EP0292070B1 - Current mirror with a high output voltage - Google Patents

Description

The present invention relates to a current mirror comprising a first branch for receiving an input current to be copied and comprising in series a first and a second diode in the forward direction and the main current path of a first transistor whose l the transmitter is connected to a common mode pole, the second diode having a first electrode for receiving the input current to be copied and a second branch for delivering an output current copying said input current and comprising in series the path of main current of a second transistor and a third diode in the forward direction, having a first electrode connected to the base of the first transistor and to the emitter of the second transistor, and a second electrode connected to the common mode pole, and also comprising a fourth transistor, the base of which is connected to the first electrode of the second diode, the collector of which is connected to the it supply voltage pole, and whose emitter is connected to the base of the second transistor.

Such a current mirror is known from European patent application EP-A-0,155,720. The output voltage that such a current mirror can produce is limited, because the copying of the input current is only precise as long as the second transistor is not in avalanche mode.

The invention provides a current mirror in which the output current accurately mirrors the input current for significantly higher output voltages.

For this purpose, a current mirror according to the invention is characterized in that the second branch comprises the main current path of a third transistor whose emitter is connected to the collector of the second transistor and whose collector delivers the output current, as well as a diode preferably Zener connected in reverse between the base of the third transistor and the emitter of the second transistor, and in that it comprises a fourth diode in the direct direction of which a first electrode is connected to a supply voltage pole and a second electrode at the base of the third transistor.

• la figure 1 un miroir de courant du type WILSON de l'art antérieur.
• la figure 2 un miroir de courant selon l'invention.

The invention will be better understood on reading the following description, given by way of nonlimiting example, in conjunction with the drawings which represent:

• Figure 1 a WILSON type current mirror of the prior art.
• Figure 2 a current mirror according to the invention.

According to FIG. 1, a WILSON type current mirror comprises an input branch receiving an input current I _E and comprising the main current path of a transistor T₁, and an output branch crossed by an output current I _s and comprising the main current path of a transistor T₂. The first branch further comprises, in series with the main current path of the transistor T₁, and directly, a diode D₁, represented here in the form of an npn transistor whose base and collector are short-circuited and connected to the base of transistor T₂, and whose emitter is connected to the collector of transistor T₁ whose emitter is connected to the common mode pole.

The second branch further comprises, in series with the main current path of the transistor T₂, and directly, a diode D₂, represented here in the form of an npn transistor whose base and collector are short-circuited and connected to the base of transistor T₁ and the emitter of transistor T₂, and whose emitter is connected to the common mode pole. Let I _b1 and I _{b2 be} the base currents of the transistors T₁ and T₂ respectively.

Ce dernier courant, du fait de l'interconnection entre la base du transistor T₁ et l'anode de la diode D₂, est le même que celui qui traverse la diode D₂ si on suppose que cette diode est réalisée à partir d'un transistor de mêmes dimensions que le transistor T₁.The current arriving at the collector of T₁ has the value I _E -I _b2 and therefore the current flowing in the emitter of T₁ has the value ${\text{I}}_{\text{E}}{\text{-I}}_{\text{b2}}{\text{+ I}}_{\text{b1}}\text{.}$

This last current, due to the interconnection between the base of the transistor T₁ and the anode of the diode D₂, is the same as that which crosses the diode D₂ if it is supposed that this diode is carried out starting from a transistor of same dimensions as the transistor T₁.

d'où :

${\text{I}}_{\text{s}}{\text{= I}}_{\text{E}}{\text{+ 2(I}}_{\text{b1}}{\text{- I}}_{\text{b2}}{\text{) ≃ I}}_{\text{E}}$

Par contre, la tension de sortie maximale qui peut être obtenue au collecteur du transistor T₂ est limitée par la structure de la branche de sortie à une valeur de l'ordre de B_VCEO + V_BE, car lorsque la tension collecteur-émetteur de T₂ atteint la valeur B_VCEO, le fonctionnement n'est plus linéaire (régime d'avalanche), et Is ne recopie plus I_E que de manière approximative.The current flowing through the emitter of transistor T₂ therefore has the value ${\text{I}}_{\text{E}}{\text{-I}}_{\text{b2}}{\text{+ 2 I}}_{\text{b1}}$

from where :

${\text{I}}_{\text{s}}{\text{= I}}_{\text{E}}{\text{+ 2 (I}}_{\text{b1}}{\text{- I}}_{\text{b2}}{\text{) ≃ I}}_{\text{E}}$

On the other hand, the maximum output voltage which can be obtained at the collector of transistor T₂ is limited by the structure of the output branch to a value of the order of B _VCEO + V _BE , because when the collector-emitter voltage of T₂ reaches the value B _VCEO , the operation is no longer linear (avalanche regime), and Is no longer copies I _E only approximately.

However, it is generally desired that the copying precision be of the order of a few%, which implies reconsidering the assembly if one wishes to obtain output voltages greater than B _VCEO .

The basic idea of the invention consists in allowing operation in B _VCB mode by conduction of a diode inducing a negative base current in a transistor of the second branch.

Figure 2 shows how such a function can be realized with npn transistors.

The first branch comprises in series and successively, a transistor D₃ mounted as a diode by short-circuiting its base and its collector which receives the input current I _E , a transistor D₁ mounted as a diode by short-circuiting of its base and its collector which are connected to the emitter of D₃, is a transistor T₁ whose collector is connected to the emitter of D₁, and whose emitter is connected to ground.

The second branch comprises in series and successively, a transistor T₃ whose collector supplies the current Is output copying the input current I _E , and whose emitter is connected (point A) to the collector of a transistor T₂ whose emitter is connected to the base and to the collector interconnected of a transistor D₂ mounted in diode and whose transmitter is connected to ground. The base and the collector of D₂ are also connected to the base of the transistor T₁.

The second branch also includes at least one reverse diode, for example a Zener diode, connected between the base of the transistor T₃ and the emitter of the transistor T₂. The base of transistor T₂ is connected to the emitter of a transistor T₄ whose collector is connected to a voltage source U and the base, to the interconnected collector and base of D₃. A transistor D₄ mounted as a diode by short-circuiting its base and its collector, connected to the supply voltage source U, has its emitter connected to the base of the transistor T₃.

Let U be the value of the supply voltage, and V _BE the value of the emitter-base voltage of a transistor (around 0.7V). Let Vs be the output voltage taken from the collector of transistor T₃.
There are three areas of operation.

1) Vs <U - 2V _BE + B _VCEO (T₃)

car la tension collecteur-émetteur VCE (T₃) est inférieure à ^BVCEO(T₃). ^B VCEO (T₃) designates the avalanche voltage of transistor T₃.
The voltage V _A at point A is constant and is equal to:

${\text{V}}_{\text{AT}}{\text{= U - 2V}}_{\text{BE}}$

because the collector-emitter voltage VCE (T₃) is less than ^B VCEO (T₃).

The voltage across diode Z is also U-2V _BE .

If the Zener voltage V _Z of the diode Z is greater than U-2V _BE , the diode Z is blocked and the current mirror operates in a conventional manner.

β désignant le gain en courant d'un transistor.We then have Is = I _E by neglecting the base current of transistor T₄ which is very close to

β designating the current gain of a transistor.

2) Vs> U - 2V _BE + B _VCEO (T₃) and Vs <V _Z + BV _CEO (T₃) + V _BE .

Le courant de base de T₃, I_b(T₃) s'annule et la tension V_A suit Vs :

${\text{VA = Vs - B}}_{\text{VCEO}}\text{(T₃).}$

La tension aux bornes de la diode Z est voisine de Vs - ${\text{B}}_{\text{VCEO}}{\text{(T₃) - V}}_{\text{BE}}$

et reste donc inférieure à V_Z, ce qui implique que la diode Z reste bloquée
On a : ${\text{Is = I}}_{\text{E}}{\text{+ I}}_{\text{B}}{\text{car I}}_{\text{B}}\text{(T₃) = 0}$

In this case, we have:

${\text{V}}_{\text{THIS}}{\text{(T₃) = B}}_{\text{VCEO}}\text{(T₃).}$

The base current of T₃, I _b (T₃) is canceled and the voltage V _A follows Vs:

${\text{VA = Vs - B}}_{\text{VCEO}}\text{(T₃).}$

The voltage across the diode Z is close to Vs - ${\text{B}}_{\text{VCEO}}{\text{(T₃) - V}}_{\text{BE}}$

and therefore remains less than V _Z , which implies that the diode Z remains blocked
We have : ${\text{Is = I}}_{\text{E}}{\text{+ I}}_{\text{B}}{\text{because I}}_{\text{B}}\text{(T₃) = 0}$

3) Vs> V _Z + B _VCEO (T₃) + V _BE

soit la tension de claquage collecteur-substrat du transistor T₃ si cette dernière est plus faible.Diode Z starts to drive. A current I _B (T₃) <0 can be established and the transistor T₃ begins to work in the area of B _VCB
The more the output voltage Vs increases, the more the current Is goes up the collector-base junction of the transistor T₃ through the diode Z.
The output current Is tends towards I _E + 2I _B.
The maximum value of Vs is either ${\text{B}}_{\text{VCBO}}{\text{(T₃) + V}}_{\text{Z}}{\text{+ V}}_{\text{BE}}\text{,}$

or the collector-substrate breakdown voltage of the transistor T₃ if the latter is lower.

It will also be noted that Vz must be such that the BV _CEO of the transistor T₂ is not reached.

Example: B _VCEO = 27V B _VCBO = 67V B _VCS = 72V V _Z = 7.2V U = 3V I _E = 100µA

Les mesures ont été effectiées avec des résistances de 1kΩ dans les émetteurs de T₁ et D₂.

The measurements were made with resistances of 1kΩ in the transmitters of T₁ and D₂.

The invention is not limited to the embodiments described and shown. Thus, the mentioned Zener diode can be replaced by a reverse diode, or by several diodes in series and in reverse. In this eventuality, it will simply follow that the described operating modes will be less clearly separated.

Claims (2)

1. A current mirror which comprises a first branch for receiving an input current (I_E) to be reproduced and comprising the series arrangement of a first and a second diode (D₁,D₃) poled in the forward direction and the main current path of a first transistor (T₁) whose emitter is connected to common-mode terminal, the second diode (D₃) having a first electrode for receiving the input current (I_E) to be reproduced, and a second branch for supplying an output current (I_S) which is a replica of said input current (I_E) and comprising the series arrangement of the main current path of a second transistor (T₂) and a third diode (D₂) which is poled in the forward direction and which has a first electrode connected to the base of the first transistor (T₁) and to the emitter of the second transistor (T₂) and which has a second electrode connected to the common-mode terminal, and also comprising a fourth transistor (T₄) whose base is connected to the first electrode of the second diode (D₃), whose collector is connected to a power-supply terminal, and whose emitter is connected to the base of the second transistor (T₂), characterised in that the second branch comprises the main current path of a third transistor (T₃) whose emitter is connected to the collector of the second transistor (T₂) and whose collector supplies the output current (Is), and a diode (Z) poled in the reverse direction between the base of the third transistor (T₃) and the emitter of the second transistor (T₂), in that it comprises a fourth diode (D₄), poled in the forward direction and having a first electrode connected to said power-supply terminal and having a second electrode connected to the base of the third transistor (T₃).
2. A current mirror as claimed in Claim 1, characterised in that the diode (Z) is a Zener diode.

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