The present invention relates to the field of current mirror circuits, and more particularly relates to method and mechanism for reducing the current variation of the output of a current reference branch circuit used as a current mirror.
There is thus provided in accordance with the invention, a current reference branch circuit, comprising a main reference branch sub-circuit operative to supply a reference voltage to current source branches and an amplifier operative to reduce output voltage variation of the main reference branch sub-circuit.
There is also provided in accordance of the invention, a current reference branch circuit, comprising a main branch sub-circuit operative to supply a first reference voltage to current source branches and a replica branch sub-circuit operative to reduce output voltage variation of the main branch sub-circuit.
There is further provided in accordance of the invention, a method of reducing current variation in a current reference branch circuit, the current reference branch circuit comprising a metal oxide semiconductor (MOS) transistor connected to a second device via a diode connection, the method comprising the steps of sampling a reference voltage at the gate terminal of the MOS transistor, first feeding said sampled voltage to an input terminal of an amplifier and second feeding the output terminal of the amplifier to a bulk terminal of said MOS transistor.
There is also provided in accordance of the invention, a method of reducing current variation in a current reference branch circuit, the current reference branch circuit comprising a first metal oxide semiconductor (MOS) transistor connected to a terminal of a second device via a diode connection, the method comprising the steps of creating a replica reference branch of the current reference branch circuit, first feeding a reference voltage from the replica reference branch to an input of an amplifier and second feeding an output signal of the amplifier to a bulk terminal of the first MOS transistor.
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
The following notation is used throughout this document:
The present invention provides a feedback mechanism to reduce current variation observed in a current reference branch circuit by using body voltage control to compensate for variations in process, temperature and supply voltage. The current reference output voltage, which is proportional to the reference current, is sampled into a feedback loop, which controls the Field Effect Transistor (FET) body voltage.
The method and mechanism of the present invention uses Corner Robust Current Reference (CRCR) in order to keep the design simple and diminish variation between Process Voltage Temperature (PVT) corners. This method exhibits superior robustness with smaller variation in the magnitude of the reference current.
The present invention is operative to aid in the design of current reference branch circuits with robust current reference, and therefore are not sensitive to variations in PVT variations. (e.g., chaos circuits and other current mode circuits). In addition, current variations are reduced for analog integrated circuits (ICs) working under strict voltage budget, and having a reduced overdrive voltage.
The present invention introduces two embodiments (replica CBD and feedback CBD) for PVT Corner Robust Current Reference. These current reference designs use body voltage control to compensate for variations in process, temperature and supply voltage. These designs also exhibit superior robustness with smaller variation in the reference current magnitude, using Corner Robust Current Reference to keep the designs simple while diminishing variation between corners.
Both replica and feedback embodiments are implemented, using either pMOS devices implemented in a separate n-well in bulk technology (the complementary alternative is nMOS with triple well in bulk technology), or MOS in Silicon on Insulator (SOI) technology. In both cases the transistor is used as a body terminal which can have a body voltage not dependent on the supply voltage or any other device body voltage.
In a first embodiment of the present invention, a reference voltage of a replica circuit (i.e. of the current reference branch) is fed into an amplifier, whose output is fed into the bulk terminal of the transistor in the main current reference branch. A schematic diagram of an example circuit implementing the replica controlled body device method of the present invention is shown in
The replica circuit 16 comprises a simple current reference branch, which is used as a replica for the main current reference branch. The reference voltage Vref1 (i.e. the voltage at the diode connection) of this branch is fed into the operational amplifier. Therefore, the variation in reference voltage of the simple current reference branch due to process threshold voltage variation is amplified at the amplifier output. In turn, the amplifier output voltage controls the body current of transistor 18, a component of the main current reference branch 12.
In the main current reference branch 12, if the threshold voltage VT is increased, the current Iref1 in the branch decreases, and as a consequence the reference voltage Vref2 drops. The transistor threshold voltage VT depends on the difference between source and bulk voltage according to:
|VT|=|VT0|+γ(√{square root over (|VSB+2φ|)}−√{square root over (|2φ|)}) (1)
where γ is the body effect coefficient, 2φ is the intrinsic silicon bend banding constant (negative for pMOS and positive for nMOS), VSB is the source to body voltage, and VTO is the threshold voltage at VSB=0. We can use this dependence to manipulate the effective threshold voltage value VT.
Consider the case of an increase in the absolute value of threshold voltage |VT0| due to process variations. This will cause a decrease in the reference voltage Vref1 of the replica branch 16, and a decrease in the amplifier output voltage, which is also connected to the body voltage of the of transistor 18. Looking at Equation (1) for transistor 18, the first term, |VT0|, is increased due to process variations, however the second term, γ(√{square root over (|VSB+2φ|)}−√{square root over (|2φ|)}), decreases due to the change in body voltage and compensates the increase in the first term, thus having smaller change in the effective threshold voltage. Therefore, the current Iref2 in the main current reference branch 12 will have less sensitivity to any variation in the threshold voltage, VT0, as opposed to current Iref1 in the replica reference branch 16 or the current Iref2 in the main reference branch 12 in the case where both source and body terminals of transistor 18 are connected to Vdd (i.e. a current reference circuit with no feedback from amplifier 14).
In a second embodiment of the present invention, the reference voltage of the current reference branch circuit is fed into an amplifier, whose output is fed back into the bulk terminal of the transistor in the current reference branch. A schematic diagram of an example circuit implementing the replica controlled body device method of the present invention is shown in
A graph showing reference current as a function of threshold voltage variations for example circuits implementing the replica branch and feedback methods of the present invention is shown in
The graph 50 shows that the current variations are about 9% for the replica method, as opposed to 32% in the simple reference branch. The graph also shows that the current variation is about 18% for the feedback method.
A graph showing reference current for PVT corner combinations in example circuits implementing the method of the present invention is shown in
A flow diagram illustrating the replica branch current variation reduction method of the present invention is shown in
A flow diagram illustrating the feedback current variation reduction method of the present invention is shown in
The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
It is intended that the appended claims cover all such features and advantages of the invention that fall within the spirit and scope of the present invention. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the invention not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present invention.
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