EP0520858A1 - Current mirror functioning at low voltages - Google Patents

Abstract

The invention relates to a current mirror keeping correct characteristics when the current to be copied corresponds to a voltage close to the lower power supply voltage Vss. …<??>This current mirror (1, T1, 2, T2) comprises a voltage feedback circuit (T5, T6, 10) which makes the first transistor (T1) of the reference arm of the mirror copy, in voltage, the second transistor (T2) of the output arm of the mirror. …<??>Applications to circuits functioning at low power supply voltages. …<IMAGE>…

Description

The present invention relates to a current mirror electronic circuit, the architecture of which has been established with a view to obtaining good operation at a low voltage close to the lower supply voltage, and a low pass resistance. By modification of a known current mirror, to which a feedback circuit is added, the output current is kept constant whatever the voltage applied to the terminals of the current mirror according to the invention.

The invention is applicable to circuits produced with different types of transistors: in order to clarify the description, the invention will be explained by relying on a circuit in N-MOS transistors, which does not limit the scope of the 'invention.

• une branche de référence composée d'une source de courant 1, qui fournit un courant I, et d'un premier transistor T1,
• une branche suiveuse ou de recopie composée d'une charge 2 et d'un second transistor T2. Les grilles de T1 et T2 sont réunies entre elles et de plus à la source de courant 1, de sorte que le courant I′ qui traverse la charge 2 recopie le courant I de la source 1.

In itself, the current mirror is well known in analog electronics, and the basic diagram is given in figure 1. In a very simplified way, between two voltage sources V _DD and V _SS are:

• a reference branch composed of a current source 1, which supplies a current I, and of a first transistor T1,
• a follower or feedback branch composed of a load 2 and a second transistor T2. The grids of T1 and T2 are joined together and in addition to the current source 1, so that the current I ′ which flows through the load 2 copies the current I from the source 1.

In fact, this type of current mirror is marred by an error (I ′ = I) due to the gain of the transistors, especially at low gain. This can be remedied by making a Wilson mirror, shown diagrammatically in FIG. 2: on the follower branch is added a transistor T3, the gate of which is connected to the reference branch, between the source 1 and T1. The transistor T3 is counter-reacted by a simple mirror. In this type of assembly, the voltage excursion of point A, between the load 2 and T3, is limited to some 100 mV + V _GS above the "lower" voltage V _SS some 100 mV "corresponds to the drop of voltage across T₃, and V _GS at the voltage drop across T2.

In the improved Wilson mirror of FIG. 3, a transistor T₄ added in the reference branch allows T₁ to work under the same conditions as T₂, by symmetrizing the circuit, because the pair T₃ T₄ imposes the same voltage at points B and C, improving current copying. But in the two cases of Wilson mirrors, there are two transistors in series in the feedback branch.

Thus, the two types of Wilson mirror described only work for output voltages (in A) greater than V _SS + V _GS + some 100 mV, which is too high a value in certain cases, if one holds account that, for MOS transistors, V _GS can reach values as high as 4 or 5 volts, while circuits operate at 1 volt.

This limitation is illustrated by the curves in FIG. 4 which give the current-voltage characteristics of a mirror as a function of different gate-source voltages V _GS , for the output transistor T₂. The dotted curves such as 5 correspond to a simple current mirror (fig. 1) and the solid lines curves to a Wilson current mirror (fig. 2 and 3). We see that Wilson mirrors reach a saturation current (therefore stable) I _{DS sat} only for a higher value of V _Ds , in 7, than for a simple mirror, in 8. Arrows 9 show the offset that exists , for a given voltage V _GS , between a simple mirror and a Wilson mirror: for the latter type, the copying is better, but at the price of a higher V _DSS .

We call V _{DS sat} or V _DSS this threshold value, which according to the prior art is too much higher than V _SS because there are in the follower branch two transistors T₂ and T₃ in series, whose pass resistance R _on is too high.

The object of the invention is to obtain that a current mirror operates with a low voltage V _DSS , above the supply voltage VSS, so as to be adapted to the circuits which themselves operate under a low potential difference between V _DD and V _SS , which does not allow the mirror to operate very above V _SS .

Another object of the invention is to produce a current mirror which has a low pass resistance R _on in its feedback branch, which is moreover a necessary condition for being able to work at a voltage close to V _ss .

These objectives are achieved, according to the invention, by means of a simple current mirror, having only one transistor in its feedback branch, but provided with a voltage feedback which has the particularity that its branch which acts on the mirror copying branch is in fact in parallel with the load, and not in series with it.

More specifically, the invention relates to a current mirror operating at low voltage, comprising, in a reference branch, a current source and a first transistor and in an output branch, a load and a second transistor, the gates of these two transistors being joined and controlled from the current source, this mirror being characterized in that it further comprises a voltage feedback circuit which, for a voltage close to the lower supply voltage requires that the first transistor copies, on its drain, the voltage existing on the drain of the second transistor.

• figures 1 à 3 : trois schémas électriques de miroirs de courant simple et de Wilson, connus, exposés précédemment,
• figure 4 : courbes de caractéristiques 1 (V) de ces miroirs connus, pour plusieurs valeurs de tension de grilles
• figure 5 : schéma électrique d'un miroir de courant selon l'invention,
• figure 6 : schéma équivalent du précédant, pour une tension élevée au dessus de V_SS
• figures 7 et 8 : courbes de caractéristiques I(V) d'un miroir de courant selon l'invention.

The invention will be better understood from the detailed description which now follows of an example of application, supported by the appended figures, which represent:

• Figures 1 to 3: three electrical diagrams of simple current and Wilson mirrors, known, exposed previously,
• Figure 4: characteristic curves 1 (V) of these known mirrors, for several values of gate voltage
• Figure 5: electrical diagram of a current mirror according to the invention,
• Figure 6: equivalent diagram of the previous one, for a high voltage above V _SS
• Figures 7 and 8: characteristic curves I (V) of a current mirror according to the invention.

• une branche de référence, composée d'une source 1, qui fournit un courant I, et d'un premier transistor T1.
• une branche de recopie, composée d'une charge 2 et d'un second transistor T2. Les grilles communes de T1 et T2 sont commandées à partir d'un point D situé entre la source de courant 1 et le transistor T1.

FIG. 5 represents the electrical diagram of a current mirror according to the invention. It comprises, as a basic element, a simple current mirror for which we recognize, by comparison with FIG. 1:

• a reference branch, composed of a source 1, which supplies a current I, and of a first transistor T1.
• a feedback branch, composed of a load 2 and a second transistor T2. The common gates of T1 and T2 are controlled from a point D located between the current source 1 and the transistor T1.

The originality of the mirror of FIG. 5 comes from the fact that it further comprises a voltage feedback circuit, formed by the transistors T5 and T6. The transistor T5 is mounted on the reference branch of the simple mirror, between the current source 1 (point D) and the transistor T1. (point C). the transistor T6 is mounted in parallel with the load 2, that is to say that its source is connected to the point B common to the load 2 and to T2, and that its drain is joined at the point A to an auxiliary current source 10 The grids of the transistors T5 and T6 are combined, and controlled from point A.

The symmetry of the circuit is remarkable: a first simple mirror 1 + T1 + T2 is counter-reacted by a second simple mirror 10 + T 6 + T5, mounted symmetrically so that the feedback branch of one constitutes the branch of other's reference. Only the load 2, mounted in parallel on the current source 10 and T6, breaks the symmetry. We can also consider that the pair of transistors T5 and T6 constitutes a voltage follower which, if neither of the two transistors is blocked, imposes the same voltage at points B and C, which means that the transistors T1 and T2 of the two branches operate under the same conditions.

V_B étant la tension au point B, défini ci-dessus,
V_DSS(T2) étant la tension V_DS à saturation pour le transistor T2.If the source 10 supplies a current I ′, the load 2 is traversed by a current II ′, since the feedback transistor T2 supplies a total current equal to I. The feedback produced by T5 and T6 makes it possible to keep the current of constant output, in the load, when $${\text{V}}_{\text{B}}{\text{- V}}_{\text{SS}}{\text{<V}}_{\text{DSS (T2)}}$$

V _B being the voltage at point B, defined above,
V _DSS (T2) being the voltage V _DS at saturation for the transistor T2.

• 1. Lorsque V_B = V_DD , T₆ est bloqué parce que son V_GS = 0 et le point A est tiré vers V_DD , par la source auxiliaire 10. T₅ se comporte dans ce cas comme un interrupteur conducteur à faible R_on. Dans ces conditions, le schéma se simplifie et devient celui représenté en figure 6. Si la résistance R_on de l'interrupteur T₅ est suffisamment faible, elle peut être négligée, et le schéma du miroir de courant selon l'invention est équivalent à celui d'un miroir simple, en figure 1.
• 2. Lorsque V_B s'abaisse et atteint $${\text{V}}_{\text{B}}{\text{= V}}_{\text{DD}}{\text{- V}}_{\text{DSS}}{\text{(10) - V}}_{\text{GS}}\text{(T6)}$$
  
  (V_DSS (10) étant la chute de tension dans la source de courant 10, qui est elle-même réalisée au moyen d'un transistor), le courant I′ débité par la source 10 est. rétabli et T6 redevient conducteur. Le courant qui traverse la charge 2 diminue et devient I-I′. Cette diminution du courant dans la charge 2 ne présente aucun inconvénient, puisque l'objet de l'invention est de travailler très près de l'alimentation négative V_SS , et non pas à proximité de l'alimentation positive V_DD.
• 3. Lorsque V_B continue de baisser, et atteint $${\text{V}}_{\text{B}}{\text{= V}}_{\text{SS}}{\text{+ V}}_{\text{DSS}}\text{(T2)}$$
  
  la paire de transistors T5 et T6 se comporte comme un suiveur de tension et recopie la tension V_B au point C, situé entre T1 et T5 sur la branche de référence du miroir de courant. Ceci garanti que les transistors T1 et T2 entrent simultanèment en fonctionnement ohmique.
  Considérons alors le comportement de T1 dans la branche de référence, illustré par la figure 7, La source de courant 1 impose un courant I, mais T5 + T6 imposent la tension au point C : la caractéristique de T1 se déplace du point P au point P′, sur la figure 7, en baisse puisque par définition V_B baisse. Il s'ensuit que la tension de grille de T1 augmente de V_GS3 à V_GS4, par exemple. Mais, par construction d'un miroir de courant, cette même tension V_GS4 est appliquée sur la grille de T2, et le courant de sortie dans la charge 2 reste constant bien que le transistor T₂ soit entré en fonctionnement ohmique, puisque le point P′ est sur la partie linéaire de la caractéristique I (V)
• 4. Si la tension V_B continue à diminuer, et donc à se rapprocher de V_SS , la tension de grille V_GS de T1 continue à augmenter, mais également la tension V_D du point D situé entre la source de courant 1 et le transistor T5. V_D est tiré à V_DD et lorsqu'il atteint cette valeur, la contre-réaction cesse de fonctionner, et le courant de sortie décroît. Le générateur de courant 1 ne fonctionne plus, ni le miroir de courant, mais néanmoins ce dernier à fonctionné jusqu'à une valeur peu élevée au dessus de V_SS.

The operation of this current mirror can be explained by considering the voltage V _B at point B, which will be assumed to decrease progressively from V _DD to V _SS .

• 1. When V _B = V _DD , T₆ is blocked because its V _GS = 0 and point A is pulled towards V _DD , by the auxiliary source 10. T₅ behaves in this case like a conductive switch at low R _on . Under these conditions, the diagram is simplified and becomes that shown in FIG. 6. If the resistance R _on of the switch T₅ is sufficiently low, it can be neglected, and the diagram of the current mirror according to the invention is equivalent to that of a simple mirror, in figure 1.
• 2. When V _B lowers and reaches $${\text{V}}_{\text{B}}{\text{= V}}_{\text{DD}}{\text{- V}}_{\text{DSS}}{\text{(10) - V}}_{\text{GS}}\text{(T6)}$$
  
  (V _DSS (10) being the voltage drop in the current source 10, which is itself produced by means of a transistor), the current I ′ supplied by the source 10 is. restored and T6 becomes conductive again. The current flowing through the load 2 decreases and becomes II ′. This reduction in the current in the load 2 presents no drawback, since the object of the invention is to work very close to the negative supply V _SS , and not close to the positive supply V _DD .
• 3. When V _B continues to fall, and reaches $${\text{V}}_{\text{B}}{\text{= V}}_{\text{SS}}{\text{+ V}}_{\text{DSS}}\text{(T2)}$$
  
  the pair of transistors T5 and T6 behaves like a voltage follower and copies the voltage V _B at point C, located between T1 and T5 on the reference branch of the current mirror. This guarantees that the transistors T1 and T2 enter ohmic operation simultaneously.
  Let us then consider the behavior of T1 in the reference branch, illustrated by Figure 7, The current source 1 imposes a current I, but T5 + T6 impose the voltage at point C: the characteristic of T1 moves from point P to point P ′, in Figure 7, decreasing since by definition V _B decreases. It follows that the gate voltage of T1 increases from V _GS3 to V _GS4 , for example. But, by construction of a current mirror, this same voltage V _GS4 is applied to the gate of T2, and the output current in the load 2 remains constant although the transistor T₂ has entered ohmic operation, since the point P ′ Is on the linear part of the characteristic I (V)
• 4. If the voltage V _B continues to decrease, and therefore to approach V _SS , the gate voltage V _GS of T1 continues to increase, but also the voltage V _D of the point D located between the current source 1 and the transistor T5. V _D is drawn at V _DD and when it reaches this value, the feedback stops working, and the output current decreases. The current generator 1 no longer works, nor the current mirror, but nevertheless the latter has operated up to a low value above V _SS .

FIG. 8 represents some characteristic curves I (V) of the current mirror according to the invention, for 4 values of V _GS different from each other. In the same figure are shown in dotted lines, the corresponding curves of a simple current mirror, for the same values of V _GS . The arrows 11 show the difference which exists between the characteristics of a known mirror (dotted lines) and those of the invention (solid lines). It can be observed that, unlike Wilson mirrors (FIG. 4) for which there is an increase in V _DSS compared to a simple mirror, there is according to the invention a reduction in V _DSS : the current mirror according to the invention operates at a voltage close to V _SS , even if V _DS of T2 is less than V _DSS .

The current mirror according to the invention is used as an interface with circuits operating under low voltage, for example TTL, or as a switch with low pass resistance.

The invention is specified by the following claims.

Claims (6)

1 - Current mirror operating at low voltage, comprising, in a reference branch, a current source (1) and a first transistor (T1) and, in an output branch, a load (2) and a second transistor ( T2), the gates of these two transistors (T1, T2) being joined together and controlled from the current source (1), this mirror being characterized in that it also comprises a voltage feedback circuit ( T5, T6, 10) which, for a voltage close to the lower supply voltage (V _SS ) requires that the first transistor (T1) copy, on its drain, the voltage (V _B ) existing on the drain of the second transistor (T₂). 2 - Current mirror according to claim 1, characterized in that the feedback circuit comprises a third transistor (T5), interposed between the current source (1) and the first transistor (T1), and a fourth transistor (T6 ), inserted between an auxiliary current source (10) and the second transistor (T2), the gates of the third (T5) and fourth (T6) transistors being joined together and controlled from the auxiliary current source (10) 3 - Current mirror according to claim 2, characterized in that the feedback circuit (T5, T6, 10) constitutes a second current mirror, the feedback branch (T5) of which controls the reference branch of the mirror. main current (T1, T2, 1). 4 - Current mirror according to claim 2, characterized in that the voltage feedback circuit (T6,10) is mounted in parallel with the load (2) 5 - Current mirror according to claim 4, characterized in that the output branch (2, T2) has only one transistor (T2). 6 - Current mirror according to claim 1, characterized in that, due to the voltage feedback circuit, the second transistor (T2) retains a drain-source current characteristic I _DS saturated for a drain-source voltage value V _DS lower than the saturation value V _{DS sat} .

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