Voltage-to-current transconductance operational amplifier with adaptive biasing

Abstract

An IC for power conversion includes bias circuitry that generates one or more bias voltages. An adaptive biasing circuit adaptively shifts an input signal having a negative value to a positive value. An operational transconductance amplifier (OTA) receives a supply bias current and the first and second bias voltages. The OTA has first and second input terminals coupled to the input signal and ground, respectively. The OTA has first and second transistors coupled to the first and second input terminals through first and second resistors at first and second internal nodes, respectively. Additional circuitry of the OTA is coupled to the second internal node. The additional circuitry insures that the voltage at the second internal node follows the voltage at the first internal node. The OTA generates an output current signal responsive to a differential input voltage applied across the first and second input terminals.

Description

TECHNICAL FIELD

This application relates generally to electronic amplifiers and integrated circuits for implementing the same; more specifically, to an operational transconductance amplifier (OTA) device that converts a differential input voltage into an output current.

BACKGROUND

Operational amplifiers are commonly utilized in electronic circuits to process analog signals. An operational amplifier is often used in integrated circuits that modify analog signals in a particular way, such for example changing amplitudes, filtering frequency components, or performing linear mathematical operations that may include summing with other signals, integration, and differentiation.

Direct current (DC) signals are often used as analog representations of measurements (such as sensed electrical signals from different nodes of a power converter or other circuitries or in other examples sensing/measuring temperature, pressure, flow, weight or motion). Often times, DC current signals are used in preference to DC voltage signals. This is because current signals are substantially equal in magnitude throughout a series circuit loop carrying current from the source (measuring device) to the load (indicator, recorder, or controller), whereas voltage signals in a parallel circuit may vary from one end to the other due to resistive wire losses. Furthermore, current-sensing instruments typically have low impedances, whereas voltage-sensing instruments typically have high impedances; that means that current-sensing instruments generally have greater electrical noise immunity.

An operational transconductance amplifier (OTA) is a well-known amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source (VCCS). Transconductance is an electrical characteristic relating the current through the output of a device to the voltage across the input of a device. That is, conductance is the reciprocal of resistance. Voltage-to-current converting OTAs are utilized in many electronic circuits. For example, OTAs may be used in instrumentation circuitry and in integrated circuit (IC) controllers that are required for operation of switched-mode power converters that power electronic devices such as laptop computers, televisions, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1A is a circuit block diagram of an example operational transconductance amplifier circuit with adaptive biasing.

FIG. 1B is a detailed circuit schematic diagram of the operational transconductance amplifier circuit with adaptive biasing shown in FIG. 1A.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various disclosed embodiments.

DETAILED DESCRIPTION

In the following description specific details are set forth, such as device types, voltages, component values, circuit configurations, etc., in order to provide a thorough understanding of the embodiments described. However, persons having ordinary skill in the relevant arts will appreciate that these specific details may not be needed to practice the embodiments described. It is further appreciated that well known circuit structures and elements have not been described in detail, or have been shown in block diagram form, in order to avoid obscuring the embodiments described.

Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or sub-combinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art.

In the context of the present application, when a transistor is in an “off state”, or “off”, the transistor does not substantially conduct current. Conversely, when a transistor is in an “on state”, or “on”, the transistor is able to substantially conduct current. By way of example, in one embodiment, a power transistor comprises an N-channel metal-oxide-semiconductor field-effect transistor (NMOS) with the high-voltage being supported between the first terminal, a drain, and the second terminal, a source. The high voltage MOSFET comprises a power switch that is driven by an integrated controller circuit to regulate energy provided to a load. For purposes of this disclosure, “ground” or “ground potential” refers to a reference voltage or potential against which all other voltages or potentials of an electronic circuit or Integrated circuit (IC) are defined or measured.

The present disclosure is directed to a voltage-to-current converter circuit and method of operation thereof. In one embodiment, an operational transconductance amplifier (OTA) is utilized in conjunction with input circuitry and an adaptive biasing circuit that adaptively shifts a negative input signal to a level that is shifted above ground (zero volts). The shifted signal is processed by the OTA. Thus, the converter circuit is capable of sensing input signals that may extend both above and below ground (oscillating positive/negative). A low impedance resistor between the input and ground may be included to provide current-to-current translation from the milliamp to the microampere range for further processing within an integrated circuit (IC) or chip package.

Persons of skill in the art will appreciate that the voltage-to-current converter disclosed herein advantageously provides for sensing of both positive and negative input signals; utilization of NMOS devices, as opposed to PMOS that typically require a higher back bias; less electrostatic discharge (ESD) susceptibility; a relatively wide input signal range (e.g., ±1 V); and improved noise rejection.

In an example application, a power supply uses a current sense leg of a power FET to provide a fraction of the switching current (e.g., in tens of mAs) into a current sense resistor located between the FET and ground. The current sense resistor may be 50 ohms or less. The voltage developed across the current sensing resistor provides an input voltage signal through an adaptive biasing circuit to a voltage-to-current (V-to-I) converter. In one implementation, the V-to-I converter is configured to handle the input voltage signals in the range of from +1 V down to −0.5 V. In one example, the power converter circuit includes ESD diodes that turn-on at a voltage of approximately −0.5 V.

In one example, level shifters in an adaptive biasing circuit utilize PMOS devices coupled as voltage followers for sensing and shifting voltage signals that are below ground to be transferred to a more suitable range within the power supply rail voltage of the IC. The adaptive biasing circuit is configured to adaptively shift any negative input signal to a shifted level above zero (above ground), which may then be processed by the V-to-I converter. Adaptive biasing reduces power consumption and increases efficiency.

FIG. 1A is a circuit block diagram of an example operational transconductance amplifier circuit with adaptive biasing. As shown, an input voltage signal Vin 145 is provided by a voltage drop across an input resistor R_in 144 due to a sensed current signal 140 from a power switch in a switching circuit/converter. Sensed current 140 is shown symbolically by current source I_in 142. Note that the voltage drop across input resistor R_in 144 generates input voltage signal Vin 145 referenced to ground 101. Input resistor R_in 144, in one example, is a relatively small (e.g., 50 ohm) resistor. Input voltage signal Vin 145 is coupled to the negative input 148 of voltage-to-current operational transconductance amplifier (V-to-I TOA) 150, and also to an adaptive biasing circuit 130. Adaptive biasing circuit 130 is configured to adaptively bias and level shift input signal V_in 145 such that negative oscillations on input signal V_in 145 become positive in reference to ground. In one example, sensed current 140 is a very small fraction of switch current sensed through a sense FET integral with a power switch.

The circuit block diagram of FIG. 1A also shows bias voltages V_bn/V_bnc 125/125C provided at node 111 coupled to input bias circuitry 110 and adaptive biasing circuit 150. Input bias circuitry 110 provides the supply and bias voltage (V_bp, V_bpc) at supply line 112 for low-side and high-side CMOS switches included in the circuitry of V-to-I OTA 150. V-to-I OTA 150 produces an output current signal I_OUT 190 responsive to the differential input voltage applied to the positive and negative input terminals of V-to-I OTA 150. Note that in the example shown the positive input terminal of V-to-I OTA 150 is connected to ground 101. Output current signal I_OUT 190 may optionally be scaled by device size scaling of transistors 184 & 188 to provide a properly scaled current signal I_out 190 for the sense and detect circuitries of the IC.

FIG. 1B is a detailed circuit schematic diagram of the operational transconductance amplifier circuit with adaptive biasing shown in FIG. 1A. The power converter circuit shown in FIG. 1B includes the three main blocks shown in FIG. 1A; namely, input bias circuitry 110, adaptive biasing circuit 130, V-to-I OTA 150.

Input bias circuitry 110 generates the bias input voltage for the high-side PMOS switches P1152, P1C 152C, P2154 and P2C 154C in V-to-I OTA 150. Input bias circuitry 110 includes two switching legs with parallel current sharing operation. It is appreciated that each of the two series-coupled cascode structure switches (e.g., PMOS transistors P_in1 116 and P_in1C 116C, at the high-side of the first current leg/branch and NMOS cascode transistor N_in1/N_in1C 122/122C, at the low-side of first current leg/branch) may include an optional cascode structure that provides high accuracy in current mirroring. In the example of FIG. 1B the cascode structure of high-side and low-side elements is repeated in other legs as well and are framed in the smaller dotted rectangles to distinguish the cascode switch structure (CSS). It is appreciated that in other examples/embodiment these low-side and high-side switching elements may include other types of single transistors and/or discrete switches.

As shown, the low-side cascode-pair NMOS transistors N_in1/N_in1C 122/122C (in the first current leg/branch) as well as cascode-pair NMOS transistors N_in2/N_in2C 124/124C (in the second current leg/branch) are referenced to ground 101 and are driven by external bias supplies V_bn 125 & V_bnc 125C.

High-side cascode-pair PMOS transistors P_in2/P_in2C 114/114C (at the high-side of the second current leg/branch) are configured to function as a diode. In other words, the gates of P_in2C 114C and P_in2 114 are coupled to the drain of the high-side PMOS cascode-pair that is also coupled to the drain of the low-side cascode-pair NMOS switching element at node B 123. The source of cascode-pair PMOS transistors P_in1/P_in1C 116/116C (at the high-side of first current leg/branch) is the same as the source of cascode-pair PMOS transistor P_in2 114/P_in2C 114C (at the high-side of second current leg/branch), both of which are coupled to a current (or voltage) supply I_sup 112, with each receiving a current I_b 113 (e.g., 2.5 μA). The voltage at node B 123 provides bias for cascode-pair PMOS transistors P_in2/P_in2C 114/114C as well as bias voltage V_bpC 115C for certain other transistors in the circuit. Voltage V_bpC 115C biases transistor P_in1C 116C and the voltage at node A 121 may provide bias V_bp 115 for transistor P_in1 116 and for certain other transistors in the circuit schematics of FIG. 1B.

Bias voltages V_bn 125 and V_bnC 125C (for cascode-pair NMOS transistors) from input bias circuitry block 110 are also coupled to adaptive biasing circuit block 130. Adaptive biasing circuit block 130 includes two current sharing legs with cascode-pair NMOS transistors N_A/N_AC 134/134C in first leg, and cascode-pair NMOS transistors N_B/N_Bc, 132/132C in second leg which are scaled to 1/k (e.g., k=5) of the size of the low-side NMOS transistors in input bias circuitry block 110. Cascode-pair NMOS transistors N_B/N_Bc, 132/132C are activated through bias voltages V_bn 125 (for N_A 134 and N_B 132) and V_bnC 125C (for N_AC 134C and N_BC 132C). The bias voltages V_bp 115 and V_bpc 115C (for PMOS transistors) generated in the input bias circuitry block 110 are coupled to adaptive biasing circuit block 130 and to V-to-I OTA block 150.

In the example of FIG. 1B, transistors N₁ 162, N2164, N3182 and the high-side PMOS transistors, are configured as cascode-pair P1/P1C 152/152C, P2/P2C 154/154C, with each being biased to conduct a fixed biasing current I_b 113. Resistor R1166 (e.g., 200K ohm) is coupled in series with transistor N1162 with the current flowing through resistor R1166 fixed at I_b113. The voltage at V1163 is thus V1=I_b×R1+Vin.

In V-to-I OTA converter block 150 transistors N1162, N2, 164 and N3182 are of the same size and carrying the same current I_b 113. As configured, the gate-to-source voltages of these transistors match, resulting in voltages V1163, V2165 and V3185 being substantially equal (V1=V2=V3). Thus, the voltages V1163, V2165 and V3185 all follow input signal V_in 145, but appear level shifted up by an offset voltage equal to I_b×R1. The level shifted version of V_in appears across resistor R2168 (In one example R2=200K ohm), hence: I_R2=I3=V_in/R₂.

The current I_b and resistor R1166 are set such that V1163 stays above ground 101 even when V_in 145 drops to its largest negative value. Thus, resistor R₁ 166 provides a voltage drop between input signal V_in 145 and voltage V1163, such that the source of transistor N1162 remains above ground 101 even when V_in<0 V (e.g., due to negative oscillations). The values of current I_b 113 and resistor R1166 are set so that I_b×R1 remains equal to, or higher than, the highest possible negative oscillation of Vin (I_b×R1≥−V_in). This ensures that the sources of transistors N1162, N2, 164 and N3182 and the drains of transistors N4169 and N5186 stay above a threshold value (e.g., ˜0.2 V) even when signal V_in is at its maximum negative value.

Persons of skill will appreciate that the current through transistor N2164 is a fixed value (=I_b; e.g., 2.5 μA); however, the current through source resistor R2168 (e.g., R2=200KΩ) drops in order for voltage V2165 to follow voltage V1163 as V1 drops. The way that the circuit of FIG. 1B bleeds off bias current through R2 is by providing a path through transistor N4169. In one embodiment, transistor N4169 is biased to carry a fixed current of I_b 113. Transistor N4169 has a common gate-source coupling with transistor N5186; therefore it and the current I_b 113 through transistor N5186. As a result, the bias current through transistor N4169 is set to the fixed current I_b 113.

In one embodiment, transistors N4169 and N5186 are sized to be relatively weak devices, such that they each require a larger gate voltage, which ensures that the drain of transistor N3182 remains in saturation mode. If voltages V1163 and V2165 rise substantially then transistor N3182 may transition out of saturation and act as a transmission gate. However, in that case the voltages on the drains of transistors N4169 and N5186 are large enough to place them into saturation such that the currents flowing through each of transistors N4169 and N5186 accurately mirror each other.

Persons of skill in the art will understand that transistor N4169 takes substantially all of the current I2 sourced by transistor N2164. This allows voltage V2165 to drop to 0V. In order for V2 to rise above ground 101, current flow through R2 is supplied by transistor P3158. Transistor P3158 is shown coupled in series with diode-connected PMOS transistor P4156 as part of an active feedback loop that monitors the drain of transistor N2164.

Note that when input signal V_in 145 is at 0 V, voltages V1163 and V2165 are matched and equal currents flow through R1166 and R2168. If voltage V1163 rises, voltage V2165 rises also, with R2168 receiving current I3. Note further that the mirroring transistor above transistor N2164 only provides a current equal to I_b 113. This causes the drain of N2164 to fall, which turns on transistor P3158 so as to deliver the additional current I3 that flows through R2168. This additional current flow raises the voltage at V2165 until it matches voltage at V1163.

Conversely, as voltage V1163 drops, voltage V2165 also drops by gradually turning off transistor P3158. If voltage V1163 reaches 0 V, voltage V2165 is also at 0 V; therefore transistor P3158 is turned off. In cases where a quick transient response is required, transistor P3158 is not allowed to turn fully off. This may be achieved by setting the values of I_b 113 and R2168 such that voltage at node V2165 does not drop below a minimum threshold (e.g., 0.2V), which means that approximately 1 μA continues to flow through transistor P3158. It is appreciated that the current that flows through R2 is supplied by transistor P3158, and not transistor N2164.

As shown, transistor P3158 is configured as a source follower which drives the mirroring transistor P4156 (configured as a diode), with a bias current I3153 (I3=I_b+V_in/R2) flowing through each device. To obtain a current that reflects V_in/R2, which is bidirectional depending on the polarity of V_in 145, current I_b 113 is subtracted. Subtracting I_b 113 is accurately accomplished by duplicating transistors N3182 and N5186, as shown in the schematic of FIG. 1B. In the last (right-most side) current leg of V-to-I OTA block 150 high-side PMOS cascode-pair transistors P5/P5C 176/176C are activated by Vbp/Vbpc 115/115C (provided by input bias circuitry 110) such that biasing current I_b flows through low-side NMOS transistors N3182 and N5186.

With continuing reference to FIG. 1B, output current scaling may be achieved by an optional mirroring ratio design (device size ratio) to generate current I_out 190 in a scaled linear relationship with input voltage signal V_in 145. Low-side NMOS transistors N3a 184 and N5a 188 mirror biasing current I_b 113 from transistors N3182 and N5186, which is then subtracted from current I3 in high-side PMOS transistor P6174 to generate output current I_out 190.

To reiterate, the main purpose of adaptive biasing circuitry 130 and resistor R1166 is to level shift (up) input signal V_in 145 via the voltage drop across resistor R1166 such that sources of transistors N1162 and N2164 stay above a minimum threshold (e.g., ˜0.2V). It is appreciated that if input signal V_in 145 is at its maximum positive value (e.g., ˜1 V) then the drain of transistors N1162 is likewise raised to its maximum value of V_in+I_b×R1+Vgs_(N1) which in one embodiment is about: 1 V+(2.5 μA*200 kΩ)+1.8V=3.3V

Practitioners will appreciate that adaptive bias circuitry 130 increases the range over which input signal V_in 145 can operate by boosting bias current I_b 113 as the input signal V_in 145 drops below ground 101 (goes negative). In addition, adaptive biasing circuitry 130 decreases current I_b 113 when V_in 145 is greater than its minimum threshold (e.g., V_in>0.2 V), which reduces the voltage drop across resistor R1166, thereby increasing the headroom for diode-connected transistor N1162. This allows the minimum supply voltage to be reduced (by ˜0.5 V).

Cascode-pair transistors N_A/N_AC 134/134C and N_B/N_BC 132/132C are sized to be a fraction of the cascode-pair transistors in input bias circuitry 110, i.e., N_in1/N_in1C 122/122C and N_in2/N_in2C 124/124C. As such, transistors N_A/N_AC 134/134C and N_B/N_BC 132/132C carry relatively little current (a fraction 1/k of I_b) when input signal V_in 145 is at 0 V. In the embodiment of FIG. 1B the fraction is about 1/k=⅕ (˜I_b/5). This increases the bias current through the transistors in adaptive bias circuitry 130 slightly, e.g., around 20% higher than the current in the NMOS transistor devices biased by V_bn 125 in input bias circuitry 113. As V_in 145 rises above ground 101, the current in transistors N_A/N_AC 134/134C and N_B/N_BC 132/132C shut off, causing the adaptive current contribution to drop to zero. On the other hand, if input signal V_in 145 falls below ground (V_in<0), the current in adaptive bias circuit 130 increases by approximately (0 V−V_in)/R_Bias; (in one example R_Bias=100 KΩ), which can cause bias current I_b 113 to increase more than double as input signal V_in 145 drops below ground 101.

In the embodiment of FIG. 1B transistors N_A/N_AC 134/134C and N_B/N_BC 132/132C are ˜⅕ of the size of the NMOS transistors in input bias circuitry 110. Note: transistors N_in1/N_in1C 122/122C and N_in2/N_in2C 124/124C are controlled by V_bn 125 and V_bnC 125C. In other embodiments, a wider size range may be achieved by doubling or tripling the size of transistors N_in1/N_in1C 122/122C and N_in2/N_in2C 124/124C, which improves accuracy and allows for a wider signal range for the input voltage signal V_in 145.—With an input voltage signal range of: −1 V<V_in<+1 V an accuracy of +/−1.2% for I_out may be achieved.

The above description of illustrated example embodiments, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms or structures disclosed. While specific embodiments and examples of the subject matter described herein are for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific example currents, voltages, resistances, device sizes, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present invention.

Claims

1. An integrated circuit (IC) for voltage-to-current conversion comprising: bias circuitry coupled to receive one or more input bias voltages referenced to ground, the bias circuitry generating one or more bias voltages;an adaptive biasing circuit coupled to receive the one or more input bias voltages and the one or more bias voltages, the adaptive biasing circuit being configured to adaptively shift an input signal having a negative value to a positive value above ground; andan operational transconductance amplifier (OTA) coupled to receive a supply bias current and the one or more bias voltages, the OTA having first and second input terminals coupled to the input signal and ground, respectively, the OTA having first and second transistors coupled to the first and second input terminals through first and second resistors at first and second internal nodes, respectively, in operation, the supply bias current flows through each of the first and second transistors, the OTA further including additional circuitry coupled to the second internal node, the additional circuitry being configured to insure that the voltage at the second internal node follows the voltage at the first internal node, the OTA generating an output current signal responsive to a differential input voltage applied across the first and second input terminals.

2. The IC of claim 1 wherein the output current signal is linearly related with the input signal.

3. The IC of claim 1 further comprising a third resistor coupled between the first input terminal and ground, the third resistor having a relatively low resistance value as compared to the first and second resistors.

4. The IC of claim 1 wherein the first and second resistors each have a substantially equal resistance value.

5. The IC of claim 1 wherein the bias circuitry includes first and second switching legs each carrying a substantially equal current in operation, each of the first and second switching legs including high-side cascode structure PMOS transistors coupled in series with low-side cascode structure NMOS transistors, the one or more input bias voltages being coupled to the gates of cascode structure NMOS transistors, and the one or more bias voltages being coupled to the gates of cascode structure PMOS transistors of the first and second switching legs.

6. The IC of claim 5 wherein during operation a voltage drop across the first resistor level shifts the input signal such that sources of the first and second transistors stay above a minimum threshold.

7. The IC of claim 1 wherein the first and second transistors comprise NMOS transistors.

8. The IC of claim 1 wherein the additional circuitry comprises third and fourth transistors, the third transistor being coupled to the second internal node, during operation the third transistor providing current flow through the second resistor, the fourth transistor being coupled between the second internal node and ground, the supply bias current flowing through the fourth transistor.

9. The IC of claim 8 wherein the third transistor comprises a PMOS transistor and the fourth transistor comprises a NMOS transistor.

10. An integrated circuit (IC) or converting a differential input voltage into an output current comprising: bias circuitry comprising first and second switching legs, each including high-side transistors coupled in series with low-side transistors, the low-side transistors coupled to receive one or more input bias voltages, the high-side transistors generating one or more bias voltages;adaptive biasing circuitry configured to adaptively shift an input voltage signal having a negative value to a positive value above ground; andan operational transconductance amplifier (OTA) having a cascode-pair of PMOS transistors coupled to receive the one or more bias voltages, the OTA having first and second input terminals coupled to the input voltage signal and ground, respectively, a difference between the input voltage signal and ground comprising the differential input voltage, the OTA having first and second NMOS transistors coupled to the first and second input terminals through first and second resistors at first and second internal nodes, respectively, in operation, a supply bias current flowing through each of the first and second transistors and through the cascode-pair of PMOS transistors, the OTA being configured to insure that the voltage at the second internal node follows the voltage at the first internal node, the OTA generating an output current signal responsive to a differential input voltage applied across the first and second input terminals.

11. The IC of claim 10 wherein the input voltage signal has a range that extends between positive and negative values.

12. The IC of claim 10 wherein the OTA has an input impedance to ground that is substantially lower than the input impedance to a supply.

13. The IC of claim 10 wherein the high-side transistors and the low-side transistors of the bias circuitry are each configured as a cascode-pair.

14. The IC of claim 10 wherein the first input terminal is coupled to ground through an input resistor having a substantially lower resistance value as compared to the first resistor.

15. The IC of claim 10 where the first and second resistors each have a substantially equal resistance value.

16. The IC of claim 10 wherein the OTA further includes third and fourth transistors, the third transistor being coupled to the second internal node, during operation the third transistor providing current flow through the second resistor, the fourth transistor being coupled between the second internal node and ground, the supply bias current flowing through the fourth transistor.

17. The IC of claim 16 wherein the third transistor is a PMOS transistor and the fourth transistor is a NMOS transistor.

18. The IC of claim 17 herein the third transistor is coupled to a supply bus through a diode.

19. The IC of claim 10 wherein the output current signal has a scaled linear relationship with the input voltage signal.