FR2482382A1 - HIGH OUTPUT AND LOW "LOSS OF VOLTAGE" CURRENT MIRROR CIRCUIT - Google Patents

Abstract

TRANSISTOR CIRCUIT, PARTICULARLY CLEAN TO BE INTEGRATED, INCLUDING A MAIN CURRENT MIRROR A, A SECONDARY CURRENT MIRROR B AND A COMPARATOR AND CURRENT AMPLIFIER MOUNTING C. THE CURRENT IA THE MAIN CURRENT MIRROR INPUT IS REFLECTED IN SEVERAL CURRENT OUTPUT, TO EACH OF WHICH CAN BE CONNECTED A USER. THE INPUT OF THE SECONDARY CURRENT MIRROR CIRCUIT B IS CONNECTED TO AN OUTPUT BRANCH WHICH THEREFORE TRANSFERS THE OUTPUT CURRENT I FROM THE MAIN CURRENT MIRROR TO ITS OUTPUT BRANCH WHICH IS CONNECTED BOTH TO THE CURRENT INPUT TERMINAL I TRANSDUCER AND AT THE INPUT OF THE COMPARATOR AND CURRENT AMPLIFIER ASSEMBLY C. THIS COMPARATOR AND AMPLIFIER ASSEMBLY IS THEREFORE AN "ERROR AMPLIFIER (E II). THE" ERROR "CURRENT IS AMPLIFIED AND SENT TO THE INPUT OF THE MAIN MIRROR A VARIATIONS IN THE OUTPUT CURRENT I CORRESPOND OPPOSITE VARIATIONS IN THE AMPLIFIED CURRENT WHICH, THEREFORE, COMPENSATE THEM.

Description

The present invention relates to current generators with transistors

and, more precisely, a current mirror circuit,

particularly suitable for use in internal circuits

  linear sandstones at low supply voltage.

  The mounting of two "current mirror" transistors is known per se and is frequently used in the design of

integrated circuits..

A circuit of this type is shown in FIG. 1. The two transistors, designated by T1 and T2, are similar and

  NPN type. The base and the transmitter of T are linked respectively.

base and issuer of T2. The collector of T1 is connected both to the common base of the two transistors and,

via a resistor R1, to a power line

  tation + V cc. The T2 collector is also connected to the supply line via a load shown

by a resistance RL.

  If we examine the operation of the circuit, we can see that it passes, through branch a ,. a current Vcc - VBE Ia =, VBE being the voltage of the base-emitter junction

R1

  of transistor T1. If one neglects the basic currents IB with respect to the collector currents, which is lawful if the gain of the transistors is sufficiently high, the current which passes through the branch b is equal to Ia. Since the two transistors a are similar and have the same base-emitter voltage (VBE), the current which flows in branch d is equal to that which flows in branch b. The branch c is traversed by a current IC which is roughly equal to that which passes in the branch d, which makes IC = Ia. Consequently, the load RL is traversed by a constant current, determined by parameters

of circuit fixed beforehand.

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A circuit of the same type can be produced with two different transistors from one another as regards the dimensions and the construction characteristics, giving Ic = KIa, K being a constant. In addition, it is obvious that a the two transistors can be of the PNP type instead of NPN. A known improvement to the current mirror circuit shown in FIG. 1 is that of FIG. 2, called "Wilson mirror", in which a "mirror circuit", comprising the transistors T1 and T3, is entrusted with the function of controlling the base current IB of the transistor T2 and, consequently, its current

  of collector IC = BI B being the current gain of T2.

The transistor T3 plays the role of current probe, sensitive to the emitter current I of the transistor T. The base of the

-E 2

  transistor T1 is connected to the base of transistor T 3.The collection

tor of T3 is connected to the emitter of T2 and directly to the base of this same transistor T3 * The collector of T1 is connected to

  the basis of T2, into which consequently the current-

  sum of the input current Ii and the collector current of T With this diagram, the reference current I can be kept constant. However, since the current of E 'collector of T2, Ic = aIE, is linked to the current emitter by the parameter which is a function of the collector-emitter voltage VCE, the output current I0 = Ic in turn depends on the value of this voltage. Such a circuit therefore does not guarantee the constancy of the output current I An ideal current generator is characterized by an infinite internal resistance and by an internal voltage drop equal to zero. A real current generator must have characteristics that are as close as possible to those of an ideal generator. A real current generator is therefore

  the better the greater this approximation.

The circuit of a real current generator must therefore have a high impedance and a very low "voltage loss" at the output, the expression "voltage loss" designating below the minimum voltage necessary for the circuit

  maintain its typical operating characteristics.

The "Wilson mirror" circuit has a much higher output impedance than that of a circuit of the type

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  to "ground transmitter" indicated in fig. i in the B / 2: 1 ratio, but its "loss of tension" is higher; indeed Vmin VCE sat T2 + VBE T

- 2 3

VCE sat T being the collector-emitter voltage of the transistor

T at the limit of saturation and VBE being-the base voltage-

2 B

  emitter of transistor T3. 3 A circuit following the diagram in fig. 1 a on the other hand Vmin VCE sat T2 We therefore sought, with other known diagrams, to approach more the characteristics of the ideal current generator. We know for example a current mirror circuit similar to that which is represented in FIG. 1, in which appropriate resistors RE are connected in series with the emitters of the transistors; such an arrangement makes it possible to increase the output impedance by a factor (1 + _ / re) by

  relative to the known circuit shown in fig. 1, re represented

  both the internal emitter resistance of the transistor of the output branch, but at the same time the increased voltage drop RIE

on the emitter resistance causes the loss of voltage to increase

lies to a corresponding extent.

Another known arrangement is the "cascade" current mirror circuit shown in FIG. 3. In this case, the operation of the transistor T2 is maintained in the active zone, by means of the transistor T3 mounted as a diode with its base short-circuited on the collector, and the transistor T1 imposes, by means of the transistor T2 , the emitter current

of transistor T4, raising the output impedance of it.

  However, this does not reduce the loss of voltage below the value

V V _

  min BE T2 VCE sat T4 VBE T2 being the base-emitter voltage of transistor T and VCE sat T4 being the saturation voltage of transistor T4o

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  The improvement of output impedance compared to the circuit of fig. 1 is the same as that obtained with the mirror

  Wilson, i.e. in the 5/2: 1 ratio.

  The object of the present invention is to produce a current mirror circuit which has the lowest voltage loss that it is possible to obtain in a current mirror transistor circuit and which at the same time has an output impedance. higher than that of known circuits with low voltage loss, so that it is particularly suitable for use in integrated circuits with low supply voltage. -This goal is achieved with a transducer circuit of

  current comprising a first connection terminal to a generator

input current generator, second and third terminals

for connection to the terminals of a supply voltage generator

  tation, and comprising a main current mirror circuit at

transistors, characterized in that it comprises a comparative assembly

  current amplifier and current amplifier, current mirror circuit

  secondary to transistors, this current-mirror circuit comprises

as an input branch connected to one of the output branches of the main mirror circuit, an output branch connected to

one, first input terminal of the comparator and amplifier assembly

  this comparator and amplifier assembly comprising a second input terminal connected to the input current generator and an output terminal connected to the input branch

of the main mirror circuit.

The invention can be clearly understood using the

detailed description which follows, given purely by way of example

  and, therefore, not limiting, with reference to the accompanying drawings which, for the first three of them, have already been

took into consideration.

  Fig. 1 is the diagram of a current mirror circuit

of known type, in its simplest form.

  Fig. 2 -is the diagram of a current mirror circuit

known, called "WVilson's mirror".

  Fig. 3 is the diagram of a known circuit, comprising

  cascading "current mirrors".

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Fig. 4 is the diagram of a current mirror circuit

according to the invention.

  If we look at the circuit diagram in fig. 4, we see that it includes six transistors designated by T1, T2, T3, T4, T5 and T6, among which T1, T2 and T3 are of the type

bipolar PNP and T4, T5 and T6 are of the bipolar NPN type.

Transistor T1 is connected to both T2 and T3

  following the current mirror scheme. The circuit thus constituted

  killed is delimited in fig. 4 by a dash line and it is designated by the letter A. The transistors T4 and T5 are interconnected according to

  the current mirror diagram designated by the letter B. -

  The collector of transistor T1 and that of transistor T2 are respectively connected to the collectors of the transistors

T6 and T4. The collector of transistor T5 is connected to a general

  input current tester Ii and at the base of transistor T6 by junction point D. The circuit formed by transistor T6 and junction point D is designated by the letter C.

  The emitters of the transistors T1, T2 and T3 are connected

  dice at the positive pole + Vcc of a supply voltage generator

tation, those of transistors T4, T5 and T6 are connected to pl1e

  negative of the same voltage generator.

According to the diagram in fig. 4, it is only associated with the transistor TL, to constitute a current mirror circuit,

  than two transistors (T2 and T3), but generally speaking, many

  suddenly other transistors can be associated with transistor T

following the same arrangement.

  Each of the transistors connected in this way to T1 according to the current mirror diagram constitutes a current generator capable of driving other circuits. One of these transistors, for example T2 in the case shown in the figure, is connected to the current mirror B which detects the collector current IC and transfers it to the base of T6, allowing control of the current IC with respect to at the input current Ii and,

by means of T6, its regulation.

Similarly, the collector current I of T3 (and

other transistors possibly connected in the indi-

identical) is automatically controlled and adjusted identically.

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  This new current mirror pe / Met therefore the

  regulation of the collector current IC by controlling it directly-

  unlike what happens in the case of the "mirror

of Wilson ".

  To facilitate the calculation, we will consider the case in which it is connected to the base of T1, in addition to the transistors T2 and T3, nl other transistors similar to T2 and T3, The base current of the transistor T is iO i 6 If - - Zc5 = i IB5 (the suffixes respectively indicating the emitter currents,

  of collector and base IE, Ic, IB, followed by the number of the transis-

tor to which they relate).

15.The base current of transistor T5 is linked to the current

Ic by the relation.

  r = = E5 + IB4 + iC4 = IB5 + IB4 + IF B4 Ic and, consequently, -if IB4 = IB5, the result is: IB5 = 2+ The base current of transistor T6 is therefore (1) iB6 îIi 2 + e TC The collector current of transistor T6 which enters the current mirror A is C6 = i 1 + + 'B2 + n IB

  IB being the base current of transistor T3 and of the other transistors

  tors similarly mounted. To simplify the example, we can set IB IB2 Par ', we designate the gain of PNP transistors (in general lower than the gain of NPN transistors of the same

  integrated circuit). With 1B2 - SC-, the base current of the transis-

tor T1 is - -Bl = + l) C

rB1 = (n +) -

7 - i2482382 The collector current of the transistor then has the value V "+% 1 rC + (n + 1) 'T" C' '; Ic6 = (n + l) - r + (n) - (n + l)% 1a IC (2) and, because IC6 = i6 it results from (1) and (2). That: - ,, 2 Ii = 2 + IC + (n + 1) I Since the gain value is much greater than 2, both for NPN transistors and for PNP transistors, we can apply the approximation i = SIc + (n + l ) IC Provided that the number of transistors mounted in current mirror (A) is negligible compared to the value,, we have Ii CIC (3) Anyway, with arrangements known to the designers of linear circuits, l The influence of the base currents on the collector currents can be made minimal and, consequently, equation (3) will be valid a fortiori. The circuit of the invention has a voltage loss equal to 0 min = VCE sat T VCE sat T designating the collector-emitter voltage of the transistor

T3 when it operates at the saturation limit.

  The voltage loss therefore has a constant value equal to the minimum value that can be obtained in a circuit

with transistor current mirror ...

  The output impedance of the circuit of the invention is higher than that of known circuits which also have a low and constant voltage loss. Indeed, the circuit of the invention has an output impedance. both larger

-8 2482 382

  than that of the current mirror circuit with ground transmitter (fig. 1), that is to say that its value is double that of the output impedance of the "Wilson mirror" and of the circuit "to cascading mirrors ". Consequently, the circuit of the invention is closer to the characteristics of an ideal current generator than the known arrangements; it therefore lends itself to the use of supply voltages lower than those required by the circuits of known embodiments, a characteristic which is very important for certain applications at low

supply voltage.

Although it has been illustrated and described a single example - of the invention, i; It goes without saying that many variants are possible without departing from the scope of the invention. For example, the direct connections * between bases and collectors of the transistors T1 and T4 of the circuit of FIG. 4 can be replaced each by a junction

  base-emitter of a transistor. In addition, the bipo- transistors

  may be replaced in whole or in part by

  field effect transistors, with appropriate modifications

required of the circuit, within the reach of the specialist in the matter. -

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Claims (6)

- CLAIMS - 1. Current transducer circuit, comprising a first connection terminal to an input current generator (Ii), a second and a third connection terminals to the terminals of a supply voltage generator, .and comprising a circuit with main current mirror (A) with transistors, presenting an input branch and - at least two output branches to each of which a user circuit can be connected, characterized in that it comprises a comparator and amplifier assembly current (C) and a secondary current mirror circuit (B) with transistors, this current mirror circuit comprising an input branch connected to one of the output branches of the main mirror circuit (A) and a branch of   output connected to a first input terminal of the comparative assembly   erator and amplifier (C), this comparator and amplifier assembly   erector (C) comprising a second input terminal connected to the generator input current generator (Ii) and a connected output terminal at. the input branch of the main mirror circuit (A).   2. The circuit as claimed in claim 1, in which the main current mirror circuit (A) comprises a first transistor (T1), a second transistor (T2) and at least a third transistor (T3) each comprising a first and a second terminals. and a control terminal, the control terminal of the second transistor (T2) and the third terminal control terminal sistor (T3) being connected to the control terminal of the first transistor (T1), and the first terminal of the first transistor (T1), the first terminal of the second transistor (T2) and the first terminal of the third transistor (T3) being connected to the same pole of a voltage generator, a coupling member being interposed between the second terminal and the control terminal of the 2482382 first transistor IT1}) characterized in that the mirror circuit secondary current (E) includes a fourth (T4) and a cin- quième (T5) transistors each having a first, a second terminal and a control terminal, the control terminal of the fifth   S transistor (T5) being connected to the control terminal of the   third transistor (T4), the control terminal and the second terminal of the fourth transistor (T4) being connected together by a coupling member, the first terminals of the fourth (T4) and   of the fifth (T5) transistors being connected to the pole of the genera-   opposite voltage to that to which the transistors are connected tors included in the main mirror circuit (A), the second terminal of the fifth transistor (T5) being the output branch of the secondary current mirror circuit (B) and the second terminal of the fourth transistor (T4) being the branch d entry to the circuit   with secondary current mirror (B).   3. Circuit according to claim 1 or 2, characterized in that the comparator and current amplifier assembly (C) comprises a comparator node (D) and a sixth transistor (T6) comprising a first terminal connected to the pole of the voltage generator to which the fourth (T4) and the fifth (T5) transistors are connected, a second terminal constituting the output branch of the assembly and a control terminal connected to the comparison node (D), this node being connected to the first and to the second input terminals of the comparator and amplifier assembly. 4. Circuit according to claim 2 or 3, characterized in that the first (T1), the second (T2), the third (T3),   the fourth (T4), the fifth (T5) and the sixth (T6) transis-   tors are bipolar transistors, the first terminal, the control terminal and the second terminal of each of them being   respectively the transmitter, the base and the collector. 5. Circuit according to claim 2 or 3, characterized in that the first (T1), the second (T2), the third (T3),   the fourth (T4), the fifth (T5) and the sixth (T6) transis- tors are field effect transistors, the first terminal, the control terminal and the second terminal of each being respectively the "source", the trigger "and the" drain ".   6. Circuit according to any one of claims 1   to 5, characterized in that the entire circuit is integrated into a   monolithic semiconductor block.