Transconductance circuit and a current digital to analog converter using such transconductance circuits

An example transconductance circuit is provided in accordance with one embodiment. The transconductance circuit can comprise: an output node; at least one transistor; a variable resistance; and a differential amplifier; wherein the at least one transistor and the variable resistance are in series connection with the output node, an output of the differential amplifier is connected to a control node of the at least one transistor, a first input of the amplifier is responsive to an input signal, and a second input of the amplifier is responsive to a voltage across the variable resistance. Such a circuit may overcome noise problems in transconductance circuits which operate over a wide range of input signals with a fixed resistor in series with the at least one transistor.

PRIORITY DATA

This application is a non-provisional of U.S. Provisional Patent Application Ser. No. 61/825,511, filed May 20, 2013, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to an improved transconductance circuit and, further, to other circuits, systems, architectures, and devices such as digital to analog converters employing such circuits.

BACKGROUND

Transconductance is a property of certain electrical components. Transconductance components can provide an electrical current output based on a voltage input. For example, if the input voltage increases then the output current may similarly increase. The ability to output an analog signal as a current can have advantages such as providing a signal over longer wire or transmission distances than is possible with a voltage signal, and also has utility in many electronic circuits. Furthermore, current mode signals can be insensitive to voltage drops or voltage differences, which provide a robustness that is desirable in many systems.

In general, transconductance devices suffer from several problems that include performance issues over an operating range. In certain systems, when an input voltage was low, noise resulting from an amplifier in the transconductance circuit would be the dominant noise source. Other amplifier related contributions may also become more significant with low input voltages, such contributions may include offsets from the amplifier. Transconductance devices or circuits may also be known as voltage to current converters.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

According to a first aspect of this disclosure, there is provided a transconductance circuit comprising: an output node; at least one transistor; a variable resistance; and a differential amplifier. The at least one transistor and the variable resistance are in series connection with the output node. An output of the differential amplifier is connected to a control node of the at least one transistor. A first input of the amplifier is responsive to an input signal, and a second input of the amplifier is responsive to a voltage across the variable resistance. Advantageously all of the components of the transconductance circuit are provided within an integrated circuit package, and preferably on a common semiconductor substrate.

The transistor may be, for example, a field effect transistor such as a metal oxide field effect transistor (MOSFET), a junction field effect transistor, multiple gate devices such as trigate FETs, FINFETs and so on. The transistor may also be a bipolar junction transistor, an insulated gate bipolar transistor (IGBT) or any other type of transistor. In an embodiment of the disclosure, MOSFETs are chosen because of the low gate current exhibited by such devices. The amplifier may be implemented as any suitable differential amplifier and is not so limited to the specific implementations discussed herein.

The variable resistance may be implemented as passive devices, active devices, an architecture to provide a dynamic or tunable resistance, or as other types of dynamically tunable resistance devices. Passive devices may include (without this list being definitive) laser trimmed resistors, film resistors, thin film resistors, polysilicon resistors, diffused resistors or carbon based resistances. Active resistive devices may include electrically tuned resistors such as polysilicon or thin film resistors where selective heating of the resistor may be used to vary its resistance. Active resistors may also include temperature coefficient tuned impedances (such devices being commercially available as iTRIM), active tuned impedances such as MOS using VGN for example as shown in U.S. Pat. No. 5,764,174. Resistive architectures may also include a digital potentiometer and resistive digital to analog converters. Furthermore, a digital potentiometers informally often known as a “digipot” (Trademark used by Analog Devices Inc.) or as an RDAC is a digitally controlled electronic component that mimics the analog function of a potentiometer, and which may also be used as the variable resistance. Such “digipots” are often used for trimming and scaling analog signals by microcontrollers. A “digipot” may be built using a ladder DAC, such as an R-2R resistive configuration. The resistive string construction is the most common form of electrically controllable resistance available at the time of writing. Each step on the resistor ladder may have its own switch that can connect this step to the output of the digipot. The selected step on the ladder determines the resistance ratio of the digital potentiometer. In certain embodiments, the resistor can be implemented with a MOS device, whose resistance can depend on the control signals supplied to the MOS device. Thus, any suitable resistor technology (or means of synthesizing a resistance) may be used. The variable resistor may be provided as part of an integrated circuit, and may be calibrated and/or trimmed during manufacture or in user controlled trimming steps.

The variable resistor may comprise a switchable array of resistors. The array of resistors may comprise a plurality of resistors, which are individually and selectively connectable between a first resistor node and a second resistor node. The first resistor node may form an input to the amplifier. Thus, the first resistor node may be connected to the inverting input of the differential amplifier. The second resistor node may be connected to a reference potential, such as a supply rail. Alternatively, a measurement amplifier may be provided to measure the voltage across the at least one resistor, and to provide this measured voltage difference as an input to the differential amplifier. The variable resistor may be provided as a plurality of resistors as part of a monolithic integrated circuit. In some embodiments, the variable resistance may be synthesized, for example by switched mode capacitor techniques.

The individually connectable resistors may be associated with respective switches in order to connect or disconnect a particular resistor from between the first and second resistor nodes. The switches may be implemented as transistor switches or as mechanical switches, for example micro-electromechanical systems (MEMS) switches or relays.

The at least one transistor may be provided as an array of transistors fabricated within an integrated circuit. The transistors may be associated with respective resistors. The resistors and transistors may be scaled such that if a resistance of one resistor (an N+1th resistor) is half that of another resistor (an Nth resistor) such that for the same voltage across the resistors twice as much current flows in the N+1th resistor compared to the Nth resistor, then the associated transistor may be scaled such that a transistor current density, for example a channel current density, remains substantially the same between devices. Thus if an Nth transistor is associated with the Nth resistor and a N+1th transistor is associated with the N+1th resistor then the width to length ratio of the N+1th transistor may be selected to be substantially twice that of the Nth transistor. Size ratios other than 2:1 may be selected. Thus radix such as 1.8 or 1.9 may be used to ensure “no missing codes” in the current range and/or additional low current (such as additional least significant bit) current sources may be provided such that, in use, the current provided by the transistors may be set slightly on the low side and augmented by the additional current sources to bring it up to a desired value. The scaling of transistors may be performed by providing “unit” sized devices in parallel. The scaling of resistors may be provided by providing “unit” sized devices in series. It should be noted that using a reduced radix could be extended to other number systems. Thus where segmented architectures are used, the scaling between the elements and between segments may be selected to ensure no missing codes.

Alternatively, or in addition to providing redundancy by way of additional current sources and/or scaling between resistors in the plurality of resistors, some or all of the transistors may be controlled by way of a connection to their back-gate to vary or trim the response of a transistor. Such trimming may be used to reduce or substantially cancel the effect of device mismatch between transistors and resistors in an array of such devices.

Additionally or alternatively small adjustments of the back-gate voltage may be made to the transistors in the transconductance DAC formed using a transconductance circuit to provide a further degree of open loop control in addition to (or indeed around) the closed loop response.

The voltage used to trim the back gate voltages may be provided by one or more digital to analog converters. In some embodiments, the back gate may be driven by a current source. This approach forward biases a base-emitter diode of a parasitic bipolar junction transistor, which exists in parallel with a FET and is formed by the drain, source, and back gate regions of the device. The forward biased diode converts a small current into a bias voltage in accordance with the I-V characteristic of the diode. The back-gate current can be provided by a transconductance circuit. The back-gate voltages of the transistors may thus be set to respective and different voltages to trim out variations between transistors in an array of transistors.

According to a second aspect of this disclosure, there is provided a digital to analog converter comprising at least one transconductance circuit constituting an embodiment of the first aspect of this disclosure. According to a third aspect of this disclosure there is provided an integrated circuit transconductance circuit in which an electrically controllable variable resistance is provided within a control loop of a digital code to analog current convertor.

In a further aspect there is provided an integrated circuit comprising a plurality of current control stages, wherein at least some of the stages comprise a field effect transistor arranged to pass a current in response to a shared voltage provided to gates of the field effect transistors, and wherein a plurality of the transistors have back gates which are connected to a circuit that is adapted to supply respective voltages or currents to the back gates of the transistors such that different transistors may have different back gate voltages.

It is this possible to trim the currents passed by the transistors to modify the response of one or more of the transistors such that the circuit exhibits a desired response characteristic or more nearly approximates the desired characteristic.

A precision transconductance device as disclosed in some embodiments herein may overcome many of the problems of former devices and, potentially, allow for the output of relatively low noise current signals in response to low voltage signals at a device input and/or provide improved linearity when dealing with small input voltages. In some embodiments, the transconductance device is in the form of a digital to analog convertor, which receives an input digital code and outputs a current. Devices of the types disclosed herein can be used in industrial, healthcare, audio, and video application spaces, to name just a few applications of such a device. Precision and reduced noise devices can allow for more robust monitoring in medical systems, better audio/video systems, and improved performance in instrumentation and control systems.

There is further provided a method of trimming current between a plurality of current control stages, where first and second stages each comprise a respective field effect transistor, said field effect transistors having top gates and back gates, wherein the top gates are connected to a shared node so as to receive a shared control voltage, and the back gates are driven to respective voltages so as to set a current ratio between the first and second stages to a desired value.

Example Embodiments and Example Implementations

FIG. 1is a circuit diagram of an example transconductance stage. The circuit comprises a transistor2in series connection with a resistor4. In this particular example the transistor2is an N channel MOSFET, having its drain6connected to an output node8, and its source10connected to a first terminal of the resistor4at a first resistor node12. A second terminal of the resistor4is connected at a second resistor node16to a reference voltage such as ground or VSS. A gate20of the transistor2functions as a control terminal for the circuit. In a basic example, an input voltage could be directly supplied to the gate20, and to a first approximation, the output current at the node8would be substantially proportional to the voltage at the gate20. However, in slightly more sophisticated implementations a differential amplifier22is provided. The amplifier22is configured such that the input signal Vin from an input node23is provided to a non-inverting input24of the amplifier22. A voltage at the first resistor node12, which effectively represents the voltage across the resistor and consequently the current flowing through the resistor4is connected to an inverting input26of the amplifier22, and an output28of the amplifier22is connected to the gate20of the transistor2. Thus, the action of the differential amplifier22is to form a negative feedback loop to linearize the operation of the circuit shown inFIG. 1by desensitizing the overall response of the circuit to non-linearities in the response of the transistor2. This desensitizing is achieved by a combination of high amplifier gain and the negative feedback.

Although this circuit works well, it is not without its own performance problems. For example, if the input voltage to the amplifier is low (close to ground), then noise on the input of the amplifier (noise generated by the amplifier which is often referred to as “input referred noise”) will be relatively large compared to the input voltage and will be amplified in the output of the amplifier. Similarly, other types of noise, for example thermal noise across the resistor4, will also be amplified. At high input voltages, which result in a high output current, the noise contribution from the amplifier is small in comparison to the output current, and probably to noise therein, and it does not pose a significant problem. However, as the input voltage and hence the current becomes smaller, then the noise contribution from the amplifier become more significant. Similarly, the proportional effect of input voltage offsets at the amplifier becomes much more significant as the magnitude of the Vin decreases relative to the amplifier input offset voltage. In addition, the magnitude of the input offset voltage may vary with the input voltage. Similarly other effects such as voltage changes due to the non-zero impedance of the circuit which supplies the input voltage Vin (not shown), and variations in the reference voltage at the second resistor node16due to signal currents in the supply rail, cross talk and thermally induced component variations may all become more significant when the input voltage is small.

FIG. 2schematically illustrates the noise performance of the circuit ofFIG. 1. The input voltage is assumed to be variable and is plotted along the abscissa (X-axis) of the graph whereas the output current noise power component is plotted along the ordinate (Y-axis) of the graph. For relatively high input voltages, the relationship between the input voltage and the output noise is substantially linear, as represented by region30ofFIG. 2. The reciprocal of the gradient of the line in region30represents the signal to noise ratio of the transconductance circuit. However, it becomes apparent that the amplifier is itself a source of noise. Similarly, the resistor4is a source of thermal noise. These noise sources set a lower noise value, as indicated by the region32inFIG. 2. The thermal noise from the resistor4is subject to the gain of the amplifier22.

The amplifier contribution to the noise can be, in part, addressed by using a chopped amplifier (also known as a chopping amplifier), such that the noise performance of the amplifier can move to a predetermined portion of the output spectrum of the transconductance circuit. However, the noise contribution from the resistor is fundamental.

Low input signal values also reveal other non-ideal artifacts of the differential amplifier, such as input offset voltages that give rise to a non-linearity in the V-I response of the transconductance circuit at lower input voltages. Techniques, such as auto-zeroing can be adopted to reduce the impact of offsets.

However, there is more opportunity to deal with the noise performance of the circuit [and also the offset performance], if the value of the resistor4could be varied. This would enable some adjustment between input voltage at the input node, and the resistor value to be achieved, with a view to improving performance.

FIG. 3schematically illustrates a first embodiment of a transconductance circuit constructed in accordance with the teachings of this document. The arrangement inFIG. 3is similar to that shown inFIG. 1, and identical reference numerals have been used to designate similar parts. The significant difference is that the fixed value resistor4ofFIG. 1has now been replaced by a variable resistor42ofFIG. 3. The variable resistor42is still in series connection with the source10of the transistor2and the reference voltage16.

The resistor42may be implemented as a digitally controlled device, for example as an array of resistors, which can be selectively switched into various configurations with one another to change the impedance between the first and second resistor nodes12and14.

FIG. 4is a circuit diagram showing an embodiment of a transconductance stage in which the variable resistor42ofFIG. 3is implemented by an array of parallel resistors42.1to42.n in series with switches44.1to44.n. Each resistor is associated with a respective series connected switch, and all of the resistors can be selectively placed in current flow communication between the resistor nodes12and14. The switches44.1to44.n may be implemented as transistors or mechanical switches, including MEMS switches and relays. The resistors42.1to42.n may be weighted so as to form a binary weighted array, or a non-binary weighted array (radix less than two), or they may be of notionally the same value (unary array) so as to form a thermometer encoded array, or grouped into decades, or a mixture of these schemes. It should be noted that a binary weighted (or indeed any) array may include repeated (additional) bits/resistors for providing redundancy. Thus the current at the output node is a function of the input voltage at the non-inverting input24of the amplifier22and whichever one or ones of the resistors42.1to42.n have been selected. The choice of scaling may be made by the designer in accordance with their specific requirements for the input to output characteristic of the transconductance stage.

Where the switches44.1to44.n are provided as transistors, it is advantageous to scale the transistors (e.g. the channel width to length ratio of a FET) in proportion to the relative currents that each transistor is expected to pass compared to the other transistors in the array such that the current density remains substantially constant across all devices, and hence the voltage drops across each switch when it is in a conducting state are substantially matched.

The provision of a variable resistor42means that there is now an opportunity to trade off resistor noise against amplifier noise.

Thermal noise from a resistor is given by
Vn=(4kβTRΔF)1/2where Vn=RMS noise voltagekβ=Boltzmann's constantT=Temperature in KelvinR=Resistance of the resistorΔF=Noise bandwidth

Thus as the resistance R of the resistor42is increased, then the noise from the resistor increases. However, a significant and probably the dominant source of thermal noise is likely to the amplifier22, with most of the noise power being attributable to the first stage of the amplifier42. At low current values, the resistor noise can be traded against supplying a larger signal to the amplifier22, and hence in influence of amplifier noise can be proportionally reduced.

In the arrangements shown inFIGS. 3 and 4, the change in the resistance values occurs outside of the feedback path between the source10of the transistor2and the inverting input26of the differential amplifier22. Thus, there are no switching actions within the feedback path, and in particular, the circuit cannot inadvertently break the feedback path.

The variable resistance42may be formed of a resistor ladder network or a combination of an resistor ladder and the parallel current paths shown inFIG. 4. Such combinations of configurations may be chosen by the designer to balance resistance-matching requirements against space (and hence cost) on an integrated circuit die.

The dynamic range of the input signal to the amplifier can be reduced if the resistor42becomes variable. In the limit, the input signal may become a fixed voltage and all of the DAC adjustment can be made by varying the resistance of resistor42. This approach means that the bandwidth at the non-inverting input24can be significantly reduced, and hence the noise power supplied from a reference voltage generating circuit can be reduced. Thus, the architecture gives the designer the choice about whether to have a variable or fixed input voltage to the non-inverting input of the amplifier.

FIG. 5schematically illustrates an embodiment of a digital to analog converter having a variable transconductance stage. The transconductance stage, of the type illustrated inFIG. 3or4is schematically designated as item50. An input voltage to the non-inverting input24of the amplifier22may be provided by a variable voltage reference, such as digital to analog converter52. The digital to analog converter52may be any suitable implementing technology such as an R-2R converter. The digital to analog converter52is responsive to a control word58supplied to it by a controller54. The controller54acts to receive an input word56, which represents the output current that it is desired to pass through the transconductance stage50, and it may remap the input word56into the control word58to be sent to the digital to analog converter52and to a second control word60to be sent to the variable resistor42. Thus, by controlling both the input voltage at the non-inverting input24of the amplifier22and the value of the variable resistance42, the output current can be controlled in such a way as to achieve improved noise performance because the value of the resistance42can be selected, in conjunction with the noise performance of the amplifier22and the DAC52in order to seek an appropriate noise performance for any given input word.

The appropriate noise performance may not necessarily be a minimum noise for the circuit, as it may need to be balanced against other parameters of the circuit. Such parameters may include seeking to operate the amplifier over a range where its input stage exhibits a minimum offset voltage. It is at the discretion of the designer whether the input DAC52has an output voltage which spans a large range, or whether its voltage is centered around a relatively small range, such that in effect the DAC52provides the least significant bit performance whereas the controllable resistor42may provide the equivalent of the most significant bit performance of the digital to analog converter shown inFIG. 5. In the limit, all of the step wise control of the current when outputting an analog current representing a digital input word can be provided by controlling the value of the variable resistance42, in which case the controller54does not necessarily need to remap the input word56, although it may continue to do so where the transconductance stage50comprises a mixture of thermometer encoded and binary weighted resistors. In such an arrangement, the DAC52may be replaced by a fixed voltage reference.

FIG. 6schematically illustrates a variation of an implementation of the circuit shown inFIG. 3andFIG. 4. WithinFIGS. 3 and 4, the series combination of the transistor2and the resistor42may be regarded as a single output stage connected to the differential amplifier22.FIG. 6takes this concept to the next stage, and has a plurality of output stages70.1,70.2, and so on up to70.n, where the stages are effectively provided in parallel and each of the stages70.1to70.n can be connected to the amplifier22such that each stage receives the signal from the output28of the amplifier22and that the voltage across each resistor within each stage is provided to the feedback circuit formed by the amplifier22. The first stage70.1comprises a first transistor, which in this example is a field effect transistor72.1which acts as an output transistor for the first stage, and which is in series with a first stage resistor74.1. A gate signal switch76.1is provided to selectively connect the gate of the transistor72.1to the output28of the amplifier22. Similarly, a feedback switch78.1is provided to selectively connect a first node of the resistor74.1to the first resistor node12, which is connected to the inverting input of the amplifier22. Similar configurations are provided in each of the other output stages70.2to70.n. The resistors74.1,74.2to74.n may be variable resistors or, as shown, they may be fixed value resistors. Alternatively, some stages may have variable resistors whereas other stages may have fixed value resistors. Furthermore, although each stage is shown as comprising only a single transistor and a single resistor, multiple transistors and resistors may be provided within a single stage in order to reduce the potential scaling problems between one stage and its neighbors. Thus, multiple transistor “units” and resistor “units” may be connected in parallel or series, as appropriate, to synthesize a correctly sized transistor and a correct resistance for any of the stages70.1to70.n. This technique need not be described further as it is common in this field.

The stages70.1to70.n may be weighted with respect to each other in various ways. For example, several of the stages may be thermometer encoded such that each stage has the same current output value as its neighbors. Thus, in an embodiment having twenty or more stages, stages70.1to70.10may be arranged to output a current having a notional value of “1.” Stages70.11to70.20may each be arranged to output a current having a value of “10.” Other ones of the stages (where they are provided) may be arranged to output binary weighted currents. The relative weighting between one stage and the next is a choice of the circuit designer, as is the number of stages. Thus, in this example where the first ten stages have the same notional current, then in order to provide an output current of “1” any single stage70.1to70.10may be energized by closing its switches76and78. For a current of “2,” any two of the stages70.1to70.10may be energized and so on. It may be beneficial to randomize or pseudo randomize which stages are selected in order to smooth out any differential non-linearity or integral non-linearity errors from the DAC shown inFIG. 6. This randomization may be done by the controller54(as shown inFIG. 5) that controls the operation of the switches76.1to76.n and78.1to78.n. Such randomization may be static or dynamic and, further, may be based on particular architecture or device needs. It should be noted that as each stage is selected it forms part of the feedback network such that the amplifier22performs voltage control to the gate of every single one of the transistors involved in supplying the output current. Thus, all the selected resistors74.1to74.n are connected at one end to the inverting input of the amplifier22, and at their other end to ground14or to Vss. In this embodiment switching of stages also causes a reconfiguration of the feedback network by virtue of switches78.1to78.n being placed between the inverting input of the amplifier22and the sources of each transistor. It may be beneficial to include means to ensure that the switches are operated in such a sequence by the controller54(FIG. 5) that prevents the amplifier22from being placed in an open loop configuration. As a further alternative an impedance may be placed between the node12and the inverting input of the amplifier22or a capacitor connected to the inverting input of the amplifier22such that the bandwidth at the inverting input is reduced so that very short breaks in the feedback loop can be tolerated and/or that the switches are fast and operated so as to minimize disturbance to the feedback loop. The input node23may be provided with a constant voltage from a voltage reference, such as a precision voltage reference80, or it may receive a variable reference voltage in response to a DAC52and controller54as described with reference toFIG. 5.

In the circuit ofFIG. 6, the current output by the DAC may be influenced by the voltage at the output node as this affects the voltage occurring across each of the transistors in the output stage.

FIG. 7shows a variation to the circuit ofFIG. 6where a further transistor has been provided between the various output stages70.1to70.n and the output node8to substantially eliminate voltage variation across the transistors72.1to72.n. Thus, the additional transistor90acts as a cascode transistor. In the arrangement shown inFIG. 7a single cascode transistor has been provided for the entire device, although it should be noted that individual cascode transistors could equally be provided within each of the output stages70.1to70.n. Other cascoding technologies or implementations may be used, such as telescopic or gain boosted cascode.

In use, the voltage applied to the non-inverting input of the amplifier22may be provided by a variable voltage source (such as a DAC22) or it may be provided from a fixed voltage source.

The switches76.1to76.n and78.1to78.n, may be controlled from a digital word to switch one or more of the stages70.1to70.n into the circuit. Each selected stage is in parallel with each other selected stage, is connected to the amplifier output, and is connected to the inverting input of the amplifier (which is the first resistor node).

Thus, the first end of each resistor74.1to74.n in a selected stage is connected to the first end of each resistor in each other selected stage, by way of the switches78. As a result, the voltage occurring at the inverting input of the amplifier is an average of the voltage at each source of the selected transistors, and is related to the current by the relative contribution of each transistor72.1to72.n that has been selected.

Such an approach enables the amplifier22to be optimized for operation over a narrow input range, and makes it easier to fabricate a circuit for use with low voltage headroom in the power supply.

The use of a shared feedback loop further causes the noise from each resistor74.1to74.n to be averaged with that of the other, so noise performance improves with increased current.

Where a cascode device90(or devices90.1to90.n are provided where cascode devices are provided within each stage) the cascode device or devices should be implemented with a bias voltage that does not significantly impact on the voltage range that can be applied across each output stage. If individual cascode devices are provided for each stage then these may be switched between on and off states by the controller54, and the gate of each transistor72.1to72.n may remain connected to the amplifier output at all times.

The feedback switches78.1to78.n may advantageously have their own resistance Ronscaled to scale with the current flow in each output stage. This, if a stage (for example stage70.11) passes10I, where I is an arbitrary unit current, the value Ronof the feedback resistor should be R/10, compared to the gain stage (such as stage70.1) which passes a current I. This weighted average provides further accuracy in the feedback loop performance compared to not scaling the resistors. This weighing reduces the impact of resistor mismatch. The feedback switches may be implemented as field effect transistors. In general, a low feedback switch impedance is desirable as it also reduces the impact of resistor mismatch.

Advantageously each stage may have its own cascode transistor, with the width to length ratio of each cascode transistor being scaled with respect to current passed by its associated output stage. Thus, the voltage dropped across each cascode transistor is substantially constant.

FETs are four terminal devices, and within an integrated circuit, it is easy to fabricate a FET such that its back gate is accessible, and such that the back gate can be driven to a desired voltage. Introducing slight variations in the back gate voltage of individual ones of the transistors provides a way of varying the amount of current passed by a transistor for a given voltage at its gate terminal. In effect applying a voltage to the back gate with differs from that of the gate is a way of modifying the drain current versus Vgsresponse to the transistor.

In a further embodiment shown inFIG. 8a correction circuit94is provided to vary a back gate voltage of one or more of the output transistors72.1to72.n. The correction circuit94may have an input96for receiving an output of the amplifier, and a plurality of outputs97connected to back gates of respective transistors72.1to72.n in the output stages. The correction circuit94may comprise memory and one or more digital to analog converters for controlling the back gate voltage of several of the transistors. This enables the response of a transistor72.1to72.n to be modified to vary the current that it passes for a given gate to source voltage. Such response trimming may be provided by a relatively small DAC associated with each transistor that has been selected for trimming. The correction circuit94may provide a fixed back gate voltage for each transistor whose response is being trimmed, or the controller may be responsive to the control voltage provided at the output of the amplifier22, and may be arranged to add a correction factor to it. Where the output voltage of the amplifier22is not expected to vary by much, then these approaches are more or less equivalent. The transistors passing the most current are the ones for which driven mismatch has the most impact, so the trimming, if provided, should be preferentially applied to the transistors passing relatively large current.

FIG. 9shows a simple resistor DAC that may be implemented within the correction circuit94to provide a modified back gate voltage to one of the transistors72.1to72.n.

The DAC comprises a string of resistors100.1to100.n arranged in series between Vdd and Vss or other suitable voltage supplies. The resistors need not be equal valued. For example if the output voltage from the amplifier22is expected to span over a narrow range, for example because it is driven from a constant Vin or a narrow range of Vin, then the values of the resistors100.1to100.n can be selected so as to set a voltage at a mid-point node102of the string of resistors to a value which sits within, and is preferably close to, the required target range of voltages for supply to the back gates, which may be similar to the expected amplifier output voltage. Switches104.1to104.n-1 are provided to selectively tap the resistor string to provide a voltage to an output node106. The first switch104.1taps off the voltage at a node formed between the first resistor100.1and the second resistor100.2. The second switch taps the voltage between the second and third resistors100.2and100.3, and so on. It can be seen that by suitable selection of the switches, a desired voltage can be supplied to the back gate of a transistor connected to the node106.

In a variation, the midpoint node102may be connected to the output of the amplifier22, either permanently of by way of a switch108. This causes the voltage at the output node106to track the amplifier output voltage, subject to an offset provided by the potential divider formed by the resistor string. Individual DACs may be provided to drive the back gate of each transistor, or a DAC may be shared in a time-multiplexed manner between a plurality of transistors. The output of the DAC may be used to charge a real or a parasitic capacitor associated with the back gate of the transistor. One resistor string may be shared concurrently by several networks of switches to provide multiple output voltages.

Instead of directly providing a bias voltage, the controller may include several current sources or current sinks110, an example of which is shown inFIG. 10, each connected to a respective back gate to force a current to flow at a connection to the back gate. This approach utilizes the existence of a parasitic diode within the FET's to convert a small current from the convertor into a back gate voltage.

The midpoint node may represent the middle of the target range, and may be used as a nominal or reset value. Values for controlling the DACs or current sources may be programmed into the controller94during a calibration process, or may be located from memory or power up. It is this possible to provide a transconductance circuit which is suitable for use within a current digital to analog converter, and similarly it is possible to provide a digital to analog converter having an improved performance.

In general, current mode signals are robust to common-mode variations unlike single ended voltage signals that are inherently referenced to a ground, or other, voltage potentials. It is therefore advantageous to use current output to drive load or sensor circuitry that may (or may not) be integrated on a same integrated circuit (IC) device. The transconductive DAC described herein serves to take advantage of the relative ease of generating voltage signals and the advantages of current mode outputs, providing a current output signal using a voltage input reference. Voltage references (e.g., bandgap voltage references, sub-bandgap voltage references, and Zener voltage) are commonplace and generally preferred in certain scenarios. This voltage input level may be a static reference level, or it can also be varied (e.g., using a DAC), and this can be used to optimize transconductance performance. The input voltage, and resultant compliance voltage, may be varied independent of the output current level for the specific load and application.

The current output from the transconductance DAC is substantially common-mode voltage insensitive, which can be useful in any number of precision applications. It is desirable to use voltage reference input. In addition, transconductance can provide for both voltage input and current output, combining two desirable attributes. Such an architecture can enable an independent control of the output compliance and the current level. The voltage input may be a stable, current independent voltage (e.g., using a voltage reference). The voltage input can be modified with the proposed configuration. Alternatively, the voltage across the impedance can be modified (e.g., increased in low current configurations versus higher current configurations) to reduce the sensitivity to error sources if/as the output compliance allows.

This precision transconductance technology can be applied, for example, in healthcare through optical vital sign measurements. Optical vital sign measurement is important because it allows measurement of vital signs of a patient in a non-invasive fashion. Some of the vital signs that can be measured include blood oxygen saturation (SpO2) and heart rate. The vital signs are measured by illuminating the surface and subcutaneous area of the skin with a light source and then measuring the optical signal, as it is reflected back and modulated by the tissue and blood flow. In order to have precise and accurate vital sign measurements, it is important to be able to provide a low noise way to produce the light to measure the vital signs. Past efforts have failed to provide a low power solution that provides sufficient photometry, colorimetry, and spectrometry in optical vital sign measurement. Photometry is a measurement of the brightness of light perceived by the human eye. Colorimetry is a measurement of human color perception of light. Spectrometry is a measurement of the wavelength of light. This precision transconductance device can allow a precision transconductance digital to analog converter to be designed and placed into a module of an optical vital sign measurement device that can be used to precisely control a light emitting source that directs the light through some part of the body, and it is received in turn by an optical detector. The optical guide can be constructed from any material capable of directing light including, for example, polycarbonate, water clear polycarbonate, plastics, glass, etc. By being able to precisely control the light source with the precision transconductance device, the wavelengths of the light transmitted in the tube are more precisely transmitted allowing for accurate readings of vital signs while consuming less power. Optical vital signs measurements can be made at many body sites, with a wide range of optical attenuation between the emitter and detector. It is common practice to adjust the intensity at the emitter in order to maintain an approximately constant signal at the detector. The present disclosure is adapted to maximize the SNR when the excitation current is small, while not necessarily needing a large input voltage when the designated current is large.

The precision transconductance device can also provide benefits in industrial applications such as industrial processes control, field transmitters, and 4 mA-20 milliamp (mA) systems. The analog 4 mA-20 mA and 10 mA-50 mA current loops are commonly used for industrial process control systems, with 4 mA representing the lowest end of the range and 20 mA the highest. The advantages of these analog current loops are that the accuracy of the signal may not be affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Hence, current output is desirable, where there is widespread usage of 4 mA-20 mA architectures, for example. Even if there is significant electrical resistance in the line, the current loop transmitter can maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (referred to as two-wire transmitters). Such instruments could be used to measure pressure, temperature, flow rates, pH or other process variables in industrial systems, for example. The precision transconductance device can also be used to control industrial systems such as a valve positioner or other output actuator, along with any number of additional industrial applications.

The precision transconductance device can provide benefits in consumer applications such as audio and video applications. For example, many mobile devices send analog audio signals to earphones. The precision transconductance device can be used to provide a better audio signal to the earphones. The mobile device could be a cell phone, a smart phone, a digital music player, a tablet, a laptop computer, or any other handheld device. The precision transconductance can be used in other audio applications to drive any type of speaker including loudspeakers and stereo speakers. The advantage of the precision transconductance device is that the speakers can be driven over a wide range and over longer wires than possible with other solutions. The same is true for video applications such as sending an analog signal to a display device, along with any number of additional audio/video applications.

The claims presented here are written in single dependency format suitable for use at the Patent office of the United States of America. However, for the avoidance of doubt, each claim may depend on any preceding claim of the same category except when that is clearly technically infeasible.

In the discussions of the embodiments above, the controllers, back gates, transistors, capacitors, arrays, switches, inductors, resistors (of any type), amplifiers, nodes, converters, digital core, and/or other components can readily be replaced, substituted, or otherwise modified in order to accommodate particular circuitry needs. Moreover, it should be noted that the use of complementary electronic chips, hardware, software, etc. offer an equally viable option for implementing the teachings of the present disclosure.

In one example embodiment, any number of electrical circuits of the FIGURES may be implemented on a board of an associated electronic chip. The board can be a general circuit board that can hold various components of the internal electronic system of the electronic chip and, further, provide connectors for other peripherals. More specifically, the board can provide the electrical connections by which the other components of the system can communicate electrically. Any suitable processors (inclusive of digital signal processors, microprocessors, supporting chipsets, etc.), memory elements, etc. can be suitably coupled to the board based on particular configuration needs, processing demands, computer designs, etc. Other components such as external storage, additional sensors, controllers for audio/video display, and other peripheral chips may be attached to the board as plug-in cards, via cables, or integrated into the board itself.

Note that the activities discussed above with reference to the FIGURES are applicable to any integrated circuits that involve signal processing, particularly those that rely on synchronization signals to execute specialized software programs, or algorithms, some of which may be associated with processing digitized real-time data. Certain embodiments can relate to multi-DSP signal processing, floating point processing, signal/control processing, fixed-function processing, microcontroller applications, etc. In certain contexts, the features discussed herein can be applicable to medical systems, scientific instrumentation, wireless and wired communications, radar, industrial process control, audio and video equipment, current sensing, instrumentation (which can be highly precise), and other digital-processing-based systems.

Moreover, certain embodiments discussed above can be provisioned in digital signal processing technologies for medical imaging, patient monitoring, medical instrumentation, and home healthcare. This could include pulmonary monitors, accelerometers, heart rate monitors, pacemakers, etc. Other applications can involve automotive technologies for safety systems (e.g., stability control systems, driver assistance systems, braking systems, infotainment and interior applications of any kind). Furthermore, powertrain systems (for example, in hybrid and electric vehicles) can apply the functionalities described herein in high-precision data conversion products in battery monitoring, control systems, reporting controls, maintenance activities, etc.

In yet other example scenarios, the teachings of the present disclosure can be applicable in the industrial markets that include process control systems that help drive productivity, energy efficiency, and reliability. In consumer applications, the teachings of the electrical circuits discussed above can be used for image processing, auto focus, and image stabilization (e.g., for digital still cameras, camcorders, etc.). Other consumer applications can include audio and video processors for home theater systems, DVD recorders, and high-definition televisions. Yet other consumer applications can involve advanced touch screen controllers (e.g., for any type of portable media chip). Hence, such technologies could readily part of smartphones, tablets, security systems, PCs, gaming technologies, virtual reality, simulation training, etc.

Other Notes, Examples, and Implementations

Note that all optional features of the apparatus described above may also be implemented with respect to the method or process described herein and specifics in the examples may be used anywhere in one or more embodiments. In a first example, a system is provided (that can include any suitable circuitry, dividers, capacitors, resistors, inductors, DACs, ADCs, arrays, logic gates, software, hardware, links, etc.) that can be part of any type of electronic device (e.g., computer), which can further include a circuit board coupled to a plurality of electronic components.

The system can include means for trimming currents between a plurality of current control stages, where first and second stages each comprise a respective field effect transistor, said field effect transistors having top gates and back gates, wherein the top gates are connected to a shared node so as to receive a shared control voltage, and the back gates are driven to respective voltages so as to set a current ratio between the first and second stages to a desired value.

In other embodiments, a voltage applied to the back gate can be provided by a DAC. Additionally, the stages can be provided as part of a transconductance DAC. The ‘means for’ in these instances (above) can include (but is not limited to) using any suitable component discussed herein, along with any suitable software, circuitry, hub, computer code, logic, algorithms, hardware, controller, interface, link, bus, communication pathway, etc. In a second example, the system includes memory that further comprises machine-readable instructions that when executed cause the system to perform any of the activities discussed above.