A MOSFET TEMPERATURE COMPENSATION CURRENT

SOURCE

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No.

60/660,728, filed March 11, 2005, entitled "A MOSFET Temperature Compensation Current Source."

FIELD OF THE INVENTION

The present invention is directed towards the field of temperature compensation

current sources.

BACKGROUND OF THE INVENTION

Current sources are components commonly used in metal oxide semiconductor

field effect transistors (MOSFETs). These current sources can be sensitive to temperature or process variations and produce an unstable and variable output current across a range of temperatures and process variations. Typically, current sources are designed to provide

a constant current across temperature or process variations using junction diodes. Figure

1 shows a conventional current source 100 implementing a junction diode 105 to achieve

band gap voltage under full complementary metal oxide semiconductor (CMOS) processes. Under CMOS processes, however, junction diodes require large areas on a circuit.

Therefore, there is a need for a constant current source that does not vary

significantly with temperature or process variations that requires less area on a circuit.

SUMMARY OF THE INVENTION

Some embodiments provide a current source that generates a constant current

using metal oxide semiconductor (MOS) transistors. A biasing circuit generates a biasing

current based on the transconductance of the MOS transistors. A load circuit generates a

load current from the biasing current. In one embodiment, the load circuit comprises a resistor coupled in parallel with a MOS device (e.g., MOS transistor) having a resistance

(R _MOS ). The load current comprises the sum of a resistor current, which flows through the

resistor, and a MOS current that flows through the MOS device.

The biasing current generated by the biasing circuit is constant except under temperature or process variations which may cause the biasing current to increase or decrease (due to the effect of temperature or process variations upon the transconductance

of the MOS transistors). An increase or decrease in the biasing current (caused by

temperature or process variations) produces an increase or decrease, respectively, in the

resistor current that flows through the resistor of the load circuit. Temperature or process

variations, however, also have an effect on the resistance value (R _MOS ) of the MOS device of the load circuit and cause the resistance value R _MOS to increase or decrease. The

changes in the resistance value R _MOS also causes a change in the MOS current that flows

through the MOS device, whereby the change in MOS current offsets the change in the

resistor current to produce a relatively constant load current.

For example, if temperature or process variations cause the biasing current to increase, the resistor current through the resistor of the load circuit increases. These

temperature or process variations, however, also cause the resistance R _MOS of the MOS device of the load circuit to increase, thereby causing the MOS current through the MOS

device to decrease. Therefore, the decrease in the MOS current approximately offsets the increase in the resistor current to produce a relatively constant load current of the load

circuit. Likewise, if temperature or process variations cause the biasing current to decrease, the resistor current through the resistor of the load circuit decreases. These

temperature or process variations, however, also cause the resistance RM _O S of the MOS

device of the load circuit to decrease, thereby causing the MOS current through the MOS

device to increase. Therefore, the increase in the MOS current approximately offsets the

decrease in the resistor current to produce a relatively constant load current of the load circuit. As such, the changes in the biasing current due to temperature or process

variations is, in effect, approximately offset by a change in the MOS current (of the MOS

device of the load circuit) due to the same temperature or process variations. Thus, the load current is relatively constant because variations of the biasing current and the resistor current are offset by variations of the MOS current across

temperature or process variations. An output current of the current source is also equal to

a sum of the resistor current and the MOS current. In one embodiment, the output current is output from a current mirror circuit that mirrors the load current as the output current.

In some embodiments, the current source produces a stable output current (I _out ) that does not vary significantly across temperatures and process variations, the current source being

implemented without use of a junction diode.

In one embodiment, the biasing circuit comprises first and second transistor pairs.

The first transistor pair generates a first transconductance between a reference voltage and the load circuit, and the second transistor pair generates a second transconductance

between the reference voltage and ground. For this embodiment, the first transistor pair

comprises a size greater than the second transistor pair (e.g., four times the size)

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a conventional current source implementing a junction diode.

Figure 2 is a block diagram illustrating a constant current source of some

embodiments.

Figure 3 is a block diagram illustrating detail of a constant current source of some

embodiments.

Figure 4 is a flowchart showing a method for generating a constant current using metal oxide semiconductor (MOS) transistors.

Figure 5 is a circuit analysis diagram of a non-constant current source.

Figure 6 is a circuit analysis diagram of the constant current source of Figure 3.

Figure 7 is a graph that shows the output currents of a constant current source and

a non-constant current source as a function of the transconductance parameter KP.

DETAILED DESCRIPTION

The disclosure of U.S. Provisional Patent Application No. 60/660,728, filed March 11, 2005, entitled "A MOSFET Temperature Compensation Current Source," is hereby expressly incorporated herein by reference.

Although the present invention is described below in terms of specific exemplary

embodiments, one skilled in the art will realize that various modifications and alterations may be made to the below embodiments without departing from the spirit and scope of

the invention. In the discussion below, Section I describes a constant current source and a

method for generating a constant current. Section II describes characteristics of a non-

constant current source and a constant current source. And Section IH discusses output currents of current sources as a function of the transconductance parameter.

I. Constant Current Source

Figure 2 is a block diagram illustrating a constant current source 200 of some

embodiments. The constant current source 200 comprises a biasing circuit 205, a load

circuit 210, and a current mirror circuit 215. The biasing circuit 205 comprises a plurality

of metal oxide semiconductor (MOS) transistors for generating a biasing current based on transconductance of the MOS transistors.

The load circuit 210 is coupled to the biasing circuit 205 and comprises at least

one resistor and at least one MOS device coupled in parallel with the at least one resistor.

As described below, the MOS device of the load circuit 210 is a MOS transistor. In other

embodiments, however, the MOS device is any other type of metal oxide semiconductor device. The load circuit 210 receives the biasing current from the biasing circuit 205 and

generates a constant load current from the biasing current. In particular, the load circuit

210 generates a resistor current (I _r ) that flows through the resistor and a MOS current (I _t )

that flows through the MOS transistor, the load current being the sum of a resistor current

and the MOS current. The load current is relatively constant because variations of the resistor current are offset by variations of the MOS current across temperature or process variations (i.e., the resistor and MOS currents balance each other to produce a constant

load current).

The current mirror circuit 215 is coupled to the biasing circuit 205 and generates

an output current (I _0Ut ) equal to a sum of the resistor current (I _r ) and the MOS current (I _t ). The current mirror circuit 215 mirrors the load current of the load circuit 210 as the output

current (I _out ). In some embodiments, the current source 200 produces a stable output

current (I _out ) that does not vary significantly across temperatures and process variations, the current source being implemented without use of a junction diode.

If the current source 200 is to produce a constant output current of value A, the

first current has a value of B, and the second current has a value of C, then A = B + C. If

the first current changes in value, the second current balances the first current so that the

output current is still approximately equal to A. For example, if temperature or processing

variations cause the first current to increase in value to equal (B + delta), the value of the

second current would thereby decrease in value to approximately equal (C - delta) so that the sum of the first and second currents is still approximately equal to A. Conversely, if

temperature or processing variations cause the first current to decrease in value to equal

(B - delta), the value of the second current would thereby increase in value to

approximately equal (C + delta) so that the sum of the first and second currents is still approximately equal to A.

Figure 3 is a block diagram illustrating detail of a constant current source 300 of

some embodiments. The constant current source 200 comprises a biasing circuit 205, a

load circuit 210, and a current mirror circuit 215.

The biasing circuit 205 comprises a plurality of metal oxide semiconductor (MOS)

transistors Q1 (305), Q2 (310), Q3 (315), and Q4 (320). As shown in Figure 3, a gate of the transistor Q3 is coupled to a drain of transistor Q3, a source of transistor Q3 is

coupled to a gate of transistor Q1, and a source of transistor Q1 is coupled to the load

circuit, a drain of transistor Q2 is coupled to the source of transistor Q3, a source of transistor Q2 is coupled to ground, a gate of the transistor Q2 is coupled to a source of

transistor Q4 and to the drain of transistor Q1, and the gate of the transistor Q3 is coupled to the gate of transistor Q4.

The biasing circuit 205 receives a starting current (I _sta ) and generates a regulated biasing current based on transconductance of the MOS transistors Q1 (305), Q2 (310), Q3

(315), and Q4 (320). In particular, the biasing circuit 205 comprises a first transistor pair Q1 (305) and Q3 (315) and a second transistor pair Q2 (310) and Q4 (320). The first

transistor pair Q1 (305) and Q3 (315) generates a first transconductance between a

reference voltage and the load circuit 210. The second transistor pair Q2 (310) and Q4

(320) generates a second transconductance between the reference voltage and ground.

The load circuit 210 is coupled to the biasing circuit 205 and comprises a resistor 325 coupled in parallel with a MOS transistor Q8 (330). The load circuit 210 receives the

biasing current from the biasing circuit 205 and generates a constant load current from the

biasing current. The load circuit 210 generates a resistor current (I _r ) through the resistor

325 and a MOS current (I _t ) through the MOS transistor 330, whereby load current equals

resistor current (I _r ) + MOS current (I _t ).

The biasing current generated by the biasing circuit 205 is constant except under

temperature or process variations which may cause the biasing current to increase or

decrease (due to the effect of temperature or process variations upon the transconductance

of the MOS transistors Q1 to Q4 of the biasing circuit 205). An increase or decrease in the biasing current (caused by temperature or process variations) produces an increase or decrease, respectively, in the resistor current (I _r ) through the resistor 325 of the load

circuit 210. Temperature or process variations, however, also have an effect on the

resistance value (R _MOS ) of the MOS device 330 of the load circuit 210 and cause the

resistance value R _MOS to increase or decrease. The changes in the resistance value R _MOS also causes a change in the MOS current (I _t ) that flows through the MOS device 330, whereby the change in MOS current (I _t ) offsets the change in the resistor current (I _r ) to

produce a relatively constant load current.

For example, if temperature or process variations cause the biasing current to

increase, the resistor current (I _r ) through the resistor 325 of the load circuit 210 increases. These temperature or process variations, however, also cause the resistance R _MOS of the

MOS device 330 of the load circuit 210 to increase, thereby causing the MOS current (I _t )

through the MOS device 330 to decrease. Therefore, the decrease in the MOS current (I _t )

offsets the increase in the resistor current (I _r ) to produce a relatively constant load current

of the load circuit 210. Likewise, if temperature or process variations cause the biasing current to decrease, the resistor current (I _r ) through the resistor 325 of the load circuit decreases. These temperature or process variations, however, also cause the resistance

R _MOS of the MOS device 330 of the load circuit to decrease, thereby causing the MOS

current (It) through the MOS device to increase. Therefore, the increase in the MOS

current (I _t ) offsets the decrease in the resistor current (I _r ) to produce a relatively constant load current of the load circuit. As such, the changes in the biasing current due to

temperature or process variations is, in effect, approximately offset by a change in the

MOS current (of the MOS device of the load circuit) due to the same temperature or

process variations. Thus, the load current is relatively constant because variations of the

biasing current and resistor current (I _r ) are offset by variations of the MOS current (I _t ) across temperature or process variations.

As shown in Figure 3, the voltage drop through transistors Q1 and Q3 and the

resistor 325 is equal to the voltage drop through transistors Q2 and Q4. This can be shown

by the following observations:

• the voltage at the gate of transistor Q3 is equal to the voltage at the gate of

transistor Q4 (indicated by point V in Figure 3);

• the gate to source voltage of Q3 (V _GS3 ) plus the gate to source voltage of Q1

(V _GS1 ) plus the voltage across the resistor 325 (I _r *R) is equal to the gate to

source voltage of Q4 (V _GS4 ) plus the gate to source voltage of Q2 (V _GS2 ) SO that:

• the drain to source voltage of Q8 (V _DSS ) is equal to the voltage across the resistor 325 (I _r *R) so that:

As such, in some embodiments, the first transistor pair Q1 (305) and Q3 (315)

comprises transistors that are larger in size than the transistors of the second transistor pair Q2 (310) and Q4 (320) to accommodate the additional voltage drop (I _T *R) across the

resistor 325 (which lies in the voltage drop path of transistors Q1 and Q3). In one

embodiment, the size of transistors Q1 and Q3 are four times the size of transistors Q2 and Q4.

The current mirror circuit 215 is coupled to the biasing circuit 205 and comprises

a plurality of MOS transistors Q5 (335), Q6 (340), and Q7 (345). As shown in Figure 3,

transistor Q5 is coupled to transistor Q3 and mirrors current flowing through transistors

Q3 and Q2; transistor Q6 is coupled to transistors Q5 and Q4 and mirrors current flowing through transistors Q4 and Q1; and transistor Q7 is coupled to transistor Q6 and mirrors current flowing through transistor Q6 and sources the output current. The current mirror

circuit 215 further comprises a resistor that couples the source of transistor Q7 to ground.

The current mirror circuit 215 mirrors the load current of the load circuit 210 to generate

an output current (I _out )- The generated output current (I _out ) is equal to a sum of the resistor current (I _r ) across the resistor 325 and the MOS current (I _t ) across transistor Q8 (330) of

the load circuit 210 so that:

Figure 4 is a flowchart showing a method 400 for generating a constant current

using metal oxide semiconductor (MOS) transistors. The method 400 begins by generating (at 405) a biasing current (e.g., using a biasing circuit comprising a plurality of MOS transistors), the biasing current being generated based on a transconductance of at

least one MOS transistor. For example, the method may generate the biasing current by

generating a first transconductance between a reference voltage and a load circuit and

generating a second transconductance between the reference voltage and ground.

The method then generates (at 410) a constant load current from the biasing

current (e.g., using a load circuit comprising a resistor and a MOS transistor in parallel),

the load current comprising a resistor current and a MOS current, wherein variations of

the resistor current are offset by variations of the MOS current across temperature or process variations. The method then generates (at 415) an output current equal to a sum of

the resistor current and the MOS current, the output current being approximately constant

in value across temperature or process variations.

II. Current Source Characteristics

Characteristics of the constant current source can be better understood by first examining the characteristics of a non-constant current source that produces an output

current that varies significantly across temperature or processing variations. As known in the art, a transconductance parameter (KP) of a current source has a high correlation to

temperature or processing variations and varies significantly along with temperature or processing variations. As such, the transconductance parameter of a current source is typically used to correlate variations in the output current of the current source to

variations in temperature or processing. Thus, a current source that produces significant

variations in its output current as the transconductance parameter of the current source

varies is also considered to produce significant variations in its output current with

temperature or processing variations (as discussed further below in relation to Figure 7).

Such a current source is sometimes referred to as a KP dependent current source.

Figure 5 is a circuit analysis diagram of a KP dependent current source 500 that

does not produce a constant current across temperature or processing variations. Note that

in comparison with the constant current source 300 of Figure 3, the KP dependent current

source 500 does not have a MOS device connected in parallel with resistor R so that there

is no offsetting MOS current. Thus the output current (I _out ) of the KP dependent current source 500 is equal to the current (I) across the resistor R. As discussed below in relation

to Figure 7, the current (I) across the resistor R varies significantly across temperature or

processing variations.

To determine temperature compensation (TC) as a function of output current (J _D )

for the KP dependent current source 500 of Figure 5:

Given:

k = gain of current source circuit;

W = width of gate channel;

L - length of gate channel;

N = transistor size multiplication factor between Q1 and Q2 and between Q4 and Q3 (e.g., if Q1 and Q4 are four times larger than Q2 and Q3 , N = 4);

KP = transconductance parameter (AJV ² );

I _D = drain current;

V _TH - threshold voltage;

V _GS = gate-source voltage;

V _DS = drain-source voltage; and

T= temperature,

then:

As such, temperature compensation (TC) as a function of output current (I _D ) can

be represented by the equation:

where:

α = the temperature compensation value at resistor R.

To determine the value of the output current (I _D ):

where:

then:

For example:

Other characteristics of the MOSFET KP dependent current source 500 of Figure 5 are

shown by the following equations:

US2006/008633

thus:

Figure 6 is a circuit analysis diagram of the constant current source 300 of Figure

3. Characteristics of the constant current source 300 are shown by the following

equations:

Given:

then:

when (/ _r = I _t ) :

III. Output Current as a Function of the Transconduεtance Parameter KP

As discussed above, the transconductance parameter KP of a current source varies

significantly with temperature or processing variations and can be used to correlate the

output current of the current source to variations in temperature or processing. A current

source that produces significant variations in its output current as the transconductance parameter KP of the current source varies is also considered to produce significant

variations in its output current with temperature or processing variations.

Figure 7 is a graph that shows the output currents of a constant current source and a non-constant current source as a function of the transconductance parameter KP of the current sources across a range of KP values from a lower boundary to an upper boundary

(assuming that temperature or process variations of KP are +/- 30%). A first graph line

705 shows the output current of a non-constant current source as a function of the transconductance parameter KP of the current source. In particular, the first graph line 705

shows the output current of the KP dependent current source 500 of Figure 5 where the

output current is equal to the current (I _r ) across the resistor R. As shown in Figure 7, the

output current of the KP dependent current source 500 varies significantly across the

various values of KP. This indicates that the output current of the KP dependent current

source 500 would also vary significantly across temperature or process variations.

A second graph line 710 shows the output current of the constant current source

300 of Figure 3 as a function of the transconductance parameter KP of the current source,

whereby the output current is equal to a resistor current (I _r ) across a resistor R and a MOS

current (I _t ) across a transistor. As shown in Figure 7, the output current of the constant

current source 300 is relatively constant within various values of KP that reflect typical operating conditions (indicated by the dashed lines). This indicates that the output current

of the constant current source 300 would also be relatively constant across temperature or

process variations. This is due to the fact that as the value of the resistor current (I _r ) varies

over various values of KP, the values of the MOS current (I _t ) varies to offset the

variations of the resistor current (I _r ) to maintain a relatively constant output current (I _out )-

One of ordinary skill will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention, even though the

invention has been described with reference to numerous specific details. In view of the

foregoing, one of ordinary skill in the art would understand that the invention is not to be

limited by the foregoing illustrative details, but rather is to be defined by the appended

claims.