EP0675422B1 - Regulator circuit generating a reference voltage independent of temperature or supply voltage - Google Patents

Description

The present invention relates to a regulator circuit providing a stabilized voltage, circuit which is connected between a supply terminal and a reference terminal and includes in particular four transistors of the same polarity each having a transmitter, a base and a collector, a first transistor whose emitter is coupled to the reference terminal through a first resistor, a second transistor whose emitter is connected to the reference terminal, the bases and the collectors of the first and second transistors being connected in cross coupling, a third transistor whose emitter is connected to the collector of the first transistor, its base and its collector connected together at one end of a second resistance, and a fourth transistor whose emitter is connected to the collector of the second transistor, and the base connected to the base and to the collector of the third transistor, circuit in which the surface emitter of the first transistor is larger than that of the third transistor.

Such a regulator circuit based on a four cell transistors of the same polarity, is known from document EP-A-0329232. In this document it says that this basic four-transistor cell can supply either a plurality of stabilized current sources or else a voltage source independent of the supply voltage and the temperature. As noted in this document, such sources stabilized current or voltage can be achieved using bipolar transistors only of NPN type. It follows that such circuit can react quickly to variations in supply voltage or to variations in the current consumed at the output.

However, the known regulator circuit does not take into account the base currents of the transistors so the accuracy of the stabilized voltage obtained remains affected by qualified errors second-order errors.

The invention proposes to provide an improvement to a regulator providing a stabilized voltage which is less sensitive still at the value of the supply voltage on either side of a rated voltage, which exhibits high rejection of noise from the supply voltage and which remains stable compared to temperature variations.

Indeed, according to the present invention, a regulating circuit of the type indicated in the introductory paragraph is characterized in that the circuit further comprises a fifth bipolar transistor similarly polarity than the aforementioned transistors, having a transmitter connected to the collector of the fourth transistor, a base coupled to its collector at through a basic resistance of value at least equal to twice the value of the second resistance, and in that the knot connecting this basic resistance to the collector of this fifth transistor is on the one hand coupled to the other end of the second resistor and secondly, coupled to the power supply terminal through a current source.

As will be discussed in more detail below, the presence of the fifth transistor, provides compensation for certain base currents, compensation which had been neglected in the circuit known. To do this, the basic resistance of the fifth transistor is chosen from a value which is related to the value of the second resistance.

According to a first embodiment of the invention, the connection between the emitter of the fifth transistor and the collector of the fourth transistor constitutes an output of the stabilized voltage.

The value of this stabilized voltage is particularly independent of the supply voltage and has a rejection rate high noise contained in the supply voltage.

Advantageously, the second, fourth and fifth transistors have an identical emitter surface. Regarding the third transistor, it is known that its emitter surface must be provided as being a submultiple of the emitter surface of the first transistor, this the latter being, in practice, constituted by the association of a plurality of identical transistors, connected in parallel, each of which is equivalent construction and matched to the third transistor.

To simplify, the third transistor can also have a emitter area equal to that of the second, fourth or fifth transistors.

According to a second embodiment of the invention, the circuit regulator is characterized in that it further comprises a sixth transistor and a seventh transistor, with the same polarity as the transistors previous, the sixth transistor, connected in diode being inserted in the direct direction between the other end of the second resistor and the source of current while the seventh transistor has its base connected to the emitter of the fourth transistor, its collector coupled to the terminal power supply, and its transmitter, which provides an output of the voltage stabilized, is coupled to the reference terminal through a resistor transmitter.

This embodiment has an output impedance of the lower stabilized voltage and therefore allows current consumption higher output, compared with the implementation mode previous. Another advantageous feature is that the collector of the seventh transistor can also be another circuit output regulator providing a stabilized reference current with respect to the supply voltage and temperature.

Since the regulator circuit according to the invention can be realized only using NPN type bipolar transistors, it is able to react at high frequency, in particular to reject the output supply voltage fluctuations, high frequency. For further increase this rejection power, vis-à-vis the voltage noise power supply, the regulator circuit according to the invention is advantageously completed with a capacity connected in parallel between the bases of the fifth transistor and the second transistor.

The capacity in question may be of low value (a few pF per example) to be integrated with the regulator circuit, its effect is finding multiplied by the gain of the second transistor. We see that the rejection power vis-à-vis the noise of the supply voltage in as a function of the frequency of this noise, increases with frequency from of a certain frequency value, of the order of 1 MHz. This property contrast with the behavior of regulating circuits of the prior art using a high gain error amplifier which must be stabilized in frequency. On the contrary, such regulating circuits have a power noise rejection which decreases beyond a limit frequency, actually corresponding to the frequency from which the amplifier error is voluntarily limited in gain.

According to a simplified embodiment of the regulator circuit according to the invention, the current source supplying the regulator circuit to from the supply terminal is reduced to a resistance. For some reasons for saving the supply current, especially in battery-powered applications, it may be advantageous to be able to completely deactivate the regulator circuit, which can be achieved when the current source is produced using a resistor in series with a MOS field effect type switch transistor.

Other types of current sources can also be used works, in particular, sources ensuring a pre-regulation of the current supplying the regulator circuit.

La figure 1 représente un schéma du circuit régulateur selon un premier mode de mise en oeuvre de l'invention.

la figure 2 représente un diagramme de réjection du bruit de la tension d'alimentation à la sortie du circuit régulateur, en fonction de la fréquence de ce bruit,

la figure 3 montre un schéma de principe d'un certain type de circuit régulateur connu et la figure 4, un diagramme du gain en fonction de la fréquence, pour un amplificateur d'erreur contenu dans un tel circuit connu,

la figure 5 représente le schéma d'un deuxième mode de mise en oeuvre du circuit régulateur selon l'invention, et

les figures 6, 7 et 8 donnent des schémas d'exemples de sources de courant qui peuvent être utilisées dans le circuit régulateur de l'invention.

The description which follows with reference to the appended drawings, given by way of nonlimiting example, will make it clear what the invention consists of and how it can be implemented.

FIG. 1 represents a diagram of the regulator circuit according to a first embodiment of the invention.

FIG. 2 represents a diagram for rejection of the noise of the supply voltage at the output of the regulator circuit, as a function of the frequency of this noise,

FIG. 3 shows a block diagram of a certain type of known regulator circuit and FIG. 4, a diagram of the gain as a function of frequency, for an error amplifier contained in such a known circuit,

FIG. 5 represents the diagram of a second embodiment of the regulator circuit according to the invention, and

Figures 6, 7 and 8 give diagrams of examples of current sources which can be used in the regulator circuit of the invention.

The regulator circuit of FIG. 1 is supplied between a positive supply voltage terminal 1 having a voltage Vcc and a reference terminal 2 carrying a voltage VEE (ground). This circuit comprises a first transistor T1 whose emitter is coupled to the terminal of reference 2 through an emitter resistor R1, a second transistor T2 whose transmitter is also connected to reference terminal 2, the transistors T1 and T2 have their bases and their collectors interconnected in cross-coupling. A third transistor T3 has its emitter connected to the collector of the first transistor T1, its base and its collector combined to form a diode configuration are connected on the one hand, to a first end of a second resistor R2, as well as at the base of a fourth transistor T4 whose emitter is connected to the collector of the second transistor T2. The four transistors T1 to T4, have the same polarity, here NPN type, and the emitter area of the first transitor T1 is n times larger than that of the third transistor T3. T2 and T4 transistors preferably have an identical emitter surface which can also be equal to that of transistor T3. The other end of the second resistor R2 is coupled to the positive supply terminal 1 through a source of current 11 which here is simply constituted by a resistor, in this example. The connection between the current source 11 and the resistor R2 forms a line 12 to which is connected a resistor R5 supplying the base of a fifth transistor T5, which has its collector connected to the line 12 and its emitter connected to the collector of the fourth transistor T4.

The node connecting the emitter of transistor T5 to the collector of transistor T4, here constitutes the output of the regulator circuit and provides a stabilized voltage Vref.

In a first analysis of operation which is rough, the basic currents of all the transistors are neglected. So we can admit that in the branch formed by the current paths of the transistors T1 and T3 and resistors R1 and R2 flows a current I1. Likewise in the branch formed by the current paths of the transistors T2, T4 and T5 flows another current I2. Furthermore, it is known that the mounting of the four transistors T1 to T4 produces a current I1 whose value is proportional to the absolute temperature and only depends on the value of resistance R1 and the emitter area ratio between transistor T1 and transistor T3.

This property will be briefly recalled by evaluating in two ways, the value of the basic voltage of the transistors T3 and T4. Let Vy be this tension: Vy = V BE (T4) + V BE (T1) + R1.I1 Vy = V BE (T3) + V BE (T2) expressions in which V _BE (Tx) indicates the base / emitter voltage of a transistor Tx.
He comes R1.I1 = V BE (T3) + V BE (T2) - V BE (T4) - V BE (T1)

As the transistors T2 and T4 are identical and traversed, as a first approximation, by the same current I2, the terms V _BE (T2) and V _BE (T4) compensate each other. He stays : R1.I1 = V BE (T3) - V BE (T1) or again, using: V BE (T3) - V BE (T1) = kT q Ln (( J (( T 3 ) J (( T1 ) ) expression in which J (T3) and J (T1) are the current densities in the emitters of T3 and T1, k is the Boltzmann constant, T the absolute temperature and q the charge of the electron. I1 = kT qR1 Ln (( J (( T 3 ) J (( T1 ) ) Let n be the ratio of the emitter surfaces of these transistors, traversed by the same current I1, the relation (1) can be written: I1 = kT qR1 Ln (( not ) Expression (2) checks the proportionality between I1 and the absolute temperature T.

The current source 11 constitutes a very current source imperfect in which flows a current which varies with the voltage Vcc supply. Thus, the tension of line 12 being practically fixed by the sum of the base / emitter voltages of transistors T2 and T3 increased by the voltage drop in resistor R2 due to the current I1, the current I2 simply results from the difference between the current delivered by the current source 11 and the current I1. Always in the same hypotesis, where the basic currents are neglected, the transistor T5 presents to its transmitter a voltage deduced from the voltage Vx by subtracting a base / emitter voltage of this transistor which outputs the current I2.

Now, transistor T5 is chosen as having a surface emitter equal to the emitter surfaces of the transistors T2 or T4 so that the base / emitter voltage drop in transistor T5 compensates for the voltage drop in transistor T2. It follows that the tension of Vref output of the circuit is substantially equal to the sum of a fall of voltage I1.R2 with a positive coefficient of temperature and a base / emitter voltage of transistor T3 traversed by a current I1, which base / emitter voltage has a negative temperature coefficient. The resistance value R2 is chosen so that the two components of the sum of the voltages have temperature coefficients that cancel each other out. In practice it is usual to use a voltage drop I1.R2 whose value is around 500mV.

From this rough initial analysis, it follows that the output voltage Vref of the regulator circuit is independent of the temperature and the value of the current I2, that is to say, independent of the value of the supply voltage Vcc. According to a more detailed analysis, which takes into account the base currents of the different transistors, it appears that the current passing through the resistor R2 is approximately equal to the current I1 passing through the transistor T1 increased by the base current of transistor T2 and the basic current of transistor T4, which lead to an increase in the voltage drop in the resistor R2 initially calculated.

The base current of transistor T5 being, firstly approximation, substantially equal to the base current of transistor T4 or of base current of transistor T2, compensation for the aforementioned effect on the voltage Vx of line 12 should be obtained when the resistance R5 inserted in the base of transistor T5 is equal to twice the value of resistance R2. Thus the increase in voltage Vx should be compensated at the output of the regulator circuit.

However, it appears in practice that this compensation is somewhat insufficient, in particular because a variation in the base current of the transistor T2 induces a very slight variation in the base / emitter voltage of transistor T3, a variation which had been neglected in the calculations previous. Improved insensitivity of the output voltage Vref at variations in the supply voltage Vcc can be obtained by increasing the value of resistance R5, the value of which is then understood between 2 and 4 times the value of resistance R2. The optimal value can be determined by an appropriate calculation and better still using a simulator.

For a reason of symmetry of operation of the circuit, we choose a value from current source 11 such as for a voltage nominal Vcc supply, the currents I1 and I2 are substantially equal. For values of the supply voltage Vcc which deviate from the nominal value and at a given temperature, the current I2 varies, but as we have seen previously, the stabilized voltage obtained Vref is only very slightly disturbed.

Like all the transistors implemented in the circuit described in example are NPN type transistors, the regulator circuit is able to react to fluctuations in supply voltages even when these fluctuations are at high frequencies.

The rejection of the noise contained in the supply voltage Vcc, can be further improved in a preferred embodiment according to which the base of transistor T5 is coupled to the base of transistor T2 at using a capacity C. This capacity can be easily integrated from the makes a low value enough. Its effect, as a first approximation, is multiplied by the gain of transistor T2.

According to this embodiment, the rejection rate R of noise at the output of the regulator circuit from the noise presented by the supply voltage Vcc is represented in FIG. 2 curve A, in function of the frequency F of this noise. We observe a peculiarity advantage of the regulator circuit according to the invention because the rate of rejection increases beyond a certain limit frequency. This property remarkable is particularly interesting when the regulator circuit is used in applications where it is integrated with switched circuits at high frequencies, for example dividers of frequencies which produce high frequency noise on the voltage feed.

Figure 3 shows very schematically the principle behind many known regulator circuits. On the one hand it has a cell 30 with two transistors whose emitter surfaces are uneven, intended for deliver a current proportional to the temperature on a resistance of compensation R. The collectors of the transistors debit on loads paired, symbolically represented by a set 31. The circuit also comprises a differential amplifier 32, with high gain, of which the output feeds the combined bases of the two transistors, all arranged so that the collector currents of the transistors are equal. The amplifier 32 is therefore an error amplifier and thus, the voltage the reference Vref at the output of this amplifier is all the more precise that the amplifier gain is high. He is well known, that such an amplifier needs to be frequency stabilized and has therefore a gain curve G whose shape is represented in the figure 4.

Correlatively, the power of noise noise rejection R power supply, for a regulator circuit of this type, has a shape inverse to that of gain, such as that indicated by curve B in dashes, of figure 2. It is clear that from the point of view of the rejection of the noise, the circuit according to the invention is very advantageous in applications where high frequency noise is present.

FIG. 5 represents the diagram of a second mode of implementation of the invention.

In this figure, the elements corresponding to those of the circuit of FIG. 1 are assigned the same reference signs. The circuit of FIG. 5 shows all the elements of the circuit of FIG. 1 to which are added a sixth transistor T6 and a seventh transistor T7 of the same polarity as the transistors T1 to T5. The transistor T6 is connected as a diode, its emitter-collector path is inserted between the resistor R2 and the line 12. The voltage Vx of the line 12 is thus increased by the value of a V _BE compared to the example previously described .

The transistor T7 has its base connected to the node joining the emitter of the transistor T5 at the collector of transistor T4. Its transmitter is coupled to the reference terminal 2 through an emitter load resistor R7. The transistor T7 is therefore arranged as a follower emitter and provides on its transmitter stabilized voltage Vref. The base / emitter voltage drop of T7 compensates, as a first approximation, for the voltage drop in the transistor T6 so that the voltage Vref is again practically identical to that obtained previously with the circuit of FIG. 1.

According to this mode of implementation, the output impedance of the circuit is lower than before and a higher current can be drawn to the output.

The collector of the transistor is represented as being supplied by a terminal 17. This can be connected directly to line 12 or to supply terminal 1. However, the circuit shown can also provide a stabilized reference current Io, absorbed by the collector of transistor T7. Terminal 17 then constitutes such an output of the regulator circuit.

It is clear that the current Io is independent of the voltage supply and temperature since it is deduced from the current emitter of transistor T7, which creates a stable voltage drop Vref, in resistance R7. The collector current of transistor T7, of NPN type whose gain is high, is little different from the emitting current and from this fact, little affected by variations in gain as a function of temperature.

Of course, the current source 11 presented as a resistor so-called limitation in Figure 1 is only a simplified example and we could also use any other current source provided with means ensuring, for example, even rough pre-regulation of the current supplying the two branches of the regulator circuit. In applications where the voltage regulator circuit is not used continuously, it it is desirable to be able to deactivate the regulator circuit when its use is not required, so as to save power consumption.

Figure 6 shows an example of substitution of the source of current 11 of FIG. 1 by a resistor 21 and transistor assembly MOS field effect 22. By an appropriate command applied to terminal 23 connected to the gate of transistor 22, a current source can be produced switchable which has a resistance equal to the sum of the value of the resistor 21 and the internal resistance of transistor 22 when it is driver.

FIG. 7 represents another example of current source 11, provided with means ensuring a pre-regulation of the supply current the entire regulator circuit.

Two resistors 31 and 32 are connected in series between the supply terminal 1 and the line 12. The common point between these resistors has its voltage V _D regulated by the effect of four diodes D1 to D4, connected in series between this point and the reference terminal 2.

Although the forward voltage of these diodes varies a little as a function of the temperature and as a function of the current flowing through them, this variation remains small so that the current delivered by the current source 11 is mainly controlled by the resistor 31 and the voltage difference V _D -Vx which varies little as a function of variations in Vcc.

Figure 8 shows yet another example of a current source 11 using at least one PNP type transistor T8, ensuring by all known means, a pre-regulation of the current delivered by its path transmitter / collector.

The use of a PNP type transistor has the disadvantage that the parasitic capacitance of such a transistor is generally large which is unfavorable from the point of view of voltage noise rejection feed. To reduce this effect, a resistor 41 is inserted between the collector of transistor T8 and line 12 so as to reduce the effect of the stray capacitance of transistor T8.

It is clear that the current sources described in connection with the Figures 6, 7 and 8 are only examples and the specialist is able to to imagine other combinations in particular which use the transistor switch 22 of Figure 6 when useful. The examples of regulating circuits of FIGS. 1 and 5 are susceptible of variants without however, depart from the scope of the invention as claimed below.

Claims (9)

1. A control circuit supplying a stabilized voltage (Vref), which circuit is connected between a supply terminal (1) and a reference terminal (2) and notably includes four transistors of the same conductivity type, each having an emitter, a base and a collector, a first transistor (T1) having its emitter coupled to the reference terminal (2) via a first resistor (R1), a second transistor (T2) having its emitter connected to the reference terminal (2), the bases and the collectors of the first and the second transistor being cross-coupled, a third transistor (T3) having its emitter connected to the collector of the first transistor (T1) and having its base and its collector connected together to one of the ends of a second resistor (R2), which second resistor has its other end coupled to the supply terminal(1), and a fourth transistor (T4) having its emitter connected to the collector of the second transistor (T2) and having its base connected to the base and to the collector of the third transistor (T3), in which circuit the emitter area of the first transistor is larger than that of the second transistor, characterized in that the circuit further comprises a bipolar fifth transistor (T5) of the same conductivity type as the afore-mentioned transistors, which fifth transistor has an emitter connected to the collector of the fourth transistor (T4), a base coupled to its collector via a base resistor (R5) whose value is at least equal to twice the value of the second resistor (R2), and in that the node (12) between this base resistor and the collector of this fifth transistor is, on the one hand, coupled to the other end of the second resistor (R2) and is, on the other hand, coupled to the supply terminal (1) via a current source (11).
2. A control circuit as claimed in Claim 1, characterized in that the second, the fourth and the fifth transistor have equal emitter areas.
3. A control circuit as claimed in Claim 1 or 2, characterized in that the connection between the emitter of the fifth transistor (T5) and the collector of the fourth transistor (T4) forms an output for the stabilized voltage (Vref).
4. A control circuit as claimed in Claim 1 or 2, characterized in that it further comprises a sixth transistor (T6) and a seventh transistor (T7) of the same conductivity type as the preceding transistors, the diode-connected sixth transistor (T6) being poled in the forward direction between the other end of the second resistor (R2) and the current source (11), while the seventh transistor (T7) has its base connected to the emitter of the fifth transistor (T5), has its collector coupled to the supply terminal (1), and has its emitter, which forms an output for the stabilized voltage (Vref), coupled to the reference terminal (2) via an emitter resistor (R7).
5. A control circuit as claimed in Claim 4, characterized in that, in addition, the collector of the seventh transistor (T7) forms an output of the control circuit, which output supplies a stabilized reference current (Io).
6. A control circuit as claimed in any one of the Claims 1 to 5, characterized in that, in addition, a capacitance (C) is connected between the base of the fifth transistor (T5) and the base of the second transistor (T2).
7. A control circuit as claimed in any one of the Claims 1 to 6, characterized in that the current source (11) comprises a so-called limiting resistor (21), (31), (41).
8. A control circuit as claimed in Claim 7, characterized in that a switching transistor (22) of the MOS-FET type is arranged between the limiting resistor (21) and the supply terminal (1).
9. A control circuit as claimed in Claim 7 or 8, characterized in that the current source further comprises means which preregulate the current applied to the control circuit.

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