DE3118617A1 - CURRENT MIRROR SWITCHING WITH HIGH OUTPUT IMPEDANCE AND LOW VOLTAGE LOSS - Google Patents

Description

description

The invention relates to current generators with transistors and in particular ein.Schaltung with current mirror, which above all is suitable to be used in linear integrated circuits with low supply voltage.

The connection of two transistors via current mirrors is known and is often used in integrated circuits.

A circuit of this type is shown in FIG. The two transistors labeled T and Tp are of the same design and are NEN transistors. The base and the emitter of the transistor T _x . are connected to the base or the emitter of the transistor Tp ". The collector of T. is connected both to the common base of the two transistors and, via a resistor R ^, to a supply line + V"

I CC

tied together. Also the collector of Tp is with the supply line connected via a load represented by the resistor R ^.

If one examines the function of this circuit, one sets

V V determines that a current I = - «- flows where-

- a SXy,

at Yqv the tension on the Bani. ^r J-Emitfcerüberp; ang of the transistor T ,, is. If one neglects the base currents I _ß with respect to the colectate currents, which is permissible if the amplification of the trans-

sistors is sufficiently high, the current flowing through the

Branch b flows, equal to I. After the two transistors ~~ a

are equal and have the same base-emitter voltage (Vdg,), is the current flowing through branch d. flows, equal to the current that "flows through the branch b_. The branch _c_ is traversed by a current I _n , which is approximately equal to the current flowing through the branch el, so that I _n = I · Thus flows through the Load Ε- a constant current determined by the predetermined circuit parameters.

A circuit of the same type can be implemented with two transistors which differ from one another with regard to dimensions and structural characteristics, as a result of which I _n = KI with K.

ο a

is reached as a constant. It will also be understood that both transistors will be PNP transistors rather than NPN transistors can.

A known improvement of the current mirror circuit shown in FIG is shown in Figure 2 and relates to the so-called "Wilson mirror", in which one of the two transistors T ^ and T ^ having the mirror circuit is assigned the task, the base current L, of the transistor T ~ and thus its collector current I = ßlgi to control, where ß is the current gain of Tp is;.

The transistor T ₅ acts as a current probe which detects the emitter current I _{E of} the transistor _{current T 2} . The base of the transistor T ^ is connected to the base of the transistor T 1. The collector of T,

is connected to the emitter of T ₀ and directly to the base of T- . The collector of T ,. is connected to the base of Tp, in which the total current of the input current T ^ and the collector current of T. flows together.

With this structure. the reference current I "can be kept constant. Since the collector current of Tp ,! "= ° Κ I", is connected to the emitter current practice "the parameter οζ , which is a function of the collector-emitter voltage Vq _E , the output current I = 1 in turn depends on the value of this voltage from. The-

O C

This circuit therefore does not guarantee the constancy of the output current I.

An ideal power generator is characterized by an infinite Internal resistance and an internal voltage drop that is 0. A real power generator must have properties which come as close as possible to the properties of an ideal generator. A real power generator is therefore the better this approximation is, the better.

The circuit of a real current generator must therefore have a high impedance and a very low "voltage loss" at the output, whereby under the designation "voltage loss" the minimum voltage should be understood that is necessary so that the circuit retains its typical operating characteristics.

A circuit with a "Wilson mirror" has a much higher output impedance than a circuit shown in FIG. 1 with a 'hate emitter', the ratio being '-x : 1,

but their "tension loss" is greater, namely

^V min ~ ^V CE.sat T ₂ ^{+ 7} BE

where V _{CE sat T is} the collector-emitter voltage of transistor T ₂ at the limit of saturation and V _{ßE is} the base-emitter voltage of transistor T-.

One after the. The circuit diagram of FIG. 1 has, however, a voltage loss

CE sat

Attempts have also been made with other known switching schemes to approximate the properties of an ideal current generator as closely as possible. For example, a current mirror circuit is known which is similar to that in Figure 1, with suitable resistors E _{E connected} in series with the emitter of the transistors: this scheme allows the output impedance by a factor (1 + Rg / r) with respect to the to increase the known circuit shown in Figure 1, where r represents the internal resistance of the emitter of the transistor in the output branch, but

-V- "■

at the same time, due to the increased voltage drop R ₁ , I · - across the emitter resistor, the voltage loss is considerable

Another known scheme is the current mirror cascade circuit shown in FIG. In this, the transistor T »is kept in the active operating range, namely by the transistor T ₃ , which is connected as a diode by short-circuiting the base and collector, and the transistor T ^ shapes the emitter current of the transistor T _{4 via the transistor T 3} increasing the output impedance.

However, this scheme does not allow the voltage drop below the value

min ^V BE T ₂ ^{+ V} CE sat

to reduce, where V _{BE T is} the base-emitter voltage of the transistor T ₂ ¹ ^ ^V GE sat T is ^the saturation voltage of the transistor T ^. are. The improvement in output impedance over that of the circuit according to FIG. 1 is equal to that achieved with the Wilson mirror, that is, the output impedance is improved

ß
in a ratio of 5 ^{: 1}

The invention is based on the object of a circuit with To create a current mirror that has a voltage loss that as low as in a current mirror circuit with transistors

is possible, and at the same time has an output impedance that is greater than in known circuits with low voltage loss and which is therefore particularly suitable in integrated Circuits with low supply voltage to be used.

This task is defined with an overxager with Current solved, as it is characterized in the claims and the description.

The invention is explained in more detail below in the description, which shows an embodiment in addition to the three figures already described. Show it:

Figure 1 shows a known current mirror circuit in its simplest Shape,

FIG. 2 shows a current mirror circuit which is known under the name "Wilson mirror",

FIG. 3 shows a known current mirror circuit in cascade form and

Figure A- the scheme of a current mirror circuit according to the invention.

The circuit shown in FIG. 4 has 6 transistors, which are designated by T ^, T ₂ , T- ,, T ^, Tr and Tg and of which T _x ., T ₂ and T ^ - bipolar PNP transistors interfere and T ^ _, T ₁ and Tg are NPN bipolar transistors. . '■

The transistor T ^ is connected to both T ₂ and T ^ according to the current mirror scheme. The circuit thus obtained is delimited in Fig. 4- by dashed lines and. marked with A.

The transistors T ^ and T ₁ - are connected to one another according to the current mirror scheme and are denoted by B.

The collectors of the transistor T ^. and the transistor T ₂ are connected to the collectors of the transistor Tg and T ^, respectively. The collector of the transistor T, - is via a node D with an input current generator I. and the base of the transistor.

T, - connected,
b

The one formed by the transistor Tg and the node D. Circuit is labeled C.

The emitters of the transistors T ^, T ₂ and T ^ are connected to the positive pole + V of a supply voltage generator, while the

Emitter of the transistors T ^, T ₁ - and Tg are connected to the negative pole of this voltage generator.

In the circuit shown in FIG. 4, only two transistors (T 1 and T ₃ ) are assigned to transistor T to form a current mirror circuit, but in general many other transistors in the same configuration can be assigned to transistor T.

Each of the in this way according to the current mirror scheme with T _x . connected transistors forms a current generator that can control other circuits. One of these transistors, for example the transistor Tp in Fig. 4-, is connected to the current mirror B, which takes over the current I _n therefrom and forwards it to the base of Tg, whereby the control of the current I _n with respect to the input current I and , via the transistor Tg, the setting of which becomes possible.

Also the collector current I of T, (and, as I said, the possible other connected transistors) is controlled and automatically set in the same way.

This new circuit with a current mirror therefore allows adjustment of the collector current Iq by controlling it directly becomes, in contrast to the "Wilson mirror".

To simplify the calculation, the EaIl should be considered, in which, in addition to the transistors T ^ u ¹¹ ^ ¹ ^ x ^m ^ ^ ^er base of further n-1 transistors, which are identical with Tp and T, are connected.

The base current of the transistor Tg is ¹ B ₆ - ¹ I - ¹ C ₅ - ¹ X - ^ßI B5

where the emitter current is denoted by L ,, the Kollelrfcor current by I _n and the base current by Lg and im.Index the number of the transistor is added to which this current relates. .

The base current of the transistor T _c is given by the relationship with the current I _n

¹ C ^{= 1} B ^ ^{+ I} B4 ^{+ 1} G ^ ^{= I} B5

linked, so that in the case of Ig ^ »Ig, - · results

¹ C

The base current of the transistor Tg is T - T
^X B6 " ^x ±

The collector current of the transistor T ^, which is in the Stromspicgel A flows is

¹ CG ^{= ßl 1} BI ^{+ 1} BI ^{+ Τ} Β2 ^{+ n 1} B '

where I ₃ denotes the base current of transistor T ^ and the other, similarly connected transistors. To simplify the example, Ig = I ^ p can be set with ß 'the gain of the PNP transistors, which is generally smaller than the gain ß of the NPN transistors of the same integrated circuit. There

I.
^Χ Β2 ⁼ "B ^T »

is the base current of transistor T "

For the collector current of the transistor T ^ - ¹ Co = < ^{n + 1} >"Β-. ¹ C ⁺ ^ ^+1} B ^? = ^ ^{N + 1} >" Β- ¹ C Da Ip ^ - = ßl-g '-, results from the relationships (4) and (2)

SB ¹ C ⁺ ^ ^{+ Λ)} "Β-

As the gain for both the NPIT transistors and

for the PNP transistors is much larger than 2, the approximation applies

ßl, = QI _n + (n + 1) I _n .

Assuming that the number of ' Transistors with respect to the value ß is negligible, results

¹ X ^ ¹ C

In any case, with measures known to those skilled in the art for linear circuits, the influence of the base currents on the Collector currents are minimized, so that relationship (3) is all the more valid.

The circuit according to the invention has a voltage drop

^V min " ^V CE sat T

where with V ^ g. m is the collector-emitter voltage of transistor T ₇ . is indicated when he is working at the saturation limit.

So the tension loss has a constant value that is equal to the minimum achievable in a current mirror circuit with transistors Is worth.

The output impedance of the circuit according to the invention is greater than that of the known circuits which also have a low and constant voltage drop. The circuit according to the invention has an output impedance which is greater by a factor of β than that of the current mirror circuit with a wet-bonded emitter (Fig. 1), 'so that it has a value twice that of the output impedance of the "Wilson mirror" and the "cascade mirror circuit". The circuit of the invention thus approximates the properties of a circuit better than the known circuits Power generator; it consequently allows the use of supply voltages that are smaller than those known for Circuits are required for what specific use cases with low supply voltage is very important.

beyond the described and illustrated embodiment Of course, numerous variants are possible within the scope of the invention. For example, instead of the. direct connections between the base and collector of the transistors T, - and T ^ of the circuit 4- in each case connections via the base-emitter-übergar-ζ; of a transistor. In addition, all or part of the bipolar transistors can be modified with appropriate circuit changes, which are known to the person skilled in the art can be replaced by field effect transistors.

The new current mirror circuit is thus comprising a transistor circuit which is particularly suitable for being integrated a primary current mirror, a secondary current mirror and a circuit device for current comparison and current amplification.

The current at the input of the primary current mirror is mirrored in a number of output branches, each of which has can be connected to a consumer. .

The input of the secondary current mirror is connected to an output branch and thus the output current of the primary current mirror transmits to the output branch, which both to the input terminal of the current to be transmitted and to the Input of the circuit device for comparing and amplifying the current is connected.

This circuit device is therefore a "false amplifier". The "fault current" is amplified and conducted to the input of the primary mirror. Correspond to the changes in the output current opposite changes in the amplified current, which therefore compensate for them.

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Claims (6)

ELISABETH JUNG dr.phil, dipl'phem .: ':: JÜRGEN SCHIRDEWAHN dr "rer.nat..dipl-phys. GERHARD SCHMITT-NIL.SON dr, ing. GERHARD B. HAGEN dr.phil. PETER HIRSCH dipl, ing. PATENT LAWYERS EUROPEAN PATENT ATTORNEYS 8000 MÖNCHEN 40, - PO BOX 40 14 68
CLEMENSSTRASSE 30 TELEPHONE: (089) 34 50 TELEGRAM / CABLE: INVENT MÜNCHEN TELEX: 5-29 686 u.z .: M 2147 M3 (HI / gu) 11. May 1981: SGS-ATES Component! Elettronici SpA Via C. Olivetti, 2nd floor
Agrate (Milan) Italy Current mirror circuit with high output impedance and low voltage loss Claims Current transformer circuit with a first terminal for connection with an input current generator (I ·) »a second and a third connection for connection to the connections of a supply voltage generator and with a main current mirror circuit (A) with transistors, which have an input branch and at least has two output branches, each of which is a consumer circuit is connectable, characterized by a switching device (C) for comparing and amplifying current and a secondary current mirror circuit (B) with transistors, this current mirror circuit having an input branch which is connected to one of the output branches the main current mirror circuit (A) is connected, and an output branch which is connected to a first input terminal the Schaltungseinrichtiang (C) is connected, which one has a second input connection to the input current generator (I.) is connected, and an output terminal, which is connected to the input branch of the main current mirror circuit (A). 2. A circuit according to claim 1, wherein the main current mirror circuit (A) has a first transistor (T,.), A second transistor (Tp) and at least one third transistor (T-,), each of which has a first and a second terminal as well has a control terminal, the control terminal of the second transistor (Tp) and the control terminal of the third transistor (T-) being connected to the control terminal of the first transistor (T ,,) and the first terminal of the first transistor (T ,,), the first terminal of the second transistor (Tp) and the first terminal of the third transistor (T ^) with the same pole of a voltage generator connected and ^being switched on a coupling device between the second terminal and the control terminal of the first transistor (T ,,) 'characterized that the secondary current mirror circuit (B) has a fourth transistor (T ₄ ) and . ^{": V:} - _': ■". " 31T8617 - ■ 3-" a fifth transistor (T ₁ -), each of which has a first and a second connection and a control connection, the control connection of the fifth transistor (Τ ^) being connected to the control connection of the fourth transistor (T ^), the control connection and the second connection of the fourth transistor (T) are connected to each other by a coupling device, the first connection of the fourth transistor (T ^) and that of the fifth transistor (Tc-) are connected to the pole of the voltage generator which is opposite to the pole to which the transistors of the main current mirror circuit (A) are connected, and the second connection of the fifth transistor (T ₁ -) the output branch of the secondary current mirror circuit (B) - and the second connection of the fourth transistor (T ^.) the input branch of the secondary current mirror circuit (B ) are. 3. Circuit according to claim Λ or 2, characterized in that the circuit device (C) for comparing and amplifying the current has a comparator node (D) and a sixth transistor (T _fi ) whose first terminal is connected to the pole of the voltage generator, with which the fourth transistor (T ^) and the fifth transistor (T ₁ -) are connected, whose second connection forms the output branch of the circuit device (C) and whose control connection is connected to the comparator node (D), which is connected to the first and the second input terminal of the circuit device is connected. 4 ·. Circuit according to claims 2 and 3 »characterized in that the first transistor (T.), the second transistor (Tp), the third transistor (T ₇ ,), the fourth transistor (T ^), the fifth transistor (Tj -) and the sixth transistor (T, -) are bipolar transistors, of which the first connection, the control connection and the second connection are the emitter "or the base" or the collector is. 5- Circuit according to claims 2 and 3 »characterized in that the first transistor (T ,,), the second transistor (T ^), the third transistor (T ^), the fourth transistor (T ^), the fifth transistor (T ₁ -) and the sixth transistor (Tg) are field effect transistors, -whose first connection, whose control connection and whose second connection are source and gate or drain. 6. Circuit according to one of the preceding claims, characterized in that the entire circuit in a monolithic Semiconductor block is integrated.