Voltage controlled adjustable current source

Abstract

A current regulating apparatus capable of regulating an electrical current with a high level of precision and over a wide range of voltages includes a first depletion mode field-effect transistor (FET), a second depletion mode FET, and a fixed resistor. The second depletion mode FET and fixed resistor are connected in series and across the gate-source terminals of the first depletion mode FET. The first depletion mode FET operates as an adjustable current source while the second depletion mode FET is controlled to operate as a voltage controlled resistor. The magnitude of current regulated by the current regulating apparatus is determined based on both the resistance of the fixed resistor and a current-setting control voltage applied to the gate of the second depletion mode FET. Various precision values of regulated current can be realized by simply changing the current-setting control voltage.

Claims

1. A voltage controlled adjustable current source, comprising: a first depletion mode field-effect transistor (FET) having a gate, a drain, and a source; a second depletion mode FET having a gate, a drain configured to receive an input drain voltage, and a source coupled to the drain of the first depletion mode FET; and a fixed resistor coupled between the source of the first depletion mode FET and the gate of the second depletion mode FET, wherein the first depletion mode FET is controlled to serve as a voltage controlled resistor, the gate of the first depletion mode FET is configured to receive a current-setting control voltage, a magnitude of a current regulated by the voltage controlled adjustable current source is determined by a magnitude of the current-setting control voltage, and the magnitude of the current-setting control voltage is controlled to reduce a deviation of an actual resistance of the fixed resistor from a designed resistance value.

2. The voltage controlled current source of claim 1, wherein the second depletion mode FET comprises a gallium-nitride high electron mobility transistor (GaN-HEMT).

3. The voltage controlled current source of claim 2, wherein the first depletion mode FET comprises a GaN-HEMT.

4. An integrated circuit, comprising: a first depletion mode field-effect transistor (FET) configured to operate as a controlled current source; a second depletion mode FET coupled to the first depletion mode FET configured to operate as a voltage controlled resistor; and a fixed resistor coupled between a gate of the first depletion mode FET and a source of the second depletion mode FET, wherein a gate of the second depletion mode FET is configured to receive a current-setting control voltage, a magnitude of a current regulated by the first depletion mode FET is determined by a magnitude of the current-setting control voltage, and the magnitude of the current-setting control voltage is controlled to reduce a deviation of an actual resistance of the fixed resistor from a designed resistance value.

5. The integrated circuit of claim 4, wherein the first and second depletion mode FETs comprise first and second gallium nitride (GaN) high electron mobility transistors or some other III-nitride transistors.

6. A method of regulating a current in an electrical circuit, comprising: configuring a first depletion mode field-effect transistor (FET) to operate in its saturation region of operation; configuring a second depletion mode FET to operate in its ohmic region of operation, the second depletion mode FET having a drain-source path connected in series with a drain-source path of the first depletion mode FET; directing an electrical current through the drain-source paths of the first and second depletion mode FETs and through a fixed resistor connected in series with the first and second depletion mode FETs; applying a current-setting control voltage to a gate of the second depletion mode FET; and regulating the electrical current to a magnitude determined by the resistance of the fixed resistor and a magnitude of the current-setting control voltage applied to the gate of the second depletion mode FET, wherein the magnitude of the current-setting control voltage is adjustable and the magnitude of the regulated current is adjustable over a range of values depending on the magnitude of the current-setting control voltage.

7. The method of claim 6, wherein the first and second depletion mode FETs are formed in an integrated circuit chip.

8. The method of claim 7, wherein the first and second depletion mode FETs comprise first and second gallium nitride (GaN) high electron mobility transistors or some other III-nitride transistors.

9. The method of claim 8, wherein the current-setting control voltage is generated by a voltage source formed in the integrated circuit chip.

10. The method of claim 8, wherein the current-setting control voltage is provided by a voltage source external to the integrated circuit chip.

11. A method of regulating a current in an electrical circuit, comprising: configuring a first depletion mode field-effect transistor (FET) to operate in its saturation region of operation; configuring a second depletion mode FET to operate in its ohmic region of operation, the second depletion mode FET having a drain-source path connected in series with a drain-source path of the first depletion mode FET; directing an electrical current through the drain-source paths of the first and second depletion mode FETs and through a fixed resistor connected in series with the first and second depletion mode FETs; applying a current-setting control voltage to a gate of the second depletion mode FET; regulating the electrical current to a magnitude determined by the resistance of the fixed resistor and a magnitude of the current-setting control voltage applied to the gate of the second depletion mode FET; and adjusting the current-setting control voltage to reduce a deviation of an actual resistance of the fixed resistor from a designed resistance value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of a conventional fixed-current current regulating (CR) diode;

(2) FIG. 2 is a schematic drawing of a conventional adjustable-current CR diode;

(3) FIG. 3 is a schematic drawing of a voltage controlled adjustable current source, according to one embodiment of the present invention;

(4) FIG. 4 is a plot of the current regulated by the voltage controlled adjustable current source depicted in FIG. 3, for various values of the current-setting control voltage V.sub.G1; and

(5) FIG. 5 is a plot of the output impedance Z.sub.OUT of the voltage controlled adjustable current source depicted in FIG. 3, when the current-setting control voltage V.sub.G1 is held at one specific setting.

DETAILED DESCRIPTION

(6) Referring to FIG. 3, there is shown a schematic drawing of a voltage controlled adjustable current source 300, according to an embodiment of the present invention. The voltage controlled adjustable current source 300 comprises a first depletion mode field-effect transistor (FET) 302, a second depletion mode FET 304, and a fixed resistor 306. The first depletion mode FET 302 has a drain connected to the source of the second depletion mode FET 304, a source connected to a first terminal of the fixed resistor 306, and a gate configured to receive a current-setting control voltage V.sub.G1. The second depletion mode FET 304 has a drain configured to receive a drain voltage V.sub.D2, a gate connected to a second terminal of the fixed resistor 306, and a source connected to the drain of the first depletion mode FET 302.

(7) The voltage controlled adjustable current source 300 is preferably implemented in an integrated circuit (IC), e.g., a monolithic microwave integrated circuit (MMIC), and in the exemplary embodiment of the invention described below and depicted in the drawings the first and second depletion mode FETs 302 and 304 comprise gallium-nitride high electron mobility transistors (GaN-HEMTs) or some other type of III-nitride transistors. While an IC implementation with GaN-HEMTs is preferred, the voltage controlled adjustable current source 300 could be alternatively constructed from discrete devices and the first and second FETs 302 and 304 could comprise other type(s) of depletion mode FET(s), as will be appreciated by those of ordinary skill in the art.

(8) The second depletion mode FET 304 in the voltage controlled adjustable current source 300 operates as an adjustable current source while the first depletion mode FET 302 is configured and controlled to operate as a voltage controlled resistor. Operating together, the first and second depletion mode FETs 302 and 304 and fixed resistor 306 regulate a current I.sub.D having a magnitude that depends on both the resistance of the fixed resistor 306 and the current-setting control voltage V.sub.G1 applied to the gate of the first depletion mode FET 302. The current-voltage (I-V) characteristics of the voltage controlled adjustable current source 300 presented in FIG. 4 illustrate how the magnitude of the regulated current I.sub.D increases as the current-setting control voltage V.sub.G1 is increased. Various values of regulated current I.sub.D are realized by simply changing the current-setting control voltage V.sub.G1, which can be provided by an on-chip voltage source or from a voltage source external to the IC.

(9) Including the first depletion mode FET 302 in series with the fixed resistor 306 affords the voltage controlled adjustable current source 300 the ability to set and control the regulated current I.sub.D with a high degree of precision. Because the magnitude of the regulated current I.sub.D can be set and controlled independent of the fixed resistor 306, the current-setting control voltage V.sub.G1 can also be exploited to fine tune the current I.sub.D and thereby overcome any deviation of the resistance of the fixed resistor 306 from its intended or designed resistance value that may have resulted due to limitations and/or variabilities in the IC manufacturing process.

(10) The presence of the first depletion mode FET 302 has the further benefit of increasing the output impedance Z.sub.OUT=1/g.sub.OUT of the voltage controlled adjustable current source 300 compared to prior art CR diodes. The increase in output impedance Z.sub.OUT can be observed in the output impedance plot provided in FIG. 5, where it is seen that for a V.sub.G1=1.25V and an equivalent V.sub.GS=1.25V in the prior art CR diode 200 the output impedance Z.sub.OUT of the voltage controlled adjustable current source 300 remains higher over almost the entire input voltage range V.sub.D2. This attribute of the voltage controlled adjustable current source 300 is also reflected in the I-V characteristics of the voltage controlled adjustable current source 300 (FIG. 4), where for V.sub.G1=1.25V the output conductance g.sub.OUT=I.sub.DS/V.sub.DS is seen to remain flat over the entire input voltage range V.sub.D2 above the knee voltage V.sub.K yet for an equivalent gate-source voltage in the prior art CR diode 202 (solid line in FIG. 4) the output conductance is not as flat and varies to a greater extent over equivalent values of input voltages V.sub.D.

(11) A final attribute that distinguishes the voltage controlled adjustable current source 300 over prior art CR diode approaches is that the knee voltages V.sub.K for the various values of control voltages V.sub.G1 that may be applied to it are lower. This attribute is desirable since for a given current-setting control voltage V.sub.G1 and equivalent V.sub.GS in the prior art CR diode 200, the voltage controlled adjustable current source 300 is able to maintain regulation over a wider voltage range V.sub.D2.

(12) The present invention operates as a transconductance (g.sub.m) circuit with g.sub.m=dI.sub.D2/dV.sub.G1. To the extent of the bandwidth available using the implemented transistors, varying the control voltage V.sub.G1 results in corresponding variation in I.sub.D2. Thus the precision control of load current in the present invention has an available dynamic characteristic. For example, this present invention can be an input for a trans-resistance amplifier (e.g. a common-gate amplifier).

(13) While various embodiments of the present invention have been presented, they have been presented by way of example and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made to the exemplary embodiments without departing from the true spirit and scope of the invention. Accordingly, the scope of the invention should not be limited by the specifics of the exemplary embodiments of the invention but, instead, should be determined by the appended claims, including the full scope of equivalents to which such claims are entitled.