Generation of a voltage proportional to temperature with stable line voltage

[0002] The present invention relates to a circuit for generating an output voltage which is proportional to temperature with a required gradient.

[0003] Such circuits exist which rely on the principle that the difference in the base emitter voltage of two bipolar transistors with differing areas, if appropriately connected, can result in a current which has a positive temperature coefficient, that is a current which varies linearly with temperature such that as the temperature increases the current increases. This current, referred to herein as Iptat, can be used to generate a voltage proportional to absolute temperature, Vptat, when supplied across a resistor.

[0004] Although this principle is sound, a number of difficulties exist in converting this principle to practical application.

[0005] One such practical difficulty is the need to maintain a stable internal line voltage in the face of significant variations in a supply voltage. This should be done without unnecessarily increasing the number of components in the circuit over and above those which are required to generate the voltage proportional to temperature.

[0006] The present invention provides a circuit for generating an output voltage proportional to temperature with a required gradient, the circuit comprising: first and second bipolar transistors with different emitter areas having their emitters connected together and their bases connected across a bridge resistive element, wherein the collectors of the transistors are connected to an internal supply line via respective matched resistive elements such that the voltage across the bridge resistive element is proportional to temperature; a differential amplifier having its inputs connected respectively to said collectors and its output connected to a control terminal of a first control element having a controllable path connected between a first power supply rail and a control node; a second control element having a controllable path connected between the control node and a second power supply rail; and a third control element having a control terminal connected to the control node and a controllable path connected between the second power supply rail and an internal supply line, whereby the differential amplifier and the first, second and third control elements cooperate to maintain a stable voltage on the internal supply line despite variations between the first and second power supply rails.

[0007] In the described embodiment the stable voltage on the internal supply line is used to power components of a second stage which allows fine adjustment of the predetermined gradient of the voltage proportional to temperature.

[0008] In the described embodiment, the voltage on the internal supply line is set from the voltage proportional to absolute temperature using that voltage in conjunction with two bipolar transistors connected in series via a resistor to an output node at which a voltage proportional to absolute temperature with a predetermined gradient is generated.

[0009] Thus, the embodiments of the invention described in the following focus on line regulation of a circuit such that if the supply voltage to a chip increases, the output of the temperature sensor does not change (or only very minutely). This is done by having a constant internal supply line for the major circuitry which is quite stable with temperature. If this does not change, then the assumption can be made that the local supply (V_ddint) is constant.

[0010] In the following described embodiments, three components in particular are discussed:

[0011] (i) The value on the internal supply line (V_ddint) is set by the voltage across the bridge resistive element and two bipolar transistors connected in series, using the current proportional to absolute temperature which is generated in the circuit.

[0012] (ii) The drop of voltage between the first and second power supply rail and the internal supply line (V_ddint) appears across the collector/emitter of the third control element. The bias for that control element is provided by the first and second control elements.

[0013] (iii) The third control element also can provide the current supply for the internal supply line. Any disturbance of current or voltage on the internal supply line loops back through the resistive bridge element, ΔVbe generator, differential amplifier to the first and second biasing control elements and to the third control element.

[0014] For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings.

[0015] FIG. 1 represents circuitry of the first stage;

[0016] FIG. 2 represents construction of a resistive chain;

[0017] FIG. 3 represents circuitry of the second stage;

[0018] FIG. 4 is a graph illustrating the variation of temperature with voltage for circuits with and without use of the present invention; and

[0019] FIG. 5 represents circuitry of another form of second stage.

[0020] The present invention is concerned with a circuit for the generation of a voltage proportional to absolute temperature (Vptat). The circuit has two stages which are referred to herein as the first stage and the second stage. In the first stage, a “raw” voltage Vptat is generated, and in the second stage a calibrated voltage for measurement purposes is generated from the “raw” voltage.

[0021] FIG. 1 illustrates one embodiment of the first stage. The core of the voltage generation circuit comprises two bipolar transistors Q0,Q1 which have different emitter areas. The difference ΔVbe between the base emitter voltages Vb(Q1)−Vb(Q0) is given to the first order by the equation (1): $\begin{array}{cc}\Delta \mathrm{Vbe}=\frac{\mathrm{KT}.}{q}\ln \frac{{\mathrm{Ic}}_1{\mathrm{Is}}_0}{{\mathrm{Ic}}_0{\mathrm{Is}}_1}& \left(1\right)\end{array}$

[0022] where K is Boltzmanns constant, T is temperature, q is the electron charge, Ic₀is the collector current through the transistor Q0, Ic₁is the collector current through the transistor Q1, Is₀is the saturation current of the transistor Q0 and Is₁is the saturation current of the transistor Q1. As is well known, the saturation current is dependent on the emitter area, such that the ratio Is₀divided by Is₁is equal to the ratio of the emitter area of the transistor Q0 to the emitter area of the transistor Q1. In the described embodiment, that ratio is 8. Also, the circuit illustrated in FIG. 1, is arranged so that the collector currents Ic₁and Ic₀are maintained equal, such that their ratio is 1, as discussed in more detail in the following. Therefore, to a first approximation, $\begin{array}{cc}\Delta \mathrm{Vbe}=\frac{\mathrm{KT}.}{q}\ln 8& \text{(1a)}\end{array}$

[0023] The difference ΔVbe is dropped across a bridge resistor R2 to generate a current proportional to absolute temperature Iptat, where: $\begin{array}{cc}\mathrm{Iptat}=\frac{\Delta \mathrm{Vbe}}{\mathrm{R2}}& \left(2\right)\end{array}$

[0024] This current Iptat is passed through a resistive chain Rx to generate the temperature dependent voltage Vptat at a node N1. A resistor R3 is connected between R2 and ground.

[0025] With R2 equal to 18 kOhms, substituting the values in equations (1) and (2) above, Iptat is in the range 2.5 μA to 3 μA over a temperature range of −20 to 100° C. The temperature dependent voltage Vptat is given by: $\begin{array}{cc}\mathrm{Vptat}=\mathrm{Iptat}\times \left(\mathrm{R2}+\mathrm{R3}+\mathrm{Rx}\right)=\frac{\mathrm{KT}\ln 8\left(\mathrm{R2}+\mathrm{R3}+\mathrm{Rx}\right)}{q\mathrm{R2}}& \left(3\right)\end{array}$

[0026] To get a relationship of the temperature dependent voltage Vptat variation with temperature, we differentiate the above equation to obtain: $\begin{array}{cc}\frac{d\mathrm{Vptat}}{dT}=K\ln 8\frac{\left(\mathrm{R2}+\mathrm{R3}+\mathrm{Rx}\right)}{q\times \mathrm{R2}}& \left(4\right)\end{array}$

[0027] With the values indicated above R2+18K, R3=36K, Rx=85K, the variation of voltage with temperature is 4.53 mV/° C.

[0028] Before discussing how Vptat is modified in the second stage, other attributes of the circuit of the first stage will be discussed.

[0029] The collector currents Ic₁, Ic₀are forced to be equal by matching resistors R0, R1 in the collector paths as closely as possible. However, it is also important to maintain the collector voltages of the transistors Q0,Q1 as close to one another as possible to match the collector currents. This is achieved by connecting the two inputs of a differential amplifier AMP1 to the respective collector paths. The amplifier AMP1 is designed to hold its inputs very close to one another. In the described embodiments, the input voltage Vio of the amplifier AMP1 is less then 1 mV so that the matching of the collector voltages of the transistors Q0,Q1 is very good. This improves the linearity of operation of the circuit.

[0030] Vddint denotes an internal line voltage which is set and stabilised as described in the following. A transistor Q4 has its emitter connected to V_ddintand its collector connected to the amplifier AMP1 to act as a current source for the amplifier AMP1. It is connected in a mirror configuration with a bipolar transistor Q6 which has its base connected to its collector. The transistor Q6 is connected in series to an opposite polarity transistor Q8, also having its base connected to its collector.

[0031] The bipolar transistors Q8 and Q6 assist in setting the value of the internal line voltage V_ddintat a stable voltage to a level given by, to a first approximation,

[0032] V_ddint=Iptat(R3+R2+Rx+Rz)+Vbe(Q6)+Vbe(Q8)  (5)

[0033] According to the principal on which bandgap voltage regulators are based, as Vptat increases with temperature, the Vbe of transistors Q6 and Q8 decrease due to the temperature dependence of Vbe in a bipolar transistor. Thus, V_ddintis a reasonably stable voltage because the decrease across Q6 and Q8 with rising temperature is compensated by the increase in Vptat.

[0034] The amplifier AMP1 has a secondary purpose, provided at no extra overhead, to the main purpose of equalising the collector voltages Q0 and Q1, discussed above. The secondary use is for stabilising the line voltage V_ddint. Imagine if V_ddintis disturbed by fluctuating voltage or current due to excessive current taken from the second stage (discussed later) or noise or power supply coupling onto it. The voltage on line V_ddintwill go up or down slightly. If V_ddintgoes higher, then the potential at resistor R2 and R3 will rise. Icl will increase slightly more than Ic₀and the difference across AMP1 increases. AMP1 is a transconductance amplifier and as the Vic increases more current is drawn through Q2, i.e. Ic₂increases. Q3 is starved of base current and switches off allowing V_ddintto recover by current discharge through the resistor bridge. The opposite occurs when V_ddintgoes low in which case AMP1 supplies less current to the base of Q2 therefore the current Ic₂decreases and mor ecurrent from Q9 can go to the base of Q3 allowing more drive current Ic₃to supply V_ddint. In effect there is some stabilisation.

[0035] The base of a transistor Q9 connected between the transistor Q2 and V_supplyis connected to receive a start-up signal from a start-up circuit (not shown). The transistor Q9 acts as a current source for the transistor Q2. An additional bipolar transistor Q5 is connected between the common emitter connection of the voltage generating transistors Q0,Q1 and has its base connected to receive a start-up signal from the start-up circuit. It functions as the “tail” of the Vptat transistors Q0,Q1.

[0036] The temperature dependent voltage Vptat generated by the first stage illustrated in FIG. 1 has a good linear variation at the calculated slope 4.53 mV/° C. However, the internal line voltage V_ddintlimits the swing in the upper direction, and also Vptat cannot go down to zero.

[0037] It will be appreciated that the resistive chain Rx constitutes a sequence of resistors connected in series as illustrated for example in FIG. 2. The slope of the temperature dependent voltage is dependent on the resistive value in the resistive chain Rx and thus can be altered by tapping off the voltage at different points P1,P2,P3 in FIG. 2.

[0038] FIG. 3 illustrates the second stage of the circuit which functions as a gain stage. The circuit comprises a differential amplifier AMP2 having a first input 10 connected to receive the temperature dependent voltage Vptat at node N1 from the first stage and a second input 12 serving as a feedback input. The output of the differential amplifier AMP2 is connected to a Darlington pair of transistors Q10, Q11. The emitter of the second transistor Q11 in the Darlington pair supplies an output voltage Vout at node 14. The amplifier AMP2 and the first Darlington transistor Q10 are connected to the stable voltage line V_ddintsupplied by the first stage. The second Darlington transistor is connected to V_supply.

[0039] The output voltage Vout is a voltage which is proportional to temperature with a required gradient and which can move negative with negative temperatures.

[0040] The adjustment of the slope of the temperature versus voltage curve is achieved in the second stage by a feedback loop for the differential amplifier AMP2. The feedback loop comprises a gain resistor R4 connected between the output terminal 14 at which the output voltage Vout is taken and the base of a feedback transistor Q12. The collector of the feedback transistor Q12 is connected to ground and its emitter is connected into a resistive chain Ry, the value of which can be altered and which is constructed similarly to the resistive chain Rx in FIG. 2. A resistor R5 is connected between the resistor R4 and ground. The gain of the feedback loop including differential amplifier AMP2 can be adjusted by altering the ratio: $\begin{array}{cc}\frac{\mathrm{R4}+\mathrm{R5}}{\mathrm{R5}}& \left(6\right)\end{array}$

[0041] This allows the slope of the incoming temperature dependent voltage Vptat to be adjusted between the gradient produced by the first stage at N1 and the required gradient at the output terminal 14. In the described example, the slope of the temperature dependent voltage Vptat at N1 with respect to temperature is 4.53 mV/° C. This is altered by the second stage to 10 mV/° C. This is illustrated in FIG. 4 where the crosses denote the relationship of voltage and temperature at N1 and the diamonds denote the relationship of voltage to temperature for the output voltage at the output node 14.

[0042] As has already been mentioned, the voltage Vptat at the node N1 cannot move into negative values even when the temperature moves negative. The second stage of the circuit accomplishes this by providing an offset circuit 22 connected to the input terminal 12 of the differential amplifier AMP2. The offset circuit 22 comprises the resistor chain Ry and the transistor Q12. Together these components provide a relatively stable bandgap voltage of about 1.25 V. The resistive chain Ry receives the current Iptat mirrored from the first stage via two bipolar transistors Q13, Q14 of opposite types which are connected in opposition and which cooperate with the transistors Q6 and Q8 of the first stage to act as a current mirror to mirror the temperature dependent current Iptat. As Iptat increases with temperature, Vbe(Q12) decreases. This offset circuit 22 introduces a fixed voltage offset at the input terminal 12, thus shifting the line of voltage with respect to temperature. This shift can be seen in FIG. 4, where the curve of the output voltage Vout at node 14 can be seen to pass through zero and move negative at negative temperatures.

[0043] From the above description it can be seen that the “bridge” network in the first stage performs a number of different functions, as follows. Firstly, it provides a temperature related voltage Vptat at the node N1. Secondly, it assists in providing a relatively fixed internal supply voltage V_ddinteven in the face of external supply variations, thus giving good line regulation for the gain circuit of the second stage. Thirdly, it provides in conjunction with the current mirror transistors Q4,Q6 current biasing for the amplifier AMP1 of the first stage. Fourthly, it provides, through the mirroring of transistors Q6,Q13 current biasing for the resistive chain Ry in the offset circuit 22 of the second stage.

[0044] Table 1 illustrates the operating parameters of one particular embodiment of the circuit. To achieve the operating parameters given in Table 1, adjustment can be made using the resistive chain Rx implemented in the manner illustrated in FIG. 2 to adjust the slope of Vptat in the first stage.

[0045] Alternatively, the slope may be adjusted in the second stage by altering the gain resistors R4, R5.

[0046] FIG. 5 represents an alternative second stage which includes a differential amplifier AMP2 in a feedback loop as in the circuit of FIG. 3. However, the second stage illustrated in FIG. 5 differs from that in FIG. 3 in that there is no offset circuit. Instead, the transistor Q12 is connected via a current mirror CM1 to the supply line V_supply. This second stage allows the gradient of the temperature dependent voltage at node N1 to be altered but does not allow it to move negative with negative temperatures. CM2 denotes a second current mirror in the circuit of FIG. 5. The second stage of FIG. 5 nevertheless still makes use of the stable internal voltage supply line V_ddintto supply the differential amplifier AMP2. Table II illustrates the operating parameters of an embodiment of the invention using the stage of FIG. 5.