MASTER BIAS CURRENT GENERATING CIRCUIT WITH DECREASED SENSITIVITY TO SILICON PROCESS VARIATION

Abstract

A master bias current generating circuit includes a current source, a first reference leg, and a second reference leg. The first reference leg includes a first transistor having a first size parameter coupled to the current source and a first diode having a second size parameter coupled to the first transistor. The second reference leg includes a second transistor having a third size parameter less than the first size parameter coupled to the current source and a second diode having a fourth size parameter greater than the second size parameter coupled to the second transistor.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:

FIG. 1 is a circuit diagram of a prior art master bias current generator;

FIG. 2 is a simplified block diagram of a mixed-signal integrated circuit device in accordance with one illustrative embodiment of the present invention;

FIG. 3 is a circuit diagram of a master bias current generator that may be used in the device of FIG. 2; and

FIG. 4 is a circuit diagram of an alternative embodiment of a master bias current generator that may be used in the device of FIG. 2.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.”

The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to FIG. 2, the present invention shall be described in the context of an integrated circuit device 200. In the illustrated embodiment, the integrated circuit device 200 is a mixed-signal device including digital blocks 205 and analog blocks 210 formed on a common substrate. A master bias current generator 215 is provided for generating a master bias current signal for use by the analog blocks 210. Exemplary analog components that may use the master bias current include operational amplifiers, digital-to-analog converters, and analog-to-digital converters, oscillators, buffers, etc. The particular size and layout of the digital blocks 205 and analog blocks 210 are provided for illustrative purposes only. An actual implementation may include different circuit arrangements. Exemplary mixed-signal analog-digital devices include imaging devices, power management devices, biomedical devices, and many others.

Turning now to FIG. 3, a circuit diagram of the master bias current generator 215 is provided. For clarity and ease of illustration, only the current generation portion of the circuit is illustrated. Any number of replicating/scaling legs (i.e., as illustrated in the prior art circuit of FIG. 1) may be used, but these are omitted to avoid obscuring the present invention. The master bias current generator 215 includes a current source 300 incorporating PMOS field effect transistors 305, 310, 315. NMOS field effect transistors 320, 325, 330 and PNP bipolar transistors 335, 340, 345 are also provided. The transistors 320, 335 define a first reference leg 350, the transistors 325, 340 define a second reference leg 355, and the transistors 330, 345 define a bias leg 360. An input voltage, V_AA, is provided at an input voltage terminal 365. The application of the present invention is not limited to the particular circuit element types shown. For example, the dopant-type of transistors may be changed or diodes may be used in place of the diode-connected transistors. The master bias current generator circuit 215 may be implemented using an NMOS current source, PMOS reference leg transistors, or NPN bipolar transistors.

The PMOS transistor 305 is diode-connected and the gates of the PMOS transistors 305, 310 are coupled to one another to form the current source 300. The gate-source voltage, V_GS, of the PMOS transistor 305 equals its drain-source voltage, V_DS, and the V_GS of the PMOS transistors 305, 310 are equal. The gate of the PMOS transistor 315 is coupled to the drain of the PMOS transistor 310, so that the PMOS transistor 315 may operate as a feedback device that forces the V_DS of the PMOS transistors 305, 310 to be equal to eliminate current imbalance and cancel the effects of the Early voltage, V_A.

The NMOS transistors 320, 325 form a voltage loop with the PNP transistors 335, 340 to generate the reference current, I_REF. In the illustrated embodiment the reference currents are equal in both legs of the master bias current generator 215 due to the transistors 305, 310 in the current source 300 having the same aspect ratios (W/L). The current in the reference legs 350, 355 may be scaled with respect to one another by varying the aspect ratios of the transistors 305, 310. The NMOS transistor 330 biases the gate voltage of the transistors with a voltage equal to the emitter-base voltage, V_EB, of the PNP transistor 345 plus the V_GS of the NMOS transistor 330.

The emitter area of the transistor 340 is a multiple (M×A) of the emitter area (A) of the transistor 335. The aspect ratios of the NMOS transistors 320, 325 are also multiples of one another. For example, if the aspect ratio of the transistor 325 is represented as W/L₁, the aspect ratio of the transistor 320 is K×(W/L₁). The transistors 330, 345 in the bias leg 360 are sized to match the corresponding pair in one of the reference legs 350, 355 (e.g., W/L₁ and M×A or K×(W/L₁) and A).

Generally, the master bias current generator 215 generates different emitter-base voltages, V_EB, for the PNP transistors 335, 340 due to the different emitter areas and causes that voltage difference to drop across the V_GS of the NMOS transistors 320, 325 having different aspect ratios. In the following equations, the NMOS transistors 320, 325 are referred to as M1 and M2, respectively, and the PNP transistors 335, 340 are referred to as Q1 and Q2, respectively. To examine how the reference current, I_REF, is set by the master bias current generator 215, consider the voltage loop equation arising from Kirchoff's Voltage Law (KVL) as follows:

V _GS(M2)+V_EB(Q2)=V_GS(M1)+V_EB(Q1)  (2)

In Equation (2), the emitter-base voltage of the PNP transistors 35, 340 and gate-source voltage of the NMOS transistors 320, 325 may be defined as follows:

$\begin{array}{cc}{V}_{\mathrm{EB}}=V_T\mathrm{Ln}\left(\frac{I_E}{J_S\times M\times A}\right)& \left(3\right)\end{array}$

where

V_EB=Emitter-Base voltage of PNP Bipolar Transistor (Volt)

V_T=Thermal Voltage=kT/q=0.0259 V at room Temperature (Volt)

I_E=Emitter current flowing into PNP emitter (A)

J_S=Emitter Current Density (A/m²)

M=Number of unit emitters in PNP

A=Area of one unit emitter in PNP (m²)

$\begin{array}{cc}{V}_{\mathrm{GS}}=V_{\mathrm{TH}}+\sqrt{\frac{2\times I_D}{{\mu }_NC_{\mathrm{OX}}\frac{W}{L}}}& \left(4\right)\end{array}$

where

V_GS=Gate-source voltage of NMOS Transistor (Volt)

V_TH=Threshold voltage of the NMOS transistor (Volt)

I_D=Drain (or Source) current of NMOS (A)

μ_N=Electron mobility in the channel of an NMOS transistor

C_OX=Gate oxide per unit area (F/m²)

W/L=Width/Length which is the aspect ratio of an MOS transistor

Expanding the KVL voltage loop equation (2) results in:

$\begin{array}{cc}{V}_{\mathrm{TH}}+\sqrt{\frac{2\times I_{\mathrm{REF}}}{{\mu }_NC_{\mathrm{OX}}\frac{W}{L}}}+V_T\mathrm{Ln}\left(\frac{I_{\mathrm{REF}}}{J_S\times M\times A}\right)=V_{\mathrm{TH}}+\sqrt{\frac{2\times I_{\mathrm{REF}}}{{\mu }_NC_{\mathrm{OX}}K\frac{W}{L}}}+V_T\mathrm{Ln}\left(\frac{I_{\mathrm{REF}}}{J_S\times A}\right)& \left(5\right)\end{array}$

where V_TH is the threshold voltage of the NMOS transistors 320, 325 (M1 and M2), and V_T is the thermal voltage.

To find a closed form solution, Equation (5) may be solved for reference current, I_REF:

$\begin{array}{cc}{I}_{\mathrm{REF}}=\frac{1}{2}{\mu }_NC_{\mathrm{OX}}\left(\frac{W}{L}\right)\times {\left[\frac{V_T\mathrm{Ln}\left(M\right)}{1-\frac{1}{\sqrt{K}}}\right]}^2& \left(6\right)\end{array}$

where μ_N is the electron mobility, C_OX is oxide capacitance per unit area; and W/L is the width/length ratio of the NMOS transistors 320, 325.

The factors M and K are integer multiples of the NMOS aspect ratio and the PNP emitter area, respectively. In the illustrated embodiment, M is 8 and K is 2; however both may vary depending on the particular implantation. For instance, M may be between 4 and 100. Generally, a larger value for M increases the accuracy of I_REF at the expense of increased chip real estate area. The values of M and K determine the amount of reference current. A larger current provides enhanced matching, but requires higher power consumption.

Both the M and K ratios may be defined precisely and their variations are typically very small in any semiconductor process. Also, in typical CMOS analog processes, the tolerance variation on oxide thickness C_OX, mobility μ_N, channel width W, and channel length L are usually well-controlled. Hence, the variability of the parameters in Equation (6) that define the value of the reference current is significantly less than the variability in the resistance parameter that defines the reference current in Equation (1) for the prior art circuit 100 of FIG. 1.

Turning now to FIG. 4, an alternative embodiment of a master bias current generator 400 is provided. In the master bias current generator 400, additional NMOS transistors 405, 410 are provided in the reference legs 350, 355 for cascading purposes. An additional PMOS transistor 415 is provided in the current source 300 for providing current to a second bias leg 420 is defined by an NMOS bias transistor 425 and an additional PNP transistor 430. Note that the aspect ratio of the bias transistor 425, W/L₂, is less than the aspect ratio W/L₃ of the bias transistor 330 to provide a higher DC voltage for biasing the transistors 405, 410. The cascoded NMOS transistors 320, 325405, 410 increase the effective resistance of the reference legs 350, 355, thereby improving power supply rejection performance. Hence, variations in the power supply voltage, V_AA, will cause less variation in the reference current.

The master bias current generator circuits 215, 400 described herein exhibit increased performance and reduced variability relative to the prior art circuit 100 of FIG. 1 that employs a resistor in its reference leg. Because the master bias current generator circuits 215, 400 do not rely on a resistor in generating the reference current, the variability in the reference current due to silicon process variation is reduced.

One aspect of the present invention is seen in a master bias current generating circuit including a current source, a first reference leg, and a second reference leg. The first reference leg includes a first transistor having a first size parameter coupled to the current source and a first diode having a second size parameter coupled to the first transistor. The second reference leg includes a second transistor having a third size parameter less than the first size parameter coupled to the current source and a second diode having a fourth size parameter greater than the second size parameter coupled to the second transistor.

Another aspect of the present invention is seen where the first reference leg includes a first field effect transistor having a first aspect ratio coupled to the current source and a first diode-connected bipolar transistor having a first emitter area coupled to the first field effect transistor. The second reference leg includes a second field effect transistor having a second aspect ratio less than the first aspect ratio coupled to the current source and a second diode-connected bipolar transistor having a second emitter area greater than the first emitter area coupled to the second field effect transistor.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

1. A master bias current generating circuit, comprising: a current source;a first reference leg including a first transistor having a first size parameter coupled to the current source and a first diode having a second size parameter coupled to the first transistor; anda second reference leg including a second transistor having a third size parameter less than the first size parameter coupled to the current source and a second diode having a fourth size parameter greater than the second size parameter coupled to the second transistor.

2. The circuit of claim 1, wherein the first and third size parameters comprise aspect ratio parameters.

3. The circuit of claim 1, wherein the first and second diodes comprise diode-connected bipolar transistors, and the second and fourth size parameters comprise emitter area parameters.

4. The circuit of claim 1, wherein the first size parameter is a first integer multiple of the third size parameter, and the fourth size parameter is a second integer multiple of the second size parameter.

5. The circuit of claim 1, wherein gate terminals of the first and second transistors are coupled to a bias voltage terminal.

6. The circuit of claim 1, further comprising a bias leg coupled to the current source and including a third diode coupled to the current source and a fourth diode coupled to the third diode, wherein the third diode is coupled to bias gate terminals of the first and second transistors.

7. A master bias current generating circuit, comprising: a current source;a first reference leg including a first field effect transistor having a first aspect ratio coupled to the current source and a first diode-connected bipolar transistor having a first emitter area coupled to the first field effect transistor; anda second reference leg including a second field effect transistor having a second aspect ratio less than the first aspect ratio coupled to the current source and a second diode-connected bipolar transistor having a second emitter area greater than the first emitter area coupled to the second field effect transistor.

8. The circuit of claim 7, wherein the first aspect ratio is a first integer multiple of the second aspect ratio, and the second emitter area is a second integer multiple of the first emitter area.

9. The circuit of claim 7, wherein gate terminals of the first and second field effect transistors are coupled to a bias voltage terminal.

10. The circuit of claim 7, wherein the current source comprises: a third diode-coupled field effect transistor coupled to the first field effect transistor; anda fourth field effect transistor coupled to the second field effect transistor, wherein gate terminals of the third diode-coupled field effect transistor and the fourth field effect transistor are coupled to one another.

11. The circuit of claim 7, further comprising a first bias leg coupled to the current source and including a third diode-connected field effect transistor coupled to the current source and a third diode-connected bipolar transistor coupled to the third field effect transistor, wherein a gate terminal of the third field effect transistor is coupled to gate terminals of the first and second field effect transistors.

12. The circuit of claim 11, wherein the current source comprises: a fourth diode-coupled field effect transistor coupled between an input voltage terminal and the first field effect transistor;a fifth field effect transistor coupled between the input voltage terminal and the second field effect transistor; anda sixth field effect transistor coupled between the input voltage terminal and the third diode-connected field effect transistor, wherein the gate terminals of the fourth diode-connected field effect transistor and the fifth field effect transistor are coupled to one another, and the gate terminal of the sixth field effect transistor is coupled to a drain terminal of the fifth field effect transistor.

13. The circuit of claim 11, wherein the first reference leg includes a fourth field effect transistor coupled between the current source and the first field effect transistor, the second reference leg includes a fifth field effect transistor coupled between the current source and the second field effect transistor, and the circuit further comprises a second bias leg coupled to the current source and including a sixth diode-connected field effect transistor coupled to the current source and a fourth diode-connected bipolar transistor coupled to the sixth field effect transistor, wherein a gate terminal of the sixth diode-coupled field effect transistor is coupled to gate terminals of the fourth and fifth field effect transistors.

14. The circuit of claim 13, wherein the sixth diode-coupled field effect transistor has an aspect ratio less than an aspect ratio of the third diode-coupled field effect transistor.

15. The circuit of claim 13, wherein the current source comprises: a seventh diode-coupled field effect transistor coupled between an input voltage terminal and the fourth field effect transistor;an eighth field effect transistor coupled between the input voltage terminal and the fifth field effect transistor;a ninth field effect transistor coupled between the input voltage terminal and the third diode-connected field effect transistor, wherein the gate terminals of the seventh diode-connected field effect transistor and the eighth field effect transistor are coupled to one another, and a gate terminal of the ninth field effect transistor is coupled to a drain terminal of the eighth field effect transistor; anda tenth field effect transistor coupled between the input voltage terminal and the sixth diode-connected field effect transistor, wherein a gate terminal of the tenth field effect transistor is coupled to a drain terminal of the eighth field effect transistor.

16. A master bias current generating circuit, comprising: an input voltage terminal;a ground terminal;a first reference leg including: a first transistor coupled to the input voltage terminal, the first transistor being diode-coupled;a second transistor having a first aspect ratio coupled to the first transistor; anda first diode-connected bipolar transistor having a first emitter area coupled between the second transistor and the ground terminal;a second reference leg including: a third transistor coupled to the input voltage terminal and having a gate terminal coupled to a gate terminal of the first transistor;a fourth transistor having a second aspect less than the first aspect ratio coupled to the third transistor; anda second diode-connected bipolar transistor having a second emitter area greater than the first emitter area coupled between the fourth transistor and the ground terminal; anda first bias leg including: a fifth transistor coupled to the input voltage terminal and having a gate terminal coupled to a source terminal of the third transistor;a sixth transistor coupled to the fifth transistor, the sixth transistor being diode-connected and having a gate terminal coupled to gate terminals of the second and fourth transistors; anda third diode-connected bipolar transistor coupled between the sixth transistor and the ground terminal.

17. The circuit of claim 16, further comprising: a seventh transistor coupled between the first and second transistors;an eighth transistor coupled between the third and fourth transistors; anda second bias leg including: a ninth transistor coupled to the input voltage terminal and having a gate terminal coupled to a source terminal of the third transistor;a tenth transistor coupled to the ninth transistor, the tenth transistor being diode-connected and having a gate terminal coupled to gate terminals of the seventh and eighth transistors; anda fourth diode-connected bipolar transistor coupled between the tenth transistor and the ground terminal.

18. A device, comprising: an analog block; anda master bias current generating circuit coupled to the analog block, comprising: a current source;a first reference leg including a first transistor having a first size parameter coupled to the current source and a first diode having a second size parameter coupled to the first transistor; anda second reference leg including a second transistor having a third size parameter less than the first size parameter and a second diode having a fourth size parameter greater than the second size parameter coupled to the second transistor.

19. The circuit of claim 18, further comprising at least one digital block formed on a common substrate with the analog block.

20. The circuit of claim 18, wherein the first aspect ratio is a first integer multiple of the second aspect ratio, and the second emitter area is a second integer multiple of the first emitter area.