Temperature compensation circuit and method of compensating

Abstract

A temperature compensation circuit converts a control signal (IG) that has an undesirable temperature coefficient to a temperature compensated control signal (I32) having a desirable temperature coefficient. In one embodiment, four transistors (60, 64, 68, and 72) are configured to convert the control signal (IG) having an undesirable temperature coefficient to the temperature compensated control signal (I32) having the desired temperature coefficient. Additional embodiments use components to refine the temperature compensation process.

Description

[0001] In general, this invention relates to a temperature compensation circuit. Specifically, this invention provides for a temperature compensation circuit and method that controls the temperature coefficient of an output signal.

[0002] Temperature compensation is often employed in situations where a control signal provided by another semiconductor device or circuit has a particular temperature coefficient and the control signal needs to be converted to a different temperature coefficient. For example, in a typical Radio Frequency (RF) application, a gain control signal is produced by a microprocessor. This gain control signal typically has an undesirable temperature coefficient, in that the gain control curve, e.g. voltage versus decibels, is subject to unwanted anomalies with temperature variation.

[0003] Prior art temperature compensation circuits, particularly those found in cellular or cordless phones, are typified by the presence of Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and an operational amplifier connected to a reference voltage for controlling the transfer characteristics of gain control input over temperature. These types of prior art circuits typically use voltage to current converters, where the reference and input voltages have an undesirable temperature coefficient and the reference and output currents have a desired temperature coefficient. One drawback of the prior art temperature compensation circuits is that the transfer characteristic does not produce a sufficiently linear result. Furthermore, the transfer characteristic produces a gain control curve where the minimum voltage is the threshold voltage (VT) of the MOSFET device, not zero. This is undesirable because the full control range is limited due to the threshold voltage. Also, the requirement for the operational amplifier adds complexity and cost to the circuit.

[0004] Therefore, a need exists to provide a temperature compensation circuit that produces an approximately linear output signal that is capable of a full range of control.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a circuit diagram of a temperature compensation circuit;

[0006] FIG. 2 is a circuit diagram of another embodiment of the temperature compensation circuit; and

[0007] FIG. 3 is a circuit diagram of still another embodiment of the temperature compensation circuit.

DETAILED DESCRIPTION OF THE DRAWINGS

[0008] Bipolar circuits have transistor base-emitter voltages and, in particular, base-emitter voltage differences that are Proportional To Absolute Temperature (PTAT). The present invention provides a circuit and method for interfacing a bipolar circuit to an external circuit having a different temperature coefficient.

[0009] FIG. 1 illustrates a four transistor model of a temperature compensation circuit. A current source 62 is connected between a power conductor that receives a voltage Vcc and the collector of a transistor 60, where current source 62 provides an input current. The emitter of transistor 60 is connected to a power conductor that receives ground potential. The base of transistor 60 is connected to an emitter of a transistor 64. Transistor 60 conducts a current I1, which is the input signal having an undesirable temperature coefficient.

[0010] Transistor 64 has a collector connected to the power conductor that receives the voltage Vcc and an emitter connected through a current source 66 to the power conductor that receives ground potential. Current source 66 provides a current I2 having the desired temperature coefficient. The base of transistor 64 and the base of a transistor 68 are connected to each other and further connected to the collector of transistor 60. The collector of transistor 68 is connected to the power conductor that receives the voltage Vcc and an emitter connected through a current source 70 to the power conductor that receives ground potential. Current source 70 provides a current I3, which is a function of current I2. Current I3 has the same undesirable temperature coefficient as current I1. A transistor 72 has a base connected to the emitter of transistor 68, an emitter connected to the power conductor that receives ground potential, and a collector connected to an output terminal. Transistor 72 conducts a current I4, which is a function of the current I1 and has a desired temperature coefficient. Thus, the current supplied by transistor 72 at the output terminal is the temperature compensated output signal.

[0011] In operation, temperature compensation circuit operates as follows. The circuit voltages are a function of the transistor base-emitter voltages (VBE) and, more particularly, the VBE of each transistor in relation to other transistors. Summing the VBE for the transistors results in the following relationship:

VBE60+VBE64=VBE68+VBE72,  (Equation 1)

[0012] where VBE60 is the VBE of transistor 60, VBE64 is the VBE of transistor 64, VBE68 is the VBE of transistor 68, and VBE72 is the VBE of transistor 72. Note that VBE is equal to (kT/q) * In(Ic/Is), where kT/q is the thermal voltage of the device, current Ic is the relevant collector current, and current Is is the saturation current of the transistor. Thus, converting equation 1 to currents, the product of the current I1 and the current I2 is equal to the product of current I3 and the current I4.

I1*I2=I3*I4  (Equation 2)

[0013] where I1 is the current conducted by transistor 60, I2 is the current conducted by transistor 64, I3 is the current conducted by transistor 68, and I4 is the current conducted by transistor 72.

[0014] Isolating for the temperature compensated output current I4 yields the following:

I4=(I1*I2)/I3  (Equation 3)

[0015] Current I2 was chosen with a desirable temperature coefficient. Currents I2 and I3 are chosen to be nominally equal at a known temperature. Current I1 has an undesirable temperature coefficient that is canceled by the undesirable temperature coefficient for the current I3 (see equation 3). Thus, current I4 supplied at output terminal 36 is equal to the current I1, but whereas input current I1 has an undesirable temperature coefficient, output current I4 has the desirable temperature coefficient. Furthermore, current ratios other than 1:1 between currents I4 and I1 are possible by simply providing an alternate ratio for currents I2 and I3 as, for example, changing the physical dimensions of the transistor emitter areas with respect to each other. It should be noted that currents I1 and I2 are interchangeable, where current I1 is the input signal and current I2 is chosen with the desirable temperature coefficient.

[0016] FIG. 2 illustrates another embodiment of a temperature compensation circuit. In this embodiment, a current source 47 supplies a current IG to the emitter of a transistor 16 and to the base of a transistor 22. The collector of transistor 16 is connected to a power conductor that receives a voltage Vcc. The collector of transistor 22 is connected to an emitter of a transistor 24 and further connected to a base of a transistor 28. The base and collector of transistor 24 are connected through a current source 26 to the power conductor that receives a voltage Vcc. The collector of transistor 28 is connected to the power conductor that receives the voltage Vcc. The emitter of transistor 28 is connected to the base of a transistor 32 and to the power conductor that receives the ground potential through a current source 33. The collector of transistor 32 is connected to an emitter of a transistor 34. The collector of transistor 34 is connected to a temperature compensated output terminal 36. The base terminals of transistors 34 and 38 are connected to the base of transistor 24. The collector of transistor 38 is connected to the power conductor that receives the voltage Vcc. The emitter of transistor 38 is connected through a current source 40 to the power conductor that receives the ground potential and to the base of transistor 16. It should be pointed out that transistor 34 may be removed from the circuit configuration.

[0017] The equations from above are modified consistent with the operation of the temperature compensation circuit. Summing the VBE for transistors 32, 28, 24, 38, 16, and 22 results in the following:

VBE32+VBE28+VBE24=VBE38+VBE16+VBE22  (Equation 4)

[0018] where VBE32 is the base-emitter voltage of transistor 32, VBE28 is the base-emitter voltage of transistor 28, VBE24 is the base-emitter voltage of transistor 24, VBE38 is the base-emitter voltage of transistor 38, VBE16 is the base-emitter voltage of transistor 16, and VBE22 is the base-emitter voltage of transistor 22. Transistors 22 and 24 conduct the same current and, therefore, the VBE22 of transistor 22 is the same as the VBE24 of transistor 24 because transistors 22 and 24 share the same current I22. Thus, equation 4 is simplified to:

VBE32+VBE28=VBE38+VBE16  (Equation 5)

[0019] The currents for transistors 32, 28, 38, and 16 can be represented by the product of currents I32 and I28 being equal to the product of currents I38 and I16.

I32*I28=38*I16,  (Equation 6)

[0020] where I32 is the current conducted by transistor 32, I28 is the current conducted by transistor 28, I38 is the current conducted by transistor 38, and I16 is the current conducted by transistor 16.

[0021] Isolating for current I32, i.e., the temperature compensated output current, provides the following equation.

I32=(I38*I16)/I28  (Equation 7)

[0022] In the preferred embodiment, transistors 16, 38, 28, 32, 24, and 22 are bipolar transistors with similar sizing. Transistors 16, 38, 28, and 32 are devices used in the basic operation of the circuit as described above in FIG. 1, while transistors 22 and 24 are included to improve the performance of the temperature compensation circuit.

[0023] This embodiment produces a temperature compensated output current at terminal 36 that is a function of the variable input current IG, but with a different temperature coefficient. By way of example, current IG may be received as a PTAT current but desired as having a zero temperature coefficient. The temperature compensation circuit illustrated in FIG. 2 converts the input PTAT current IG to an output current I32 having the zero temperature coefficient. In this embodiment, current I40 is chosen as having a zero temperature coefficient and the output current I32 will have the same temperature coefficient as the current I40. Thus, the current I32 supplied at output terminal 36 is equal to current IG at a given temperature, but having a zero temperature coefficient.

[0024] FIG. 3 illustrates another embodiment of the temperature compensation circuit. It should be pointed out that like elements in the figures are denoted by the same reference numerals. The temperature compensation circuit receives a control voltage VG from a voltage source 12. A microprocessor, microcontroller, or other device capable of producing a variable voltage may supply the voltage VG. Alternatively, the voltage VG may be generated on the same integrated circuit as the temperature compensation circuit. The voltage VG received at one terminal of resistor 13 is converted to a current IG. The other terminal of resistor 13 is commonly connected to the emitter of transistor 16, a collector of a transistor 52, and a base of transistor 22. A reference voltage generator 42 is connected to one terminal of a resistor 44. The other terminal of resistor 44 is connected to the base and collector of a transistor 46, and to the base of transistors 52 and 30. The emitters of transistors 46, 52 and 30 are connected to the power conductor that receives a ground potential. In this embodiment, a resistor 31 connects the power conductor that receives a ground potential to the common connection that includes the emitter of transistor 28, the base of transistor 32, and the collector of transistor 30. In the preferred embodiment, resistors 13, 31, and 44 have matching resistance values.

[0025] Equations 4, 5, 6 and 7 set forth above are applicable to the embodiment of the temperature compensation circuit illustrated in FIG. 3. This embodiment of the temperature compensation circuit produces a temperature compensated output current at terminal 36 that is a function of the variable input current IG, but with a different temperature coefficient. The temperature compensation circuit illustrated in FIG. 3 compares the input voltage VG to the reference voltage VR that is received having an unknown temperature coefficient and a current I32 is supplied at output terminal 36 having a desired and known temperature coefficient.

[0026] By now it should be appreciated that a circuit is provided that receives a signal having a particular temperature coefficient and generates an output signal having a different temperature coefficient.

Claims

1. A temperature compensation circuit, comprising:

first, second, and third current sources;

a first transistor having an emitter coupled to a first power conductor and a collector coupled through the first current source to a second power conductor;

a second transistor having a base coupled to the collector of the first transistor, a collector coupled to the second power conductor, and an emitter coupled to a base of the first transistor and further coupled through the second current source to the first power conductor;

a third transistor having a base coupled to the base of the second transistor, a collector coupled to the second power conductor, and an emitter coupled through the third current source to the first power conductor; and

a fourth transistor having a base coupled to the emitter of the third transistor, an emitter coupled to the first power conductor and a collector coupled to an output terminal.

2. The temperature compensation circuit of

claim 1, wherein a first temperature coefficient of a current in the first current source matches a first temperature coefficient of a current in the third current source.

3. The temperature compensation circuit of

claim 2, wherein a second temperature coefficient of a current conducted by the fourth transistor matches a second temperature coefficient of a current in the second current source, the first temperature coefficient being different from the second temperature coefficient.

4. The temperature compensation circuit of

claim 3, wherein the current provided by the second current source has a fixed ratio to the current provided in the third current source at a given temperature.

5. A temperature compensation circuit, comprising:

a first transistor having an emitter coupled to a first power conductor and a collector coupled to an output terminal;

a second transistor having an emitter coupled to a base of the first transistor and a collector coupled to a second power conductor;

a third transistor having a base and a collector coupled to the second power conductor, and an emitter coupled to the base of the second transistor;

a fourth transistor having a base coupled to the base of the third transistor, an emitter coupled to the first power conductor, and a collector coupled to the second power conductor;

a fifth transistor having a base coupled to the emitter of the fourth transistor and a collector coupled to the second power conductor; and

a sixth transistor having a base coupled to an emitter of the fifth transistor, a collector coupled to the emitter of the third transistor, and an emitter coupled to the first power conductor.

6. The temperature compensation circuit of

claim 5, further comprising:

a seventh transistor having a collector coupled to the base of the sixth transistor and an emitter coupled to the first power conductor; and

a eighth transistor having a base coupled to a base of the seventh transistor, a collector coupled to the base of the first transistor, and an emitter coupled to the first power conductor.

7. The temperature compensation circuit of

claim 6, further comprising a first resistor that couples the base of the first transistor to the first power conductor.

8. The temperature compensation circuit of

claim 7, further comprising a second resistor having a first terminal coupled for receiving a signal and a second terminal coupled to the base of the sixth transistor.

9. The temperature compensation circuit of

claim 8, further comprising:

a third resistor having a first terminal coupled for receiving a reference signal; and

a ninth transistor having a commonly coupled collector and base coupled to a second terminal of the third resistor and further coupled to the base of the seventh and eighth transistors.

10. The temperature compensation circuit of

claim 9, further comprising a tenth transistor having a base coupled to the base of the third transistor, a collector coupled to the output terminal, and an emitter coupled to the collector of the first transistor.

11. The temperature compensation circuit of

claim 5, further comprising a current source coupled between the emitter of the fifth transistor and the first power conductor.

12. The temperature compensation circuit of

claim 5, further comprising a current source coupled between the collector of the third transistor and the second power conductor.

13. The temperature compensation circuit of

claim 5, further comprising a current source coupled between the emitter of the second transistor and the first power conductor.

14. The temperature compensation circuit of

claim 5, further comprising a current source coupled between the emitter of the fourth transistor and the first power conductor.

15. A method of generating a temperature coefficient of an output signal that is different from an input signal, comprising:

generating a sum of base-emitter voltages of first and second transistors that matches a sum of base-emitter voltages of third and fourth transistors;

conducting a current in the first transistor proportional to the input signal;

conducting a current in the second transistor having a desired temperature coefficient;

conducting a current in the third transistor having a known relation to the current in second transistor and having a same temperature coefficient as the current in the first transistor; and

outputting a current from the fourth transistor that is proportional to the current in the first transistor, where the current from the fourth transistor has the desired temperature coefficient.

16. The method of

claim 15, further comprising the step of conducting a current in the third transistor that equals the current conducted in the second transistor at a given temperature.