generate an output current based on the gain current. For example, the output stage 18 can be coupled to a second of the gain stage(s) 14 that includes at least one current mirror that can be configured to conduct the gain current from the first transistor in each of the pairs of cross-coupled transistors in the transistors 16, such that the mirrored gain current can control at least one transistor in the output stage 18. The output stage 18 can include a source-follower arrangement of transistors that are controlled by the mirrored gain current, such that the source-follower arrangement of transistors can be configured to conduct the output current through an output node 20, such as sourced by a voltage VP or sunk to a voltage VN, respectively, to provide the output voltage VOUT corresponding to an increased magnitude of the input voltage VIN. For example, the voltage VP can have an amplitude that is greater than the amplitude of the voltage VDD, such as approximately 17.5V. As an example, the output current can be proportional to the gain current based on respective gate-widths of the transistors between the transistors in the gain stage(s) 14 and the source-follower arrangement of transistors in the output stage 18. Additionally, the source-follower arrangement of the transistors in the output stage 18 can provide a low output impedance with respect to the output node 20, such as can be required in certain voltage amplifier applications.
In the example of FIG. 1, the amplifier system 10 further includes at least one current limit stage 22 that is configured to limit the amplitude of the output current during a current limit condition to substantially prevent damage to the amplifier system 10 or associated electronics. As described herein, a “current limit condition” is defined as a condition in which the output current flows from the amplifier system 10 via the output node 20, or into the amplifier system via the output node 20, via the output stage 18 at a maximum amplitude, as described herein, such as in response to a short-circuit or a very low resistance current path from the output node 20 to a low-voltage rail (e.g., ground). The current limit stage(s) 22 can include a current mirror that is configured to provide the gain current that the gain stage(s) 14 conduct, and to define an amplitude limit of the gain current, which can thus limit the amplitude of the output current based on the proportionality of the output current with respect to the gain current.
For example, the current limit stage(s) 22 can include a sourcing current limit stage and a sinking current limit stage that are respectively configured to conduct the gain current via the gain stage(s) 14. Each of the sourcing and sinking current limit stage(s) 22 can include a current mirror that conducts a predetermined current that can define a maximum amplitude of the gain current. Therefore, because the output current is proportional to the gain current, the output current can have a maximum amplitude that is proportional to the amplitude limit of the gain current, as defined by the current limit stage(s) 22. Accordingly, in response to a current limit condition, the output current can have a maximum amplitude that is set by the current limit stage(s) 22.
FIG. 2 illustrates an example of an amplifier circuit 50. The amplifier circuit 50 can correspond to a voltage amplifier to provide an output voltage VOUT that has an increased amplitude relative to a differential input voltage, demonstrated in the example of FIG. 2 as a first voltage VIN_N and a second voltage VIN_P. As an example, the amplifier circuit 50 can be implemented in a dual-stage actuator driver in an HDD system. For example, the amplifier circuit 50 can correspond to the amplifier system 10 in the example of FIG. 1. Therefore, reference is to be made to the example of FIG. 1 in the following description of the example of FIG. 2.
In the example of FIG. 2, the first input voltage VIN_N and the second input voltage VIN_P are provided to respective gates of P-channel metal-oxide semiconductor field-effect transistors (MOSFETs) P1 and P2, described hereinafter as “P-FETs”. The P-FET P1 interconnects a current source 52 and a first control node 54, and the P-FET P2 interconnects the current source 52 and a second control node 56. The current source 52 is configured to conduct a current I1 from the voltage VDD. The input voltages VIN_P and VIN_N can be approximately equal during a steady-state normal operating condition of the amplifier circuit 50, such as based on feedback to establish the normal operating condition. As another example, the amplifier circuit 50 can be configured as an inverting amplifier, such that the input voltage VIN_P can be substantially fixed at a predetermined amplitude, such as less than 1V (e.g., approximately 900 mV), and the input voltage VIN_N can have an amplitude that varies about the amplitude of the input voltage VIN_P (e.g., +/−700 mV of the input VIN_P). The input voltages VIN_N and VIN_P can operate the P-FETs P1 and P2 in the saturation region to provide a voltage VSW1 on the first control node 54 and a voltage VSW2 on the second control node 56, respectively.
The amplifier circuit 50 includes a reference stage 58 that includes an N-channel MOSFET (hereinafter, “N-FET”) N1 and a P-FET P3 that are each diode-connected in series and interconnect a first reference node 60 and a second reference node 62. A current source 64 provides a reference current IREF from the voltage VDD through the N-FET N1 and the P-FET P3, as well as a diode-connected N-FET N2 that is coupled to the P-FET P3 and a diode-connected N-FET N3 that is coupled to the N-FET N2 to ground. Based on the diode-connection of the N-FET N1, the P-FET P3, and the N-FETs N2 and N3 between the voltage VDD and a low-voltage rail, demonstrated in the example of FIG. 2 as ground, the reference nodes 60 and 62 have respective reference voltages VREF1 and VREF2 that are substantially constant.
In addition, the gate of the N-FET N2 is coupled to a gate of an N-FET N4 and an N-FET N5, and the gate of the N-FET N3 is coupled to a gate of an N-FET N6 and an N-FET N7. Therefore, the N-FETs N6, and N7 are arranged as current mirrors with respect to the N-FET N3, and the N-FETs N2, N4, and N5 are arranged as cascode stages with respect to the N-FETs N3, N6, and N7, respectively. The first control node 54 interconnects the source of the N-FET N4 and the drain of the N-FET N6, and the second control node 56 interconnects the source of the N-FET N5 and the drain of the N-FET N7. The drain of the N-FET N4 is coupled to a diode-connected P-FET P4 having a common gate connection and a common source connection (e.g., at the voltage VDD) with a P-FET P5, such that the P-FET P5 is arranged as a current mirror with respect to the P-FET P4. As a result, the reference current IREF is mirrored through the P-FET P4 and the N-FET N4 as a current IM1 and through the N-FET N5 as a current IM2, such that the currents IM1 and IM2 are mirrored versions of the current IREF minus one half of the current I1 in normal operating conditions. However, a relative magnitude of the voltages VSW1 and VSW2 at the respective control nodes 54 and 56 can control activation of the N-FETs N4 and N5 based on the coupling of the respective control nodes 54 and 56 to the sources of the N-FETs N4 and N5. Thus, the relative amplitudes of the currents IM1 and IM2 can be controlled based on the relative amplitudes of the respective voltages VSW1 and VSW2.
The amplifier circuit 50 also includes a first gain stage 66. The first gain stage 66 includes a diode-connected N-FET N8 having a common gate connection at a first bias node 68 with an N-FET N9, and includes a diode-connected P-FET P6 having a common gate connection at a second bias node 70 with a P-FET P7. Therefore, the N-FET N9 and the P-FET P7 operate as a current mirror with respect to the N-FET N8 and the P-FET P6. In the example of FIG. 2, the first bias node 68 is also coupled to the drain of the P-FET P5 and has a voltage VB1, and the second bias node 70 is also coupled to the drain of the N-FET N5 and has a voltage VB2. As described in greater detail herein, the voltages VB1 and VB2 are controlled by the currents IM1 and IM2 via the voltages VSW1 and VSW2, and are thus controlled by the differential input voltage VIN_P and VIN_N. In addition, the first gain stage 66 includes an N-FET N10 and a P-FET P8 that are controlled by the reference voltages VREF1 and VREF2, respectively, and are thus arranged as a current mirror with respect to the N-FET N1 and the P-FET P3 in the reference stage 58. The N-FET N8 is coupled via a common source connection to the P-FET P6 at a node 72, and the sources of the N-FETs N9 and N10 and the sources of the P-FETs P7 and P8 are all coupled at a node 74 having a voltage VCOM. Therefore, the N-FETs N9 and N10 and the P-FETs P7 and P8 are arranged as cross-coupled pairs of transistors, with the N-FET N9 and the P-FET P8 constituting a first pair of the cross-coupled transistors and the N-FET N10 and the P-FET P7 constituting a second pair of the cross-coupled transistors, as described in greater detail herein.
The amplifier circuit 50 also includes a first current limit stage 76 and a second current limit stage 78. The first current limit stage 76 is configured to generate a gain current as a sourcing current ISRC that is provided to the first gain stage 66, demonstrated in the example of FIG. 2 as being provided to a drain of the N-FET N9. In the example of FIG. 2, the first current limit stage 76 includes a diode-connected P-FET P9 having a common gate connection with a P-FET P10 and a common source connection to the voltage VDD. Therefore, the P-FETs P9 and P10 cooperate as a current mirror. The first current limit stage 76 also includes a current source 80 that is configured to conduct a substantially constant current IL1 from the voltage VDD through the P-FET P9 to ground. Therefore, the substantially constant current LL1 can define a maximum amplitude for the current ISRC that is based on the amplitude of the current LL1 and based on relative gate widths of the transistors in the current path of the current ISRC (e.g., including the P-FET P10).
Similarly, the second current limit stage 78 is configured to generate the gain current as a sinking current ISNK that is provided from the first gain stage 66, demonstrated in the example of FIG. 2 as being provided from a drain of the P-FET P7. In the example of FIG. 2, the second current limit stage 78 includes a diode-connected N-FET N11 having a common gate connection with an N-FET N12 and a common source connection to ground. Therefore, the N-FETs N11 and N12 cooperate as a current mirror. The second current limit stage 78 also includes a current source 82 that is configured to conduct a substantially constant current IL2 from the voltage VDD through the N-FET N11 to ground. Therefore, the substantially constant current IL2 can define a maximum amplitude for the current ISNK that is based on the amplitude of the current IL2 and based on relative gate widths of the transistors in the current path of the current ISNK (e.g., including the N-FET N12).
The amplifier circuit 50 also includes a second gain stage 84 and an output stage 86 that are coupled via an output control node 88. The second gain stage 84 is coupled to the first gain stage 66 via a capacitor C1 that interconnects the node 72 and the output control node 88, as well as the N-FET N10 and the P-FET P8. In the example of FIG. 2, the second gain stage 84 includes a diode-connected N-FET N13 having a common gate connection with an N-FET N14 at a drain of the P-FET P8 in the first gain stage 66, and having a common source connection to the voltage VN (e.g., a negative voltage, such as approximately −17.5V). Similarly, the second gain stage 84 includes a diode-connected P-FET PP11 having a common gate connection with a P-FET P12 at a drain of the N-FET N10 in the first gain stage 66, and having a common source connection to the voltage VP (e.g., a voltage that is equal and opposite the voltage VN, such as approximately 17.5V). The drains of the N-FET N14 and the P-FET P12 are coupled to the output control node 88. Therefore, the N-FETs N13 and N14 and the P-FETs P11 and P12 each respectively cooperate as current mirrors that are configured to conduct the gain current through the first gain stage 66 and mirror the gain current through the output control node 88. As an example, the N-FET N14 and the P-FET P12 can have a larger gate width relative to the N-FET N13 and the P-FET P11, respectively, such as based on a fixed proportion (e.g., a gate width that is twice the gate width of the N-FET N13 and the P-FET P11, respectively).
The output stage 86 includes an N-FET N15 and an N-FET N16 that are configured in a source-follower arrangement, and thus can provide a low output impedance with respect to an associated output node 90. In the example of FIG. 2, the N-FET N15 is diode-connected at the output control node 88, with the N-FETs N15 and N16 having a common gate and a common source to provide a current mirror configuration. The source of each of the N-FETs N15 and N16 are coupled to the output node 90 having the output voltage VOUT, and the drain of the N-FET N16 is coupled to the voltage VP. The output stage 86 also includes an N-FET N17 that has a gate coupled to the gates of the N-FETs N13 and N14 in the second gain stage 84 and which interconnects the output node 90 at a drain and the voltage VN at a source. Therefore, the N-FET N17 is arranged as a current mirror with respect to the N-FET N13. The N-FETs N16 and N17 are therefore configured to conduct the output current based on the gain current that is conducted through the first gain stage 66 and the second gain stage 84. As an example, the N-FET N16 can have a gate width that is significantly greater than the gate width of the N-FET N15 (e.g., approximately twenty-five times the gate width of the N-FET N15). Similarly, the N-FET N17 can have a gate width that is significantly greater than the gate width of the N-FET N13 (e.g., approximately fifty times the gate width of the N-FET N13). Additionally, in the example of FIG. 2, a diode D1 interconnects the source and the drain of the N-FET N15 from anode to cathode, respectively. As an example, the diode D1 can be configured as a parasitic diode with respect to the N-FET N15. The diode D1 is configured to conduct the output current from the output node 90, as described in greater detail herein.
In the example of FIG. 2, additional currents are demonstrated in the amplifier circuit 50, particularly in the first gain stage 66, the second gain stage 84, and the output stage 86. The currents described hereinafter have current flow directions that are based on positive amplitudes. Therefore, as described herein, a negative amplitude of the currents indicates a current flow in the opposite direction of that demonstrated in the example of FIG. 2.
As a first example, the input voltages VIN_P and VIN_N can be approximately equal (e.g., each having an amplitude of approximately 900 mV). In response, with reference to the example of FIG. 2, the voltages VSW1 and VSW2 can be approximately equal to provide an amplitude of the currents IM1 and IM2 that are likewise approximately equal. Therefore, the first and second bias voltages VB1 and VB2 can be approximately equal to the voltages VREF1 and VREF2, respectively, and the voltage at the node 72 can be approximately equal to the voltage VCOM, to provide for saturation region activation of the N-FET N9 and the P-FET P7. Therefore, based on the amplitude of the voltage VCOM at the node 74 at the sources of the N-FET N10 and the P-FET P8, the N-FET N10 and the P-FET P8 are also each activated in the saturation region.
In response to the activation of the N-FETs N9 and N10 and the P-FETs P7 and P8 in the saturation region, the current ISRC (e.g., approximately 5 μA) is provided from the first current limit stage 76 through the N-FET N9 and a current ICC1 (e.g., approximately 39 μA) is provided from the voltage VP via the P-FET P11. A first portion of the current ICC1 flows through the P-FET P8 and the N-FET N13 as a current ICC2 (e.g., approximately 20 μA) to the voltage VN, and a second portion of the current ICC1, demonstrated in the example of FIG. 2 as a current ILK (e.g., approximately 19 μA), combines with the current ISRC to flow through the P-FET P7 as the current ISNK (e.g., approximately 24 μA) to the second current limit stage 78. Additionally, based on the current mirror configuration of the P-FET P12 relative to the P-FET P11 and the N-FET N14 relative to the N-FET N13, the current ICC1 causes a current IGS1 (e.g., approximately 78 μA) to flow through the P-FET P12 and the current ICC2 causes a current IGS2 (e.g., approximately 40 μA) to flow through the N-FET N14. The current IGS2 can be a first portion of the current IGS1, and a second portion of the current IGS1 can flow through the N-FET N15, demonstrated in the example of FIG. 2 as a first output current IOT1 (e.g., approximately 38 μA), based on the respective activation of the P-FET P12 and the N-FET N14.
Additionally, because the N-FET N16 is arranged as a current mirror with respect to the N-FET N15, and because the N-FET N17 is arranged as a current mirror with respect to the N-FET N13, a second output current IOT2 flows through the N-FET N16 and a third output current IOT3 flows through the N-FET N17. Additionally, a respective portion of the output currents IOT2 and IOT3 is provided as an output node current IOUT that is demonstrated in the example of FIG. 2 as flowing from the output node 90, such as to external circuit components (e.g., a capacitor). The third output current IOT3 and the output node current IOUT can be equal to a sum of the second output current IOT2 and the first output current IOT1. In the example of the input voltages VIN_P and VIN_N being approximately equal in a negative feedback condition (e.g., the output voltage VOUT can be coupled as the input voltage VIN_N in a voltage-follower configuration), the amplifier circuit 50 can provide the output voltage VOUT without sourcing any current from or sinking any current into the output node 90 from external circuit components. Therefore, in the example of the input voltages VIN_P and VIN_N being approximately equal, the output node current IOUT can be approximately equal to zero, such that the third output current IOT3 (e.g., approximately 1 mA) can be equal to a sum of the second output current IOT2 (e.g., approximately 962 μA) and the first output current IOT1.
As another example, the output node 90 can be short-circuited to ground or a reference voltage and the input voltages VIN_P and VIN_N can have amplitudes that are not equal. For example, the input voltage VIN_P can be greater than the input voltage VIN_N (e.g., by approximately 700 mV). Based on the difference between the input voltages VIN_P and VIN_N, the voltage VSW1 can be greater than the voltage VSW2, which can thus result in the voltages VB1 and VB2 having a relatively low amplitude. As the voltages VB1 and VB2 decrease in amplitude, the P-FET P7 of the cross-coupled pair of the N-FET N10 and the P-FET P7 has a stronger activation (e.g., the P-FET P7 operates in the linear region and the current ISNK achieves a maximum (i.e., limit) amplitude, resulting in a lower activation resistance RDS_ON) than in the normal operating condition (e.g., described previously), and the N-FET N10 of the cross-coupled pair of the N-FET N10 and the P-FET P7 operates as a cascode stage to conduct the ISNK as a current ILK, since the difference between the voltages VREF1 and VB2 increases, while the voltage VREF1 remains at the same amplitude and the voltage VCOM decreases, resulting in the gate-source voltages of both the N-FET N10 and the P-FET P7 increasing. On the other hand, a lower amplitude of the voltages VB1 and VB2 results in deactivation of the N-FET N9 and the P-FET P8, since the difference between the voltages VB1 and VREF2 decreases, which results in the gate-source voltages of both the N-FET N9 and the P-FET P8 decreasing.
Based on the cross-coupling of the N-FET N10 and the P-FET P7 with respect to the N-FET N9 and the P-FET P8, the voltage VCOM decreases in response to the low amplitudes of the voltages VB1 and VB2, thus likewise activating the N-FET N10 and the P-FET P7 and deactivating the N-FET N9 and the P-FET P8. Therefore, the current ISRC is deactivated and the entirety of the gain current ICC1 is sunk to ground as the currents ILK and ISNK (e.g., the currents ICC1, ILK, and ISNK can be equal with an amplitude of approximately 1 mA). As an example, the amplitude of the currents ICC1, ILK, and ISNK can have an amplitude that is a maximum amplitude defined by the current IL2 that flows through the N-FET N11 in the second current limit stage 78 and based on the respective gate widths of the transistors through which the respective currents ICC1, ILK, and ISNK flow.
Based on the current mirror configuration of the P-FET P12 relative to the P-FET P11, the current ICC1 causes a current IGS1 (e.g., approximately 2 mA) to flow through the P-FET P12. Based on the deactivation of the P-FET P8, the N-FETs N13, N14, and N17 can also be likewise deactivated. Therefore, the currents ICC2, IGS2, and IOT3 can be zero. The current IGS1 can therefore flow in its entirety through the N-FET N15 as the first output current Ian to be combined with the second output current IOT2 (e.g., approximately 50 mA based on a relative gate width of the N-FET N16). Therefore, the output node current IOUT can flow from the output node 90 at an amplitude that is limited to a maximum amplitude (e.g., approximately 52 mA) based on the limit amplitude of the gain current ISNK (e.g., 1 mA), as defined by the second current limit stage 78. Accordingly, the amplitude limit of the output node current IOUT defined by the second current limit stage 78 can substantially mitigate damage to the amplifier circuit 50 and/or to circuit components coupled to the output node 90.
As yet another example, the output node 90 can be short-circuited to ground or a reference voltage and the input voltage VIN_N can be greater than the input voltage VIN_P (e.g., by approximately 700 mV). Based on the difference between the input voltages VIN_P and VIN_N, the voltage VSW2 can be greater than the voltage VSW1, which can thus result in the voltages VB1 and VB2 having a relatively high amplitude. As the voltages VB1 and VB2 increase in amplitude, the N-FET N9 of the cross-coupled pair of the N-FET N9 and the P-FET P8 has a stronger activation (e.g., the N-FET N9 operates in the linear region and the current ISRC achieves a maximum (i.e., limit) amplitude, resulting in a lower activation resistance RDS_ON) than in the normal operating condition (e.g., described previously), and the P-FET P8 of the cross-coupled pair of the N-FET N9 and the P-FET P8 operates as a cascode stage to conduct the ISRC as the current ILK (negative with respect to the example of FIG. 2), since the difference between the voltages VREF2 and VB1 increases, while the voltage VREF2 remains at the same amplitude and the voltage VCOM increases, resulting in the gate-source voltages of both the N-FET N9 and the P-FET P8 increasing. Additionally, a higher amplitude of the voltages VB1 and VB2 results in deactivation of the N-FET N10 and the P-FET P7, since the difference between the voltages VB2 and VREF1 decreases, which results in the gate-source voltages of both the N-FET N10 and the P-FET P7 decreasing,
Based on the cross-coupling of the N-FET N10 and the P-FET P7 with respect to the N-FET N9 and the P-FET P8, the voltage VCOM increases in response to the high amplitudes of the voltages VB1 and VB2, thus likewise activating the P-FET P8 and the N-FET N9 and deactivating the N-FET N10 and the P-FET P7. Therefore, the current ISNK is deactivated and the gain current ISRC is provided through the first gain stage 66 as the currents ILK (i.e., negative with respect to the example of FIG. 2) and ICC2 (e.g., the currents ISRC, ILK, and ICC2 can be equal with an amplitude of approximately 1 mA). As an example, the amplitude of the currents ICC2, ILK, and ISRC can have an amplitude that is a maximum amplitude defined by the current IL1 that flows through the P-FET P9 in the first current limit stage 76 and based on the respective gate widths of the transistors through which the respective currents ICC2, ILK, and ISRC flow.
Based on the current mirror configuration of the N-FET N14 relative to the N-FET N13, the current ICC2 causes a current IGS2 (e.g., approximately 2 mA) to flow through the N-FET N14. Based on the deactivation of the N-FET N10, the P-FETs P11 and P12 can also be likewise deactivated. Accordingly, the N-FETs N15 and N16 can also be deactivated. Therefore, the currents ICC1, IGS1, and IOT2 can be zero. The current IGS2 can therefore flow as a portion of the output node current IOUT (i.e., negative with respect to the example of FIG. 2) from the output node 90 as the first output current IOT1 (i.e., negative with respect to the example of FIG. 2) through the diode D1. The remainder of the output node current IOUT can be provided as the third output current IOT3 (e.g., approximately 50 mA based on a relative gate width of the N-FET N17). Therefore, the output node current IOUT can flow into the output node 90 at an amplitude that is limited to a maximum amplitude (e.g., approximately 52 mA) based on the limit amplitude of the gain current ISRC (e.g., 1 mA), as defined by the first current limit stage 76. Accordingly, the amplitude limit of the output node current IOUT defined by the first current limit stage 76 can substantially mitigate damage to the amplifier circuit 50 and/or to circuit components coupled to the output node 90.
FIG. 3 illustrates an example of an HDD system 150. The HDD system 150 can be implemented in a variety of computer applications for writing data to and reading data from a hard-disk drive. The HDD system 150 includes a magnetic disk 152 that acts as a spinning magnetic storage medium to which data can be written and from which data can be read. The HDD system 150 also includes a spindle motor (SPM) 154 that is configured to spin the magnetic disk during disk write/read operations. A head 156 is configured to perform the read/write operations with respect to the magnetic disk 152 based on positioning over the magnetic disk 152. The positioning of the head 156 can be provided in a precise manner by a voice coil motor (VCM) 158 and a dual-stage actuator (DSA) 160. The DSA 160 can receive actuator signals from a servo controller 162, such as can be implemented as at least a portion of an integrated circuit (IC) as part of control electronics 164 that are configured to control the HDD system 150. The servo controller 162 includes a VCM driver 166 configured to control the VCM 158, an SPM driver 168 configured to control the SPM 154, and a DSA driver 170 configured to control the DSA 160. In the example of FIG. 3, the DSA driver 170 includes a voltage amplifier 172. As an example, the voltage amplifier 172 can be configured substantially similar to the amplifier system 10 in the example of FIG. 1 or the amplifier circuit 50 in the example of FIG. 2.
As an example, the DSA driver 170 can be configured to provide the actuator signals that can be amplified by a voltage amplifier 172 to the DSA 160 to provide precise positioning of the head 156 over the magnetic disk 152. Therefore, the voltage amplifier 172 can provide an amplified output voltage that corresponds to an input voltage, such as a differential input voltage. Furthermore, the voltage amplifier 172 can be configured to exhibit current limiting with respect to an output current, such as based on including at least one current limit stage. Therefore, damage to the voltage amplifier 172 and/or the DSA 160 from excessive current, such as based on an associated output of the voltage amplifier 172 being short-circuited, can be substantially mitigated.
What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or method for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims.

## Claims

1. A hard-disk drive (HDD) system comprising:
a magnetic disk configured to store data;
a spindle motor configured to control rotation of the magnetic disk;
a head configured to write data to and read data from the magnetic disk;
a gain stage configured to conduct a gain current in response to an input voltage;
a current limit stage coupled to the gain stage and configured to one of source and sink the gain current and to define a limit amplitude of the gain current during a current limit condition;
an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the gain current, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude; and
a dual-stage actuator configured to cooperate with a voice coil motor to position the head with respect to the magnetic disk in response to an output voltage generated at the output node.

2. The system of claim 1, wherein the input voltage is a differential input voltage comprising a first input voltage and a second input voltage, wherein the current limit stage comprises:
a first current limit stage configured to sink the gain current through the gain stage based on a positive difference between the first input voltage and the second input voltage; and
a second current limit stage configured to source the gain current through the gain stage based on a negative difference between the first input voltage and the second input voltage.

3. The system of claim 1, wherein the output stage comprises a source-follower arrangement of transistors, such that the output stage one of sources and sinks the output current provided out from or into the output node during the current limit condition.

4. The system of claim 1, wherein the current limit stage comprises a first transistor and a second transistor being arranged as a current mirror and a current source, wherein a predetermined current is provided from the current source through the first transistor, and wherein the second transistor is configured to one of source and sink the gain current based on the predetermined current through the first transistor.

5. The system of claim 1, wherein the gain stage comprises a plurality of transistors that are configured as a cross-coupled transistor arrangement configured to sink the gain current through a first pair of transistors of the cross-coupled transistor arrangement during a sinking current limit condition and to source the gain current through a second pair of transistors of the cross-coupled transistor arrangement during a sourcing current limit condition.

6. The system of claim 5, wherein a first transistor of the first pair of transistors and a first transistor of the second pair of transistors are controlled by respective predetermined reference voltages, and wherein a second transistor of the first pair of transistors and a second transistor of the second pair of transistors are controlled by the input voltage.

7. The system of claim 6, further comprising a reference stage coupled to the first transistor of the first pair of transistors and the first transistor of the second pair of transistors as respective current mirrors, the reference stage being configured to set a magnitude of the predetermined reference voltages based on a reference current.

8. The system of claim 7, wherein the input voltage is a differential voltage comprising a first input voltage and a second input voltage, the system further comprising:
a first control node associated with the first input voltage and being coupled to a source of a first reference transistor that is controlled via a second of the predetermined reference voltages; and
a second control node associated with the second input voltage and being coupled to a source of a second reference transistor that is controlled via the second of the predetermined reference voltages;
wherein the second transistor of the first pair of transistors is controlled via activation of the first reference transistor in response to the first input voltage and the second transistor of the second pair of transistors is controlled via activation of the second reference transistor in response to the second input voltage.

9. The system of claim 5, wherein the gain stage is a first gain stage, the system further comprising a second gain stage comprising a first gain stage current mirror and a second gain stage current mirror, wherein the first transistor of the first pair of transistors is coupled to the first gain stage current mirror and the first transistor of the second pair of transistors is coupled to the second gain stage current mirror, wherein the output stage is coupled to the first gain stage current mirror and the second gain stage current mirror, such that the gain current controls the magnitude of the output current via the respective first and second gain stage current mirrors.

10. The system of claim 9, wherein the output stage comprises a first output transistor, a second output transistor, an output control transistor, and a diode, wherein the first output transistor and the output control transistor are arranged in a source-follower configuration, wherein the first output transistor and the output control transistor are configured to conduct the output current to flow from the output node in response to the sourcing current limit condition, and wherein the second output transistor and the diode are configured to conduct the output current that is provided from the output node in response to the sinking current limit condition.

11. A hard-disk drive (HDD) system comprising:
a magnetic disk configured to store data;
a spindle motor configured to control rotation of the magnetic disk;
a head configured to write data to and read data from the magnetic disk; and
a dual-stage actuator configured to cooperate with a voice coil motor to position the head with respect to the magnetic disk in response to an output voltage generated by an amplifier system, wherein the amplifier system includes:
a gain stage configured to conduct a gain current in response to an input voltage;
a current limit stage coupled to the gain stage and being configured to one of source and sink the gain current and to define a limit amplitude of the gain current during the current limit condition; and
an output stage coupled to the gain stage and configured to conduct an output current through an output node in response to the gain current to provide the output voltage at the output node, the output current having a maximum amplitude during the current limit condition that is proportional to the limit amplitude.

12. The system of claim 11, wherein the input voltage is a differential input voltage comprising a first input voltage and a second input voltage, wherein the current limit stage comprises:
a first current limit stage configured to sink the gain current through the gain stage based on a positive difference between the first input voltage and the second input voltage; and
a second current limit stage configured to source the gain current through the gain stage based on a negative difference between the first input voltage and the second input voltage.

13. The system of claim 11, wherein the output stage comprises a source-follower arrangement of transistors, such that the output stage one of sources and sinks the output current provided out from or into the output node during a current limit condition.

14. The system of claim 11, wherein the current limit stage comprises a first transistor and a second transistor being arranged as a current mirror and a current source, wherein a predetermined current is provided from the current source through the first transistor, and wherein the second transistor is configured to one of source and sink the gain current based on the predetermined current through the first transistor.