Low temperature coefficient voltage output circuit and method

Abstract

A low temperature coefficient (TC) voltage output circuit compensates for the TC of a base circuit's output voltage with a compensation circuit that includes first and second current sources with different TCs. The differential between the current sources is applied to a voltage drop circuit that generates a temperature dependent compensation voltage with a TC of opposite polarity to the TC of the base circuit's output. To provide compensation over a desired temperature range, the two current sources are set equal to each other at one temperature within the range, and the compensated voltage output is trimmed at another temperature within the range to the desired output value. The result is a compensated voltage, with a TC of opposite polarity to that of the base circuit's output, which combines with the base circuit output to yield a low TC compensated output. The compensation scheme is particularly useful for voltage references.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to low temperature coefficient voltage output circuits, and more particularly to providing temperature compensation to a base circuit which produces a preliminary output voltage having a temperature coefficient of known polarity.

2. Description of the Related Art

Voltage reference circuits provide a fixed voltage for purposes such as regulated power supplies and comparison with other voltages. It is important that the output voltage of the reference circuit be made temperature invariant, to the extent practical. This is typically done by measuring the reference's output voltage at high and low temperatures, and then trimming the circuit to compensate for changes in the output voltage between the two measurement temperatures.

In practice, multiple voltage references in integrated circuit (IC) form are typically provided on a single wafer, the voltage measurements at the high and low temperatures are taken at the wafer level, and the individual reference circuits are then trimmed to reduce their temperature dependency, also at the wafer level. Other circuits also require some form of temperature compensation to maintain a substantially temperature invariant voltage level. One example is the offset voltage for an operational amplifier.

There are several problems with this approach. First, the wafers are typically held on a chuck, and the chuck temperature is commonly directly sensed to obtain an indication of the wafer temperature. This requires that the chuck temperature at which the voltage is read be accurately known, which is difficult to do in a production environment. Furthermore, although the chuck temperature is generally close to that of the wafer, the two temperatures are usually not exactly equal, which leads to further inaccuracies. While the wafer temperature can be read directly, this is expensive to do. Also, it is necessary to keep track of which voltage and temperature measurements go with which die on a given wafer, and among the dies of different wafers. Again, this is difficult to do in a production environment.

SUMMARY OF THE INVENTION

The present invention seeks to provide a voltage output circuit with a low temperature coefficient (TC), and an associated temperature compensation method which eliminates the need for both accurate temperature measurement, and for keeping track of the different dies and wafers and their associate voltage and temperature measurements.

These goals are achieved with a voltage output circuit that includes a base circuit which produces a preliminary output voltage having a TC of one polarity, and a compensation circuit having a pair of current sources with different TCs that are connected to compensate for the TC of the base circuit's output voltage (Vo) so as to yield a compensated Vo with a lower TC than the preliminary Vo TC. The differential between the currents of the two current sources is transformed to a voltage drop which has a TC of opposite polarity to the TC for the base circuit's Vo. The voltage drop is combined with the base circuit Vo to yield the compensated Vo.

In a preferred embodiment, the voltage drop circuit is implemented with a resistor connected to receive a current that varies with the differential between the two current sources. The resistor produces a compensation voltage with a TC that approximates, but is of opposite polarity to, the preliminary Vo TC.

To provide a compensated Vo over a predetermined temperature range, the current sources are first trimmed at one temperature level within the range, preferably at one end of the range, so that they are substantially equal in absolute value and thereby cancel, producing zero compensation voltage. Then, at another temperature within the range, preferably at the opposite end of the range, the voltage drop resistor is trimmed to set the compensated Vo at the desired value.

One of the current sources can have a positive TC and the other a negative TC, or the current sources can both have positive or negative TCs, so long as the TC of one source differs from that of the other in a direction and by an amount that produces a compensation voltage of the desired polarity and magnitude. In the case of a voltage reference circuit, the compensated Vo is amplified by an output buffer amplifier. The amplifier is trimmed to produce a desired reference voltage at the first temperature setting, at which the current sources are set equal, and continues to provide the same or a close reference voltage as the temperature varies.

These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a voltage reference circuit which incorporates the invention;

FIG. 2 is a flow diagram of a procedure for establishing a correct TC compensation level;

FIGS. 3 ( a ), 3 ( b ) and 3 ( c ) are vector diagrams illustrating the nature of the temperature compensation performed first at a low temperature and then at a higher temperature to obtain a compensation voltage with a positive TC, for three different combinations of current source TC polarities;

FIGS. 4 ( a ), 4 ( b ) and 4 ( c ) are vector diagrams illustrating the nature of the temperature compensation performed first at a low temperature and then at a higher temperature to obtain a compensation voltage with a negative TC, for three different combinations of current source TC polarities;

FIGS. 5 ( a ), 5 ( b ) and 5 ( c ) are vector diagrams illustrating the nature of the temperature compensation performed first at a high temperature and then at a lower temperature to obtain a compensation voltage with a positive TC, for three different combinations of current source TC polarities;

FIGS. 6 ( a ), 6 ( b ) and 6 ( c ) are vector diagrams illustrating the nature of the temperature compensation performed first at a high temperature and then at a lower temperature to obtain a compensation voltage with a negative TC, for three different combinations of current source TC polarities; and

FIGS. 7 and 8 are schematic diagrams of known current sources having positive and negative TCs, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The invention uses the difference in current from two current sources having different TCs to compensate for temperature induced changes in a voltage output from a base circuit. FIG. 1 illustrates the invention as applied to a voltage reference circuit 2 , which produces an uncompensated Vo at a preliminary output terminal 4 . Several different kinds of voltage reference circuits are in common use, such as buried zener, band-gap and MOS (IGFET) references. These types of circuits are well known and are described, for example, in Fink and Christiansen, ed., Electronics Engineers' Handbook, third edition, McGraw-Hill, 1989, pages 8-47 to 8-52.

The voltage reference 2 produces a preliminary Vo at terminal 4 with a TC of known polarity. For example, a buried zener reference usually has a positive TC, while a band-gap reference can be given either a positive or a negative TC by resistor scaling. After temperature compensation by the invention in the manner described below, the reference voltage Vo is applied to an operational amplifier 6 which includes a gain setting feedback circuit consisting of resistors R 2 and R 3 , with R 2 connected between the op amp output terminal 8 and its inverting input, and R 3 connected between the inverting input and ground. The amplifier's gain is set by the usual formula (R 2 +R 3 )/R 3 . One or both of R 2 and R 3 are made trimmable so that the gain can be adjusted.

The temperature compensation circuit consists of a pair of current sources I 1 and I 2 , at least one of which is trimmable, connected in series between two voltage supply buses, illustrated as V+ and ground. The node 10 between the two current sources establishes a compensated Vo terminal, at which a TC compensated Vo from the voltage reference is provided and fed into the non-inverting input of op amp 6 . A voltage drop circuit, preferably implemented as a trimmable resistor R 1 , is connected between preliminary and compensated Vo terminals 4 and 10 . The current sources I 1 and I 2 are oriented so that I 1 delivers current to, and I 2 draws current from, the compensated Vo terminal 10 . One of the current sources has a TC that is more positive than the TC for the other current source; whether this is I 1 or I 2 depends upon whether the voltage reference 2 produces a preliminary Vo with a positive or a negative TC.

A voltage drop circuit, preferably in the form of a trimmable resistor R 1 , is connected between preliminary and compensated Vo terminals 4 and 10 to provide the desired TC compensation. When I 2 exceeds I 1 , the current differential is drawn through R 1 from preliminary Vo terminal 4 to compensated Vo terminal 10 , in the direction indicated by arrow 12 for the compensation current Ic, and a compensation voltage drop Vc is produced from preliminary Vo terminal 4 to compensated Vo terminal 10 . This causes the voltage at terminal 10 to be less than that at terminal 4 . Conversely, when I 1 exceeds I 2 , Ic flows in the opposite direction through R 1 , from terminal 10 to terminal 4 . The Vc drop is also from terminal 10 to terminal 4 , causing the voltage at terminal 10 to exceed that at terminal 4 . In one particular implementation to compensate a voltage reference having a TC of about +125 ppm/°C., I 1 and I 2 had nominal room temperature values of 15 microamperes and 7.5 microamperes, respectively, both had a nominal value of 10 microamperes at 125° C., and R 1 had a trimming range of 500-1,000 ohms.

FIG. 2 is a block diagram illustrating the sequence used to establish a TC compensated Vo over a desired temperature range, typically 25° C.-125° C. for voltage references. At one temperature T 1 within the range, preferably at one end of the range, I 1 and/or I 2 is trimmed so that I 1 and I 2 are equal. The current flowing into compensated Vo terminal 10 from I 1 is therefore balanced by the current flowing out of the terminal to I 2 , and both Ic and Vc equal zero (block 14 ). Then, for the case for a voltage reference in which the ultimate output voltage is the output of op amp 6 , rather than the compensated Vo at terminal 10 , feedback resistor R 2 or R 3 is trimmed at temperature T 1 to set the op amp output voltage at terminal 8 at the desired reference voltage value (block 16 ). The temperature is then changed to a difference level T 2 within the desired range, preferably at the opposite end of the range from T 1 . At T 2 , compensation resistor R 1 is trimmed to set the compensated Vo at terminal 10 so that the op amp Vo is at the same desired level as that established for T 1 (block 18 ). However, since zero current flows through R 1 and Vc is zero at T 1 , the change in R 1 due to trimming the resistor does not change the compensated Vo at terminal 10 from its value at T 2 . Thus, the same compensated value of Vo is established at both ends of the desired temperature range.

This setup process is performed at the wafer level. At no time during the process is it necessary to know the exact temperature, since the precise value of neither T 1 or T 2 is critical. An indication of the wafer temperature can therefore be obtained inexpensively by sensing the temperature of the wafer support chuck.

To the extent the voltage reference TC is linear, the compensated Vo will remain at the same desired value for all intermediate temperatures between T 1 and T 2 . In practice, a voltage reference's output will typically exhibit a slight “bow” between two different temperatures, rather than a perfectly linear TC.

The compensation process can be performed in numerous ways. FIG. 3 ( a ) illustrates a procedure that can be used to compensate for a negative TC voltage reference. For purposes of this invention, a “negative” TC is one at which the absolute value of a parameter decreases with increasing temperature, while a “positive” TC is one at which the absolute value of the parameter increases with increasing temperature. All values of I 1 and I 2 in FIGS. 3 ( a )- 6 ( c ) are assumed to be positive, regardless of the sign of their TCs.

I 1 and I 2 are set equal to each other at a low temperature T 1 within the desired range; both I 1 and I 2 have positive TCs, with the I 1 TC more positive than I 2 . I 1 will therefore exceed I 2 at the higher temperature T 2 at which R 1 is trimmed to set the compensated Vo at the desired value. At T 2 Ic flows through R 1 from right to left (in FIG. 1 ), indicated by a downward directed arrow in FIG. 3 ( a ). This produces a Vc that causes the compensate Vo at terminal 10 to exceed the preliminary Vo at terminal 4 . Since I 1 increases with temperature more rapidly than I 2 , Vc has a positive TC to compensate the voltage reference's negative TC.

The procedure illustrated in FIG. 3 ( b ) is similar to FIG. 3 ( a ), except the TC for I 2 is now negative while the TC for I 1 remains positive but at a lower level. Ic has the same magnitude and polarity as in FIG. 3 ( a ), resulting in the same value of compensated Vo.

In FIG. 3 ( c ) both I 1 and I 2 have negative TCs, but again the TC for I 1 is more positive than for I 2 , and by the same amount as before. Ic will therefore have the same magnitude and polarity as in FIGS. 3 ( a ) and 3 ( b ), resulting in the same value of compensated Vo.

The procedure illustrated in FIGS. 4 ( a )- 4 ( c ) establishes a Vc with a negative TC, and is useful in compensating a positive TC voltage reference. I 1 and I 2 are again set equal to each other at a low temperature T 1 , and RI is trimmed to obtain the desired value for the compensated Vo at the higher temperature T 2 . In this case, however, I 2 has a TC that is more positive than the TC of I 1 , resulting in Ic flowing through R 1 from left to right (in FIG. 1 ) at T 2 . This produces a voltage drop across R 1 that causes the compensated Vo at terminal 10 to be less than the preliminary Vo at terminal 4 . The voltage reference output at terminal 4 will be equal to the desired Vo at T 1 and, since no current flows through R 1 at this temperature, the compensated Vo at terminal 10 will also be at the desired level. As the voltage reference preliminary Vo at terminal 4 progressively diverges from the desired value with increasing temperature, the Ic current through R 1 also progressively increases to provide an increasing level of compensation.

FIGS. 4 ( b ) and 4 ( c ) are similar to FIGS. 3 ( b ) and 3 ( c ), respectively, but again with I 2 having a more positive TC than I 1 . The result is an Ic of equal magnitude and polarity to FIG. 4 ( a ).

In FIG. 5 ( a ) the setup procedure is reversed, with I 1 and I 2 first set equal to each other at a high temperature T 1 within the desired range, and R 1 then trimmed to set the compensated Vo at the desired value at a lower temperature T 2 . (In this nomenclature T 1 is always the first temperature, at which I 1 is set equal to I 2 , and T 2 is always the second temperature at which R 1 is trimmed, regardless of which temperature is higher or lower.) In this case I 1 has a more positive TC than I 2 . The result is an Ic that flows from left to right through R 1 at T 2 , yielding a compensated Vo that is greater than the preliminary Vo from the voltage reference. This procedure is suitable for compensating a voltage reference with a negative TC.

In FIG. 5 ( a ) both I 1 and I 2 have positive TCs. FIGS. 5 ( b ) and 5 ( c ) illustrate the situation with I 1 having a positive and I 2 a negative TC, and both I 1 and I 2 having negative TCs, respectively. In both cases Ic has the same magnitude and polarity as in FIG. 5 ( a ) (assuming equal differences between the TCs of I 1 and I 2 in the three cases).

FIGS. 6 ( a ), 6 ( b ), and 6 ( c ) illustrate situations similar to FIGS. 5 ( a ), 5 ( b ) and 5 ( c ), respectively, but with I 2 having a higher TC than I 1 . This results in Ic having the same magnitude as in FIGS. 5 ( a )- 5 ( c ), but with a reversed polarity flowing through Rl from right to left at T 2 . This results in an increase in the compensated Vo level at terminal 10 relative to the preliminary Vo level at terminal 4 at the lower temperature T 2 , which then progressively reduces to a zero differential as the temperature increases back to T 1 . This negative TC for Vc is suitable for compensating a voltage reference having a positive TC.

Trimmable current sources with positive and negative TCs are well known. FIG. 7 illustrates one such circuit with a positive TC. A similar circuit, but with npn transistors, is described in U.S. Pat. No. 3,930,172 to Dobkin assigned to National Semiconductor Corp. A pair of pnp transistors Q 1 and Q 2 are connected with their bases and collectors cross-coupled and their emitters connected to a positive voltage bus V+, with a trimming resistor R 4 between V+ and the emitter of Q 2 . The emitter of Q 1 is scaled to a unity area of 1x while the emitter of Q 2 is scaled to an area of mx, either by implementing the transistor with a single larger emitter or with multiple emitters. The collectors of Q 1 and Q 2 are cascoded respectively with additional pnp transistors Q 3 and Q 4 , with the base and collector of Q 3 connected together in a diode configuration and a common base connection for Q 3 and Q 4 . The emitter area of Q 3 is scaled to nx, while the emitter area of Q 4 is 1x.

A current source IA draws current from the common base/collector connection for Q 3 . The magnitude of IA is not critical, so long as it is sufficient to keep all of the transistors conductive. The current source I 1 is provided from the collector of Q 4 . I 1 is approximately equal to kTln(nm)/qR 4 , where k is Boltzmann's constant, T is temperature in Kelvin and q is the electronic charge. R 4 can be trimmed to obtain a desired value for I 1 . Since I 1 is directly proportional to T, it has a PTAT (proportional to absolute temperature) characteristic. Its TC can be varied by making the resistance value of R 4 temperature dependent, with the value of TC for I 1 varying with the degree of temperature dependence for R 4 . Temperature dependent resistors are well known. If R 4 is given a negative TC the TC of I 1 will become more positive, and conversely if R 4 has a positive TC the TC of I 1 will become less positive. A similar circuit could be used to generate I 1 with a positive TC by substituting npn transistors for the pnp devices and “flipping” the circuit over.

FIG. 8 illustrates a known circuit that can be used to generate I 2 with a negative TC. A pair of npn transistors Q 5 and Q 6 have a common base connection, with the collector and base of Q 5 connected together in a diode configuration. The emitter of Q 5 supplies another diodeconnected npn transistor Q 7 , while the emitter of Q 6 is connected to a trimming resistor R 5 . The emitter of Q 7 and the opposite side of R 5 are connected to a negative voltage bus V−. A current source IB is connected the common base/collector connection for Q 5 to keep all of the transistors conductive, while I 2 is established as the collector current for Q 6 . The exact value of I 2 at a given temperature is set by trimming R 5 .

The value of I 2 is given by the following expression, in which Vbe is the transistor base-emitter voltage:

( Vbe Q5 +Vbe Q7 −Vbe Q6 ) R 5 ,

which is approximately equal to Vbe/R 5 , with Vbe for an npn transistor typically about 600 mV and having a TC of about −2 mV/°C. In a manner similar to the circuit of FIG. 7 , the TC for I 2 can be varied by making the resistance value of R 5 temperature dependent. If R 5 is given a negative TC the TC of I 2 will become less negative, and conversely if R 5 has a positive TC the TC of I 2 will become more negative.

While several embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. For example, the same approach could be used to provide TC compensation for the input offset voltage (Vos) of an operational amplifier, which typically can have either a positive or a negative TC. The amplifier could be skewed to force a TC that is always of the same polarity. In this case the input Vos prior to compensation corresponds to the “preliminary Vo” of the voltage reference for purposes of this description. Accordingly, it is intended that the invention be limited only in terms of the appended claims.

Claims

1. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity, and a compensation circuit connected to compensate the base circuit's Vo TC to yield a compensated Vo having a lower TC than said preliminary Vo TC, said compensation circuit comprising first and second current sources having different TCs and connected to provide said Vo TC compensation, said compensation circuit further comprising a voltage drop circuit connected to receive a current which varies with the differential between said current sources, and to produce a compensation voltage which cooperates with said base circuit Vo to yield said compensated Vo, wherein said current sources provide a current to said voltage drop circuit that causes said compensation voltage to have a TC substantially equal in absolute value, but of opposite polarity, to said preliminary Vo TC.

2. The low TC Vo circuit of claim 1, said voltage drop circuit comprising a resistor circuit.

3. The low TC Vo circuit of claim 1, for providing a compensated Vo over a predetermined temperature range, wherein the currents from said first and second sources are substantially equal in absolute value at a first temperature within said range.

4. The low TC Vo circuit of claim 3, wherein said compensation circuit produces a compensation voltage at a second temperature within said range which combines with said preliminary Vo to produce a compensated Vo at said second temperature substantially equal to the compensated Vo at said first temperature.

5. The low TC Vo circuit of claim 1, for providing a compensated Vo over a predetermined temperature range, wherein said compensation circuit yields substantially equal compensated Vo's at opposite ends of said range.

6. The low TC Vo circuit of claim 1, wherein said base circuit comprises a voltage reference circuit.

7. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity, and a compensation circuit connected to compensate the base circuit's Vo TC to yield a compensated Vo having a lower TC than said preliminary Vo TC, said compensation circuit comprising first and second current sources having different TCs and connected to provide said Vo TC compensation, wherein both of said current sources have positive TC's.

8. The low TC Vo circuit of claim 1, wherein one of said current sources has a positive TC and other has a negative TC.

9. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity, and a compensation circuit connected to compensate the base circuit's Vo TC to yield a compensated Vo having a lower TC than said preliminary Vo TC, said compensation circuit comprising first and second current sources having different TCs and connected to provide said Vo TC compensation, wherein both of said current sources have negative TC's.

10. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity at a preliminary Vo terminal, a voltage drop circuit that is connected to said preliminary Vo terminal and responds to a current to produce a voltage drop that combines with said preliminary Vo to yield a compensated Vo, and first and second current sources that produce currents with different TC's, said current sources connected in circuit with said voltage drop circuit to cause said voltage drop circuit to produce a voltage drop with a TC polarity opposite to the polarity of said preliminary Vo TC polarity.

11. The low TC Vo circuit of claim 10, wherein said voltage drop circuit is connected between said preliminary Vo terminal and a compensated Vo terminal, one of said current sources is connected to supply current to said compensated Vo terminal and the other of said current sources is connected to draw current from said compensated Vo terminal.

12. The low TC Vo circuit of claim 11, said voltage drop circuit comprising a resistor circuit.

13. The low TC Vo circuit of claim 12, wherein said base circuit produces a preliminary Vo with a positive TC, and the current source which draws current from said compensated Vo terminal has a TC that is more positive than the TC of said first current source.

14. The low TC Vo circuit of claim 12, where said base circuit produces a preliminary Vo with a negative TC, and the current source which draws current from said compensated Vo terminal has a TC that is more negative than the TC of said first current source.

15. The low TC Vo circuit of claim 12, for providing a compensated Vo over a predetermined temperature range, wherein said first and second current sources produce a substantially zero current flow through said resistor circuit at a first temperature within said range.

16. The low TC Vo circuit of claim 15, wherein said first and second current sources provide a current through said resistor circuit at a second temperature within said range to produce a voltage drop across said resistor circuit which is substantially equal in magnitude, and of opposite polarity, to the change in said preliminary Vo from said first to said second temperatures.

17. The low TC Vo circuit of claim 12, for providing a compensated Vo over a predetermined temperature range, wherein said first and second current sources provide currents through said resistor circuit, at the opposite ends of said range, to produce a change in the voltage drop across said resistor circuit, between the opposite ends of said range, which is equal in magnitude, and of opposite polarity, to the change in the preliminary Vo between said opposite ends of the range.

18. The low TC Vo circuit of claim 10, wherein said base circuit comprises a voltage reference circuit.

19. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity at a preliminary Vo terminal, a voltage drop circuit that is connected to said preliminary Vo terminal and responds to a current to produce a voltage drop that combines with said preliminary Vo to yield a compensated Vo, first and second current sources that produce currents with different TCs, said current sources connected in circuit with said voltage drop circuit to cause said voltage drop circuit to produce a voltage drop with a TC polarity opposite to the polarity of said preliminary Vo TC, and a buffer amplifier having an input connected to receive said compensated Vo.

20. The low TC Vo circuit of claim 10, wherein both of said current sources have positive TC's.

21. The low TC Vo circuit of claim 10, wherein one of said current sources has a positive TC and the other has a negative TC.

22. The low TC Vo circuit of claim 10, wherein both of said current sources have negative TC's.

23. A temperature compensation circuit, comprising:a voltage drop circuit having an input terminal for receiving a voltage to be temperature compensated, and an output terminal, said voltage drop circuit producing a voltage drop that varies with the current flowing through it, and first and second current sources having different respective temperature coefficients (TC's), one of said current sources connected to supply current to and the other of said current sources connected to draw current from said voltage drop circuit, said current sources producing a net current flow through said voltage drop circuit that produces a voltage drop with a non-zero TC across said circuit, wherein both of said current sources have positive TC's.

24. A temperature compensation circuit, comprising:a voltage drop circuit having an input terminal for receiving a voltage to be temperature compensated, and an output terminal, said voltage drop circuit producing a voltage drop that varies with the current flowing through it, and first and second current sources having different respective temperature coefficients (TC's), one of said current sources connected to supply current to and the other of said current sources connected to draw current from said voltage drop circuit, said current sources producing a net current flow through said voltage drop circuit that produces a voltage drop with a non-zero TC across said circuit, wherein both of said current sources have negative TC's.

25. A method of compensating for temperature induced changes in the output voltage (Vo) of a base circuit which has a temperature coefficient (TC) of one polarity, comprising:establishing two currents that have different TCs, establishing a differential between said two currents, transforming said current differential to a voltage drop having a TC of opposite polarity to said base circuit Vo TC, and combining said base circuit Vo and said voltage drop to yield a compensated Vo having a lower TC than said base circuit Vo TC.

26. The method of claim 25, wherein said voltage drop is established by applying said current differential to resistor circuit.

27. The method of claim 26, further comprising the step of trimming said resistor circuit to obtain a desired Vo value at a predetermined temperature.

28. The method of claim 27, for providing compensation over a predetermined temperature range, wherein said current differential establishes a substantially zero voltage drop at a first temperature within of said range, and said resistor circuit is trimmed at a second temperature within said range.

29. The method of claim 28, wherein said two currents are established by first and second current sources, respectively, further comprising the step of trimming at least one of said current sources at said first temperature to establish said substantially zero voltage drop.

30. A method of compensating, over a predetermined temperature range, for temperature induced changes in the output voltage (Vo) of a voltage reference circuit which has a temperature coefficient (TC) of one polarity, comprising:establishing two currents that have different TCs by first and second current sources, respectively, establishing a differential between said two currents, transforming said current differential to a voltage drop having a TC of opposite polarity to said voltage reference circuit Vo TC by applying said current differential to a resistor circuit, trimming said resistor circuit to obtain a desired Vo value at a predetermined temperature, wherein said current differential establishes a substantially zero voltage drop at a first temperature within said range, said resistor circuit is trimmed at a second temperature within said range, and at least one of said current sources is trimmed at said first temperature to establish said substantially zero voltage drop, combining said voltage reference circuit Vo and said voltage drop to yield a compensated Vo having a lower TC than said voltage reference circuit Vo TC, providing an operational amplifier having a resistive feedback circuit to amplify said voltage reference circuit Vo, and trimming said resistive feedback circuit at said first temperature to establish a desired output voltage from said amplifier.

31. A method of compensating for temperature induced changes in the output voltage (Vo) of a base circuit which has a temperature coefficient (TC) of known polarity, comprising:establishing a differential between the currents of two current sources which have different TCs, using said differential to establish a compensation voltage having a TC of opposite polarity to the TC of said base circuit, and compensating said base circuit Vo with said compensation voltage.

32. The method of claim 31, wherein said current sources are set equal to each other at one temperature, and the compensated base circuit Vo is set equal to a desired value at another temperature.

33. The low TC Vo circuit of claim 1, wherein said first and second current sources have TCs with different absolute values.

34. The low TC Vo circuit of claim 10, wherein said first and second current sources have TCs with different absolute values.

35. The method of claim 25, wherein said two currents are established with TCs of different respective absolute values.

36. The method of claim 31, wherein said two current sources produce currents that have TCs of different absolute values.

37. The low TC Vo circuit of claim 1, said compensation circuit further comprising a voltage drop circuit connected to receive a current which varies with the differential between said current sources, and to produce a compensation voltage which cooperates with said voltage reference circuit Vo to yield said compensated Vo.

38. A low temperature coefficient (TC) voltage output circuit, comprising:a base circuit which produces a preliminary output voltage (Vo) having a TC of one polarity, and a compensation circuit connected to compensate the base circuit's Vo TC to yield a compensated Vo having a lower TC than said preliminary Vo TC, said compensation circuit comprising first and second current sources having different TCs and connected to provide said Vo TC compensation, said compensation circuit further comprising a voltage drop circuit connected to receive a current which varies with the differential between said current sources, and to produce a compensation voltage which cooperates with said voltage reference circuit Vo to yield said compensated Vo, wherein said current sources provide a current to said voltage drop circuit that causes said compensation voltage to have a TC substantially equal in absolute value, but of opposite polarity, to said preliminary Vo TC.

39. A low temperature coefficient (TC) voltage output circuit, comprising:a voltage reference circuit which produces a preliminary output voltage (Vo) having a non-zero TC of one polarity at a preliminary Vo terminal, a voltage drop circuit that is connected to said preliminary Vo terminal and responds to a current to produce a voltage drop that combines with said preliminary Vo to yield a compensated Vo, and first and second current sources that produce currents with different TCs, said current sources connected in circuit with said voltage drop circuit to cause said voltage drop circuit to produce a voltage drop with a TC polarity opposite to the polarity of said preliminary Vo TC polarity.

40. The low TC Vo circuit of claim 39, wherein said voltage drop circuit is connected between said preliminary Vo terminal and a compensated Vo terminal, one of said current sources is connected to supply current to said compensated Vo terminal and the other of said current sources is connected to draw current from said compensated Vo terminal.

41. The low TC Vo circuit of claim 40, said voltage drop circuit comprising a resistor circuit.

42. The low TC Vo circuit of claim 41, for providing a compensated Vo over a predetermined temperature range, wherein said first and second current sources produce a substantially zero current flow through said resistor circuit at a first temperature within said range.

43. The low TC Vo circuit of claim 42, wherein said first and second current sources provide a current through said resistor circuit at a second temperature within said range to produce a voltage drop across said resistor circuit which is substantially equal in magnitude, and of opposite polarity, to the change in said preliminary Vo from said first to said second temperatures.

44. The low TC Vo circuit of claim 41, for providing a compensated Vo over a predetermined temperature range, wherein said first and second current sources provide currents through said resistor circuit, at the opposite ends of said range, to produce a change in the voltage drop across said resistor circuit, between the opposite ends of said range, which is equal in magnitude, and of opposite polarity, to the change in the preliminary Vo between said opposite ends of the range.

45. The low TC Vo circuit of claim 39, wherein said first and second current sources have TCs with different absolute values.

46. A method of compensating for temperature induced changes in the output voltage (Vo) of a voltage reference circuit which has a non-zero temperature coefficient (TC) of one polarity, comprising:establishing two currents that have different TCS, establishing a differential between said two currents, transforming said current differential to a voltage drop having a TC of opposite polarity to said base circuit Vo TC, and combining said voltage reference circuit Vo and said voltage drop to yield a compensated Vo having a lower TC than said base circuit Vo TC.

47. The method of claim 46, wherein said voltage drop is established by applying said current differential to a resistor circuit.

48. The method of claim 47, further comprising the step of trimming said resistor circuit to obtain a desired Vo value at a predetermined temperature.

49. The method of claim 48, for providing compensation over a predetermined temperature range, wherein said current differential establishes a substantially zero voltage drop at a first temperature within of said range, and said resistor circuit is trimmed at a second temperature within said range.

50. The method of claim 49, wherein said two currents are established by first and second current sources, respectively, further comprising the step of trimming at least one of said current sources at said first temperature to establish said substantially zero voltage drop.

51. The method of claim 46, wherein said two currents are established with TCs of different respective absolute values.

52. A method of compensating for temperature induced changes in the output voltage (Vo) of a voltage reference circuit which has a non-zero temperature coefficient (TC) of known polarity, comprising:establishing a differential between the currents of two current sources which have different TCs, using said differential to establish a compensation voltage having a TC of opposite polarity to the TC of said voltage reference circuit, and compensating said voltage reference circuit Vo with said compensation voltage.

53. The method of claim 52, wherein said current sources are set equal to each other at one temperature, and the compensated voltage reference circuit Vo is set equal to a desired value at another temperature.

54. The method of claim 52, wherein said two current sources produce currents that have TCs of different absolute values.