GB2267404A - Current mirror circuit with cascode output to alleviate Early effect - Google Patents

Abstract

A current mirror circuit includes: a first transistor (Q1) to which an input current is inputted; a second transistor (Q2), which is diode-connected and a base of which is connected to a base of the first transistor; a third transistor (Q3) for taking out an output current, which is connected in series with the second transistor such that a base of the third transistor is connected to a collector of the first transistor, and an emitter of the third transistor is connected to a collector of the second transistor; a fourth transistor (Q4) which is cascode-coupled to the third transistor, for outputting the output current therethrough; and a constant voltage source (Vconst) connected between a base of the fourth transistor and the emitter of the third transistor. <IMAGE>

Description

2267404 CURRENT MIRROR CIRCUIT The present invention is related with a

current mirror circuit used for various kinds of amplifier circuits.

There is a current mirror circuit as a basic circuit consisting of bipolar transistors. A basic current mirror circuit is constructed such that a standard input current is flown f rom the side of a collector of a transistor which collector and base are shorted to form diode connection, and the connection point where the collector and the base are shorted, is connected to a base of another transistor which is arranged in the vicinity and has the same characteristic.

The above mentioned basic current mirror circuit theoretically operates by the right mirror coefficient. However, in f act, this circuit has the problem that a change of the output current is caused by the influence of the Early effect. The Early ef fect is a phenomenon as following. Namely, when the collector-emitter voltage VCE of the transistor increases, the depletion layer is spread by the decrease of the electronic density, so that the substantial base width becomes narrow. Thus, as the collector voltage increases, the increasing tendency of the collector current becomes outstanding.

There is a feedback type current mirror circuit as shown in FIG. 5, as a highly precise current mirror circuit, which is intended to restrain the change of the output current by the influence of the Early effect. This current mirror circuit is based on a current mirror circuit of Willson type, such that a transistor Q4 of the PNP type of the base ground type, is cascode-coupled to an output transistor Q3 of the PNP type. A constant voltage sourceVC111ST 'Sconnected between the emitter of a transistor Q2 and the base of the transistor Q4. This type of circuit is often used for audio amplifier circuits. In this manner, by connecting the constant voltage source VCONST with the transistor Q4, and by clamping the change of the collector- emitter voltage VCS3 of the transistor Q3 by use of the constant value of the constant voltage source VCONST and the constant value of the base-emitter voltage V,,, (about 0. 6 V) of the transistor Q4 and the transistor Q21 the circuit is intended to restrain the change in the output current due to the Early effect.

However, since the current mirror circuit shown in FIG. 5 is equipped with the transistor Q4 of the base ground type at the output stage, the input and-output characteristic is not necessarily sufficient from the view point of the linearity, although it can obtain a high output impedance. Especially, this non-linear distortion becomes a serious problem in the audio circuits.

Namely, in the current mirror circuit of FIG. 5, the -2- forward voltage drop V,, of the diode-connected transistor Q2, and the base-emitter voltage V1,E4 of the transistor Q,, are approximately constant (about 0. 6 V). Thus, with the constant bias voltage given by the constant voltage source VCONST 'the collector voltage Ve of the transistor 03 is to be clamped. However, strictly speaking, the base- emitter voltage VBE is not constant, but it is dependent on the output current as expressed by the following expression, and has a nonlinear characteristic with respect to the emitter current I..

VIE = (kT/q)ln(IE/IS) Wherein.k is a BoltzmannIs constant, T is an absolute temperature at the junction section, q is an electric charge of an electron, and IS is a reverse direction saturation current. The same thing can be said about the forward direction voltage drop VF of the transistor Q2, since it is d iode- connected.

And, the change of the output current 12. which follows the change of the input current 11. appears as the change of the forward direction voltage drop VF Of the transistor Q2 as a diode and the base-emitter voltage V... of the transistor Q4. This change of the forward direction voltage drop VF appears as the change of the col lector- emitter voltage VCE3 of the output transistor Q3 - By the change of this voltage VCE31 the change of the output current by the aforementioned Early ef f ect, arises in a small degree. At the same time,. the capacity Cob by the -3- depletion layer of the PN junction between the base and the collector of the output transistor Q3, changes.

Thereby, the frequency characteristic is changed by the amplitude of the output current, i.e. the output signal, resulting in the increase of the distortion.

With reference to FIG. 6, the distortion by the Early effect will be explained hereinbelow.

The VCE _ IC characteristic, which adopts the base currents i.1 to iBr, of the transistor Q3 as the parameters, is shown in FIG. 6 as an comparison example. Each curve does not become a flat characteristic due to the Early effect. The collector current IC3 of the transistor Q3 changes according to the change of the output current 12, by the amount of 2AVBE (about AVF + AVU4) "e' the amount of 2AIC3- This change distorts the amplified wave form, which is treated as the problem of the degradation of the linearity of the amplifier circuit.

It is therefore an object of the present invention to provide a current mirror circuit, which has a high output impedance and can restrain the non-linear distortion.

According to the present invention, the above object can be achieved by a current mirror circuit including: a f irst transistor to which an input current is inputted; a second transistor, which is diode-connected and a base of which is -4- connected to a base of the first transistor; a third transistor for taking out an output current, which is connected in series with the second transistor such that a base of the third transistor is connected to a collector of the f irst transistor, and an emitter of the third transistor is connected to a collector of the second transistor; a fourth transistor which is cascode-coupled to the third transistor, for outputting the output current therethrough; and a constant voltage source connected between a base of the fourth transistor and the emitter of the third transistor.

In the present invention, since the constant voltage source is connected between the emitter of the third transistor and the base of the fourth transistor, the collector -emitter voltage of the third transistor is clamped. As a result, the collector -emitter voltage of the third transistor, is not influenced by the change of the forward direction voltage drop of the second transistor. Therefore, the influence of the Early effect, and the change of the PN junction capacity Cob between the collector and the base of the third transistor, are reduced. It becomes possible to reduce the nonlinear component included in the output current.

In this manner, the high output impedance by the base-grounded fourth transistor, can be kept, and the reduction of the non-linear distortion by the constant voltage source, can be made possible, according to the present invention.

The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.

In the accompanying drawings:- FIG. 1 is a diagram for explaining the principle of a current mirror circuit according to the present invention; FIG. 2 is a circuit diagram of a first embodiment of the current mirror circuit according to the present invention; FIG. 3 is a circuit diagram of a second embodiment of the current mirror circuit according to the present invention; FIG. 4 is a circuit diagram of a third embodiment of the current mirror circuit according to the present invention; FIG. 5 is a circuit diagram of a feedback type current mirror circuit; and FIG. 6 is a graph showing a V,,: - Ic characteristic of an output transistor of a current mirror circuit.

Nextly, the preferred embodiment of the present invention will be explained hereinbelow, with reference to FIGs. 1 to 4 and 6.

FIG. 1 is a diagram for explaining the principle of a current mirror circuit according to the present invention.

In FIG. 1, a current mirror circuit 1 includes: a f irst transistor Q1 to which an input current I1 is inputted; a second transistor Q2, which is diode-connected and a base of which is connected to a base of the first transistor; a third transistor Q3 f or taking out an output current, which is connected in series with the second transistor such that a base of the third transistor is connected to a collector of the f irst transistor, and an emitter of the third transistor is connected to a collector of the second transistor; a fourth transistor Q4 which is cascode-coupled to the third transistor, for outputting the output current 12 therethrough; and a constant voltage source Vc.n., connected between a base of the f ourth transistor and the emitter of the third transistor.

In this manner, according to the present invention, since the constant voltage source V,,,,., is connected between the emitter of the third transistor Q3 and the base of the fourth transistor Q41 which is connected to the collector of the third transistor Q3, the col lector- emitter voltage VC13 of the third transistor Q3 is clamped. As a result, the collector-emitter voltage VCE3 of the third transistor Q3, is not influenced by the change of the forward direction voltage drop V. of the second transistor Q2, which is generated in company with the change of the output current 12 following the change of the input current Ii. Therefore, the inf luence of the Early ef f ect, and the change of the PN junction capacity Cob between the collector and the base of the third transistor Q3 (shown in FIG. 5), are -7- reduced. It becomes possible to reduce the nonlinear component included in the output current 12.

In this manner, the high output impedance by the base-grounded f ourth transistor Q,, can be kept, and the reduction of the non-linear distortion by the constant voltage source VCONSTO can be made possible.

Hereinbelow, f irst to third embodiments using the above explained principle of the present invention, will be explained.

[I] First embodiment FIG. 2 indicates the first embodiment of the present invention. In FIG. 2, 1 the same constitutional elements as those in FIG. 1, carry the same reference numerals and the explanation thereof are omitted.

In FIG. 2, a current mirror circuit has a voltage/current conversion circuit A and a circuit B. The voltage/current conversion circuit A is a circuit which converts an input voltage signal into a current. In the voltage/current conversion circuit A, the drain of a field effect transistor (FET) Q5 of the N type, is connected to the collector of a transistor Q, of the circuit B. The source of the transistor Q5 is grounded through a source resistor RS. A gate resistor Rg is connected between the gate of the transistor Q. and the ground. An input voltage signal is given to the gate of the transistor Q5 through an input terminal IN. -8-

The transistor Q5 supplies a current corresponding to the inputted signal voltage as the input current I, to the transistor Q, of the circuit B. The circuit B has the input transistor Q,, a diodeconnected transistor Q2 for feedback, an output transistor Q31 and a cascode transistor Q..

In FIG. 2, the circuit of the present embodiment, is different from the related art circuit of FIG. 5 in the following points. Namely, a constant voltage source VCONST is connected between a connection point of the emitter of the transistor Q3 and the collector of the transistor Q2 i.e. a node N2, and a node N4 which is connected to the base of the transistor Q4. A constant current source 1CONST 'S connected to the constant voltage source VCONST. The constant current source ICONST makes the constant voltage source VCONST generate a constant voltage by absorbing or applying a constant current. As the constant voltage source VCONST1 for example, a constant voltage diode, such as a Zener diode etc., may be utilized. As the constant current source ICONST. f or example, a constant current diode may be utilized.

The collector of the transistor Q4 is grounded through a load resistor RL. The output signal voltage is taken out f rom a node N3 of the connecting point between the load resistor RL and the collector of the transistor Q41 through an output terminal OUT.

Nextly, the operation of thus constructed first -9- embodiment will be explained.

When the input voltage signal is given to the input terminal IN, the voltage of the input voltage signal is converted to the corresponding current by the voltage/current conversion circuit A. The output current 12 following the input current I1 flows in the course of: the voltage source VCC the transistor Q2 -> the transistor Q3 -> the transistor Q4 the load resistor RL -> the ground. The amount of the output current 12 changes as following the change of the input current Il. The output voltage corresponding to the change, is taken out from the node N3 of the load resistor RL through the output terminal OUT.

At this time, the forward direction voltage drop VF Of the transistor Q2, is changed by the change of the output current 12 which follows the change of the input signal. However, the collector-emitter voltage V CE3 of the transistor Q3 is not influenced by the change of the voltage drop VF - The changing component con sists of only a changing component of the base-emitter voltage V..4 of the transistor Q4. Therefore, as shown in FIG. 6, the change of the current 1C3 by the change of the voltage VCE, becomes only a component corresponding to AvBE4f i. e. AIc3 As shown in FIG. 6, the influence of the Early effect and the change of the capacity Cob. are reduced, so that, as compared with a comparison example according to the related art device of FIG. 5, the non-linear wave form of the output current 12. can be reduced, according to the f irst -10embodiment.

In this manner, since the f irst embodiment generates the constant voltage by the combination of the constant voltage source VCONST and the constant current source I CONST, the output current 12 is reduced, by the amount of the current 1 3, which is branched to the side of the constant voltage source VCOUST, as compared with the current 1 1. which f lows through the transistor Q2 (12 = 1 11 - 1.3). However, this reduced component of the current 13 consists of the component of direct current bias. Therefore, the changing component A12 (i.e. the signal component) of the current as an alternating component, does not change. Consequently, the first embodiment well operates as a current mirror circuit, from a view point of the al ternating- current operation, without the reduction of gain.

[III Second Embodiment FIG. 3 shows a second embodiment of the present invention.

The second embodiment is an example in which a push-pull amplifier circuit is constituted by use of the current mirror circuit of the first embodiment of FIG. 2.

Namely, as shown in FIG. 3, the second embodiment is constituted by arranging current mirror circuits 1A and 1B, each of which is equivalent to the circuit B of FIG. 2, in a symmetrical manner with respect to the upperand-lower direction. Each of transistors q, to q. constituting the 11- current mirror circuit 1B on the lower side, consists of a transistor of electrical conductive type opposite to that of transistors Q1 to Q4 on the upper side, 1.e. an NPN type transistor, in order to obtain a symmetrical nature. For the same reason, a P type FET is utilized for the transistor q5.

Each of the current mirror circuits 1A and 1B, act in the same manner as the current mirror circuit of FIG. 2, so that it restrains the distortion of the even order wave form to be amplified. Consequently, an amplifier circuit having a good linearity, can be obtained according to the second embodiment.

[III] Third Embodiment FIG. 4 indicates a third embodiment of the present invention.

The third embodiment is another concrete example of the current mirror circuit of FIG. 3.

In FIG. 4, the third embodiment is provided with a drive circuit 100, and a power amplifier circuit 200. In the third embodiment, the current mirror circuit of the present invention, is applied to the drive circuit 100, which is arranged at the stage previous to the power amplifier circuit 200.

The drive circuit 100 includes a constitution of a push-pull amplifier circuit in the same manner as the second embodiment of FIG. 3, and a DC servo-control circuit 3. The DC servo-control circuit 3 is a circuit for removing the direct current offset generated at the output terminal OUT of the drive circuit 100 by feedback. Namely, the DC servo-control circuit 3 has a field effect transistor T1 of the N type, which constitutes a constant current source ICONSTO and a f ield ef f ect transistor T2 of the P type. A resistor r, and a capacitor C, are connected to the gates of these transistors T1 and T2. The other end of resistor r. is connected to the output end OUT of the drive circuit 100. The other end of capacitor C is grounded. An integration circuit (or low path filter), is constituted by the resistor ri and the capacitor C, which removes the alternating current component generated at the output terminal OUT, and feeds back only the direct current component. A resistor r2 is a variable resistor for adjusting the direct offset voltage. Both ends of the resistor r2 are connected to the sources of the transistors T1 and T2. respectively. The middle point of the resistor r2 is grounded through a resistor r3. The direct current offset component, which is generated at the output terminal OUT, is fed back to the gates of the transistors T1 and T2, respectively, through the resistor ri. The direct current offset is controlled, such that the ratio of the drain currents of the transistors T. and T2 i. e. the constant current to f low through the upper and lower constant voltage source V CONSTs, respectively, are controlled according to the amount of this direct current offset.

In this manner, the current mirror circuit having a high output impedance and a good linearity with little distortion.. can be adapted to a practical circuit, and it is possible to combine it as a part of the DC servo-control circuit 3, which can remove the direct current of f set, so that the amplifier circuit having a good characteristic can be constituted, according to the present embodiment.

As described above in detail, since the col lector- emitter voltage of the third transistor, which constitutes the output transistor, is fixed by the constant voltage source, the inf luence of the Early ef f ect can be reduced, and the change of the capacity between the collector and the base is restrained, resulting in the improvement of the linearity of the output signal, according to the present embodiment.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (8)

1 A current mirror circuit comprising: a first transistor to which an input current is inputted; a second transistor, which is diode-connected and a base of which is connected to a base of said first transistor; a third transistor for taking out an output current, which is connected in series with said second transistor such that a base of said third transistor is connected to a collector of said first transistor, and an emitter of said third transistor is connected to a collector of said second transistor; a fourth transistor which is cascode-coupled to said third transistor, for outputting the output current therethrough; and a constant voltage source connected between a base of said fourth transistor and the emitter of said third transistor.

2. A circuit according to claim 1, wherein said constant voltage source comprises a constant voltage diode.

3. A circuit according to claim 1 or 2, further comprising a constant current source, connected to a connection point between the base of said fourth transistor and said constant voltage source, f or applying a constant current to said constant voltage source to generate a constant voltage.

4. A circuit according to claim 3, wherein said constant current source comprises a constant current diode.

5. A circuit according to any one of claims 1 to 4, wherein a collector of said fourth transistor is grounded through a load resistor, and an output voltage is generated at a connection point between the collector of said fourth transistor and said load resistor.

6. A circuit according to claim 1, further comprising a voltagelcurrent conversion circuit coupled to the collector of said first transistor, for converting an input voltage signal to the input current to supply the input current to said first transistor.

7. A circuit according to claim 3, wherein said circuit is constructed in a push-pull construction.

8. A circuit according to claim 1, substantially as described with reference to any of the examples shown in the accompanying drawings.

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