Amplifiers and related biasing methods and devices

[0001] 1. Field

[0002] The present teachings relate to amplifier biasing. In particular, the present teachings relate to amplifiers and related methods and devices.

[0003] 2. Description of Related Art

[0004] In low voltage (<4 volt) semiconductor technology such as silicon on insulator (SOI), typical transistors can only withstand a limited voltage across the drain-source terminals before experiencing reliability issues. A typical maximum voltage is around 4 volts for an SOI technology with a minimum feature size of 0.5 μm for the gate electrode and decreases as the minimum feature size is decreased. This limitation poses a challenge when designing circuitry such as an operational amplifier intended to operate at a much higher power supply voltage (such as 10 V or greater). Typical applications using high voltage amplifiers, involve generating output common mode voltages approaching the limits of the positive and negative supply. In a careful design, the constituent transistors of such amplifiers do not experience excessive voltage across their drain and source terminal as to incur reliability issues. Moreover, a high voltage operational amplifier is designed to deliver a wide range of input common mode levels approaching the positive and negative supply.

[0005] FIG. 1 shows an output stage (100) of a folded cascode topology high voltage operational amplifier. The folded cascode refers to the folding of N-channel cascode active loads of a differential pair and then changing the MOSFETs to P-channel. The amplifier output stage (100) is designed to provide an output voltage which approaches a positive and negative supply, VDD and VSS in the Figure, by using a series of stacked transistors VCascode_x (x ranging from 1 to 24 in the example of the Figure) of complementary devices (N-channel FETs and P-channel FETs in the example of the Figure) in a push-pull fashion. The push-pull comes from the ability to drive either a positive or negative current into a load by using a complementary pair of transistors (N-channel and P-channel). Transistors (110), (111), (112), and (113) are used to set a bias current through the output stage (100) transistors via gate bias node voltage (120) and (130).

[0006] Referring to FIG. 1, a number of transistor devices needed in series depends on a total voltage that the operational amplifier output stage (100) is able to withstand. For a given stress voltage level supported by a technology, a higher voltage supply implies the use of a higher number of transistors stacked.

[0007] Further referring to FIG. 1, it is desirable that the series stacked cascode transistors VCascode_x be biased in a way to ensure that the total voltage across the stack is evenly distributed among the transistors VCascode_x. In this way, it is ensured that each transistor is not over stressed with a voltage between drain and source terminals which exceeds the limits of the fabrication technology.

[0008] According to a first aspect of the present disclosure, a biasing method is provided, comprising the steps of: providing an amplifier having an amplifier output node; sensing an output voltage at the amplifier output node; and generating one or more bias voltages proportional to the output voltage to bias the amplifier.

[0009] According to a second aspect of the disclosure, a bias circuit is provided, the bias circuit being connectable, during operation, to an output stage of an amplifier comprising a plurality of amplifier MOSFET devices to produce an output voltage at an amplifier output node, the bias circuit comprising: a sensing section to sense the amplifier output voltage; and a biasing section connected with the sensing section to provide, during operation, bias voltages proportional to the output voltage to gate terminals of the amplifier MOSFET devices.

[0010] According to a third aspect, a bias circuit is provided, the bias circuit being connectable, during operation, to an amplifier comprising a plurality of amplifier MOSFET devices to produce an output voltage at an amplifier output node, the bias circuit comprising a plurality of biasing MOSFET devices arranged in a cascode configuration and serially connected with an in-series arrangement of a plurality of resistors wherein during operation: a gate terminal of one of the MOSFET devices is connected with the amplifier output node; and a current proportional to the amplifier output voltage is generated through the plurality of resistors to provide bias voltages to gate terminals of the plurality of amplifier MOSFET devices.

[0011] According to a fourth aspect, a biasing method is provided, the method comprising the steps of: providing a differential amplifier with an input common mode node; sensing an input common mode voltage at the input common mode node; and producing one or more bias voltages proportional to the input common mode voltage to bias the differential amplifier.

[0012] According to a fifth aspect, a bias circuit is provided, the bias circuit being connectable, during operation, to an input stage of a differential amplifier comprising a plurality of amplifier MOSFET devices to produce an input common mode voltage at an amplifier input common mode node, the bias circuit comprising: a sensing section to sense the amplifier input common mode voltage; and a biasing section connected with the sensing section to provide, during operation, bias voltages proportional to the input common mode voltage to gate terminals of the amplifier MOSFET devices.

[0013] According to a sixth aspect, a bias circuit is provided, the bias circuit being connectable, during operation, to a differential amplifier comprising a plurality of amplifier MOSFET devices to produce an output voltage at an amplifier input common mode node, the bias circuit comprising a plurality of biasing MOSFET devices arranged in a cascode configuration and serially connected with an in-series arrangement of a plurality of resistors wherein during operation: a gate terminal of one of the MOSFET devices is connected with the amplifier input common mode node; and a current proportional to the amplifier input common mode voltage is generated through the plurality of resistors to provide bias voltages to gate terminals of the plurality of amplifier MOSFET devices.

[0014] According to a seventh aspect, an amplifier is provided, comprising: an amplifier differential output stage having two complementary sides, each side having an output node and comprising a stack of N-type and P-type MOSFET transistors arranged in series; and two biasing circuits each comprising: a first stack comprising: a plurality of MOSFET devices of a first type arranged in a cascode configuration; and a plurality of series resistor arranged in series with the plurality of MOSFET devices of the first type; and a second stack comprising: a plurality of MOSFET devices of a second type, opposite of the first type, arranged in a cascode configuration; and a plurality of series resistor arranged in-series with the plurality of MOSFET devices of the second type; wherein in each biasing circuit: a gate terminal of one of the plurality of MOSFET devices of the first type and a gate terminal of one of the plurality of MOSFET devices of the second type are connected with the output node of one of the two complementary sides and wherein during operation: currents proportional to output voltages generated at the output nodes flow through the series resistors within the biasing circuits to produce bias voltages for gate terminals of the N-type and P-type MOSFET transistors of the amplifier differential output stage.

[0015] According to an eighth aspect, an amplifier is provided, comprising: a) an amplifier differential input stage having an input common mode voltage node and a plurality of amplifier MOSFET devices; and b) a biasing circuit comprising: b1) a first stack comprising: a plurality of biasing MOSFET devices of a first type arranged in a cascode configuration; and a plurality of series resistors arranged in series with the plurality of biasing MOSFET devices of the first type; and b2) a second stack comprising: a plurality of biasing MOSFET devices of a second type, opposite of the first type, arranged in a cascode configuration; and a plurality of series resistors arranged in-series with the plurality of biasing MOSFET devices of the second type; wherein: a gate terminal of one of the plurality of biasing MOSFET devices of the first type and gate terminal of one of the plurality of biasing MOSFET devices of the second type are connected with the input common mode node of the amplifier input differential stage and wherein during operation: currents proportional to an input common mode voltage generated at the input common mode node flow through the series resistors within the biasing circuit to produce bias voltages to gate terminals of the plurality of the amplifier MOSFET devices.

[0016] Further aspects of the disclosure are shown in the specification, drawings and claims of the present application.

[0020] Throughout this description, embodiments and variations are described for the purpose of illustrating uses and implementations of the inventive concept. The illustrative description should be understood as presenting examples of the inventive concept, rather than as limiting the scope of the concept as disclosed herein.

[0021] In particular, biasing methods and devices for use in high power amplifiers in accordance with the embodiments of the present disclosure are described. The described high power amplifiers can be fabricated in a low voltage technology and can safely operate from a high voltage supply which is significantly larger than the maximum voltage allowed across a single device for the given technology.

[0022] FIG. 2 shows a circuit (200) comprising an amplifier output stage (262) and two biasing networks (261, 263) according to an embodiment of the present disclosure. Each of the biasing networks (261) and (263) comprises series transistors Pb, Nb (the first letter ‘P’ or ‘N’ represents the type of transistor, i.e. P-type and N-type, and the second letter ‘b’ intends to indicate the term ‘bias’) and series resistors R2. As shown in FIG. 2, the amplifier output stage (262) further comprises cascode devices (263, 264, 265, 266). The cascode devices (263, 265) are of P-type and the cascode devices (264, 266) are of N-type. The biasing network (261) provides gate biasing for the cascode devices (263, 265) and, in the same way, the biasing network (263) provides gate biasing for the cascode devices (264, 266). Output nodes (240) and (250) are also shown in FIG. 2. VDD and VSS represent respectively a positive and a negative power supply for the circuit (200).

[0023] Referring to the embodiment of FIG. 2, the output node (240) voltage of the amplifier output stage (262) is fed into gate terminals of the series transistors (216) and (217). If the output node (240) voltage increases, the series transistor (216) is biased at a lower gate to source voltage resulting in a reduction of a bias current I_bias—pmos through a biasing transistor-resistor stack (205). The reduced bias I_bias—pmos current results in less voltage drop across each series resistor R2 in the biasing transistor-resistor stack (205). According to an embodiment of the present disclosure, the series resistors R2 are chosen to have equal resistance values such that there is equal voltage division among the series resistors R2. The series resistors R2 in the biasing transistor-resistor stack (205) are used to generate the cascode bias voltages to the P-channel cascode devices (264) in the output stage (262) of the amplifier as well as to the series stacked N-channel devices (207) in a biasing transistor-resistor stack (206).

[0024] Further referring to FIG. 2 and continuing with the discussion above, if the output node (240) voltage increases, an opposite effect will occur in the transistor-resistor stack (206). In other words, the series transistor (217) is biased at a higher gate voltage resulting in an increased I_bias—nmos through biasing transistor-resistor stack (206). As a result, the voltage drop across the series resistors R2 in the biasing transistor-resistor stack (206) is increased. According to an embodiment of the present disclosure, the series resistors R2 are chosen to have equal resistance value such that there is equal voltage division among the series resistors R2 in the biasing transistor-resistor stack (206). The series resistors R2 in the biasing transistor-resistor stack (206) are used to generate cascode bias voltages to the N-type cascode devices (266) in the output stage (262) of the amplifier as well as P-type series stacked devices (208) in the biasing transistor-resistor stack (205).

[0025] With continued reference to FIG. 2, if the output node (240) voltage is decreased, the behavior of the biasing transistor-resistor stacks (205) and (206) are swapped relative to the scenario where the output node (240) voltage is increased, as described above. In other words, when the output node (240) voltage is decreased, the series transistors (216) are biased at a lower gate bias voltage and, as a result, the current I_bias—pmos flowing in the biasing transistor-resistor stack (205) is increased. This implies a larger voltage drop across any of the series resistors R2 in the transistor-resistor stack (205) resulting in larger bias voltages to bias the P-type cascode devices (264) in the output stage (262) of the amplifier as well as series stacked N-channel devices (207) in a biasing transistor-resistor stack (206). At the same time, a decreased output node (240) voltage results in a lower gate bias to the series transistor (217). As a result, the current I_bias—nmos through the device as well as the series resistors R2 in the biasing transistor-resistor stack (206) is decreased. This generates smaller bias voltages to bias the N-type cascode devices (266) in the output stage (262) of the amplifier as well as the P-type series stacked devices in the biasing transistor-resistor stack (205).

[0026] Based on the description set forth above, the person skilled in the art will appreciate that the biasing transistor-resistor stacks (205, 206) of FIG. 2 work together to sense the output node (240) voltage of the amplifier and adjust the biasing voltages to the cascode transistors (264, 266) through series resistors R2 of the transistor-resistor stacks (205, 206). As a result of scaling bias voltages of the cascode devices (264, 266) proportional to the output node (240) voltage, each transistor within the output stage (262) will operate within a safe drain-source voltage level.

[0027] Referring to FIG. 2, the person skilled in the art will understand that, depending on the magnitude of the positive supply voltage VDD and the negative supply voltage VSS to the circuit (200), a number of transistors and resistors placed in the transistor-resistor stacks (205, 206) can be adjusted in various embodiments of the present disclosure. In this way, for a given technology, acceptable voltage ranges across each transistor can be provided to avoid transistor breakdown and operational failures.

[0028] Although in some applications one may use only one of the output nodes (240, 250) in a single-ended fashion, according to an embodiment of the present disclosure, the circuit (200) of FIG. 2 is designed differentially comprising two sides (201, 202) that are complementary. As such, the cascode transistors (264, 266) within the side (201) play a substantially similar role to the cascode transistors (263, 265) within the complementary side (202). As a consequence, the same mechanism as described above is implemented to avoid over-stressing the cascode transistors (263, 265). This is accomplished by utilizing the biasing transistor-resistor stacks (203, 204) in a substantially similar way as previously described with reference to the biasing transistor-resistor stacks (205, 206). This time, the output node (250) is sensed by the transistor-resistor stacks (203, 204) to adjust gate biasing of the cascode devices (263, 265) in such a way that over-stressing of the cascode devices (263, 265) is avoided.

[0029] Methods used to bias devices and methods for increasing an input common mode range of high voltage amplifiers are now described. The input common mode is the range of voltage that can be applied to the input terminals while ensuring that the amplifier behaves as expected.

[0030] FIG. 3 shows a circuit (300) comprising an input stage (360) of an amplifier and biasing transistor-resistor stacks (303, 304) in accordance with an embodiment of the present disclosure. The input stage (360) of the amplifier comprises input transistors (301, 302) paired differentially. As shown in FIG. 3, cascode transistors (361, 362) are used in series with the input transistors (301, 302). A cascoded current source (320) is also shown in FIG. 3. The current source (320) comprises cascode transistors (321) and provides bias current for the input stage (360).

[0031] Referring to FIG. 3, in order to avoid over-stressing and break down, bias voltages for the cascode devices (321, 361, 362) are adjusted as an input common mode voltage at an input common mode sensing node (311) to the input transistors (360) varies during operation of the circuit (300). In order to accomplish this, a pair of biasing transistor-resistor stacks (303, 304) is used to sense the input common mode voltage through two equal value resistors (310) at the input common mode sensing node (311). Once the input common mode voltage is sensed, the bias voltages for the cascode devices (321, 361, 362) are adjusted. Adjustment of the bias voltages for the cascode devices (321, 361, 361) is performed in a substantially similar way as the one described with reference to FIG. 2, the difference being that in the embodiment of FIG. 3, the input common mode sensing node (311) is sensed in contrast with the output node (240) of the circuit (200) of FIG. 2.

[0032] Referring again to FIG. 3, the transistor-resistor stack (303) generates bias voltages for the cascode transistors (321) and the transistor-resistor stack (304) generates bias voltages for the cascode devices (361, 362). The person skilled in the art will appreciate that by tracking voltage levels and the input common mode sensing node (311), the bias voltages provided to the cascode devices (361, 362, 321) are adjusted to avoid over-stressing of each individual transistor of the cascode transistors (361, 362, 321). The person skilled in the art will also appreciate that the biasing methods described herein allow a design of amplifiers capable of operating with high voltage supplies while being fabricated using a technology such as silicon on insulator (SOI) which inherently allows for low stress voltages for transistors.

[0033] Accordingly, what has been shown are biasing methods and devices for amplifiers. While the devices and methods have been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the disclosure. It is therefore to be understood that, within the scope of the claims, the disclosure may be practiced otherwise than as specifically described herein.

[0034] A number of embodiments of the present inventive concept have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the inventive teachings.

[0035] Accordingly, it is to be understood that the inventive concept is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims. The description may provide examples of similar features as are recited in the claims, but it should not be assumed that such similar features are identical to those in the claims unless such identity is essential to comprehend the scope of the claim. In some instances the intended distinction between claim features and description features is underscored by using slightly different terminology.