Current generating circuit.

0001This invention relates generally to a current generating (dynamic element matching) circuit, and more particularly to such a current generating circuit which is suitable for use in current switching for a digital-to-­analog (D/A) converter.

0002First, a previously proposed current generating circuit 4, disclosed in Japanese patent publication number 54-24098, will hereinafter be explained with reference to fig 1.

0003The current generating circuit 4 comprises a current dividing circuit 3 in a current mirror configuration which is formed of transistors Q₁ and Q₂. Currents I₁ and I₂ passing through the transistors Q₁ and Q₂ are respectively obtained by dividing a current 2I substantially by two, so that the currents I₁ and I₂ are substantially equal to each other. A pair of differential switching circuits 1a and 1b, which form a current change-over circuit 2, respectively switch and deliver the currents I₁ and I₂ alternately to a pair of output terminals T₃ and T₄. The differential switching circuit 1a is formed of a pair of transistors Q₃ and Q₄ which have their emitters connected with each other, and their connecting point is connected with the collector and the base of the transistor Q₁. The other differential switching circuit 1b is formed of a pair of transistors Q₅ and Q₆ which have their emitters connected with each other, and their connecting point is connected with the collector of the transistor Q₂.

0004The collectors of the transistors Q₃ and Q₅ are connected to the output terminal T₃ in common, while the collectors of the transistors Q₄ and Q₆ are connected to the other output terminal T₄ in common. The output terminals T₃ and T₄ are grounded respectively through capacitors Cx and Cy constituting low pass filters.

0005The input terminals T₁ and T₂, for switching, are fed with a pair of opposite-phased switching signals E₁ and E₂ with a predetermined frequency. The switching input terminal T₁ is connected to the bases of the transistors Q₃ and Q₆ in common, while the other switching input terminal T₂ is connected to the bases of the transistors Q₄ and Q₅ in common.

0006Next, the operation of the current generating circuit 4 will be explained. When the switching signal E₁ supplied to the input terminal T₁ is at a high level, the switching signal E₂ supplied to the input terminal T₂ is at a low level, so that the transistors Q₃ and Q₆ are turned on while the transistors Q₄ and Q₅ are turned off. Therefore, the current I₁ from the current dividing circuit 3 is delivered to the terminal T₃ and the current I₂ is delivered to the terminal T₄, respectively.

0007When the switching signal E₂ supplied to the switching input terminal T₂ is at the high level, the switching signal E₁ supplied to the input terminal T₁ is at the low level, so that the transistors Q₄ and Q₅ are turned on while the transistors Q₃ and Q₆ are turned off. Therefore, the current I₁ from the current dividing circuit 3 is delivered to the terminal T₄ and the current I₂ is delivered to the terminal T₃, respectively.

0008Thus, the output terminal T₃ is alternately fed with the currents I₁ and I₂, while the output terminal T₄ is alternately fed with the currents I₂ and I₁. These currents I2 and I₁ are averaged by the capacitors Cx and Cy constituting the low pass filters, so that both of the output terminals T₃ and T₄ are equally fed with a current I (I = (I₁ + I₂)/2), as a result. In other words, the current generating circuit 4 can provide at its two output terminals the output current I which is derived by accurately dividing the input current 2I supplied thereto by two.

0009Let it now be assumed that the current generating circuits 4a, 4b and 4c, each being explained with reference to fig 1 as disclosed in Japanese patent publication number 54-24098, are connected upward in a cascading manner as shown in fig 2. The current generating circuit 4a at the first stage is disposed to divide the input current 2I by two to obtain the two output currents Is, one of which is delivered to the output terminal T₄, and the other of which is supplied to one input terminal of the current generating circuit 4b at the next stage.

0010Then, the current generating circuit 4b is disposed to divide the input current I by two to obtain two output currents (I/2)s, one of which is outputted to an output terminal T₅, and the other of which is supplied to one input terminal of the current generating circuit 4c at the third stage.

0011By repeating the above operation, the current generating circuit 4c at the third stage derives a current I/4 at its output terminal T₆. Incidentally, reference letters T₃, T₃′ and T₃˝ designate the other output terminals of the current generating circuits 4a, 4b and 4c, respectively.

0012If each of the output currents delivered to the output terminals T₄, T₅ and T₆ are controlled independently by different switches which are turned on and off in accordance with binary combinations of a 3-bit digital signal, a current generator type D/A converter can be formed.

0013The current generating circuit 4 shown in fig 1 can derive substantially equal currents at the output terminals T₃ and T₄ by averaging the input currents I₁ and I₂ supplied to the differential type switching circuits 1a and 1b. However, if the current generating circuits are arranged as shown in fig 2, to form, eg, an n-bit D/A converter, the total number of the transistors Q₃ and Q₄, for example, in the differential switching circuit 1a is increased by a factor of n, so that the voltage necessary to operate all these transistors Q₃ and Q₄ or the like becomes higher as the bit number of the D/A converter is increased.

0014To solve the above-mentioned problem, Japanese patent publication number 57-31809 proposes to derive from the single current generating circuit 4 the output currents I, I/2, I/4 ... which have been delivered one by one from the current generating circuits 4a, 4b, 4c ... That is, the single current generating circuit provided with a plurality of output terminals for deriving a plurality of output currents, for example I and I/2, is disclosed. The construction shown in this document can reduce the number of the current generating circuits 4a, 4b, 4c ... which are piled by connecting in multiple stages, as shown in fig 2, thereby making it possible to reduce the source voltage.

0015There will now be explained a single-stage current generating circuit equivalent to the above-mentioned two-­stage current generating circuit and the operating waveforms thereof with reference to figs 3 and 4, by using the principle of Japanese patent publication number 57-31809.

0016In fig 3, reference numeral 4 designates an overall current generating circuit in which an input terminal T₁₅ is connected, eg, to a current generating circuit for the upper digit, and an output terminal T₁₄ to a current generating circuit for the lower digit, in a manner that they are piled in multiple stages, similarly to fig 2. The current dividing circuit 3 is formed of a current-mirror circuit comprising transistors Q₁₁, Q₁₂, Q₁₃ and Q₁₄, for dividing an input current I supplied to the input terminal T₁₅ substantially equally by four. That is, the input current is divided into 2ⁿ (n = 2, 3, 4 ...). The collectors of the transistors Q₁₁ to Q₁₄ are connected to switching circuits 11, 12, 13 and 14, respectively, while the bases and the emitters thereof are respectively connected in common. The base of the transistor Q₁₁ is connected to the collector of the same. The switching circuits 11, 12, 13 and 14 are respectively formed of switching transistors Q₁₅ to Q₁₈, Q₁₉ to Q₂₂, Q₂₃ to Q₂₆ and Q₂₇ to Q₃₀ comprised in the switching circuits 11, 12, 13 and 14 are respectively connected in common. A control terminal T₁₀ is connected to the respective bases of the transistors Q₁₅, Q₁₉, Q₂₃ and Q₂₇, while a control terminal T₁₁ is connected to the respective bases of the transistors Q₁₆, Q₂₀, Q₂₄ and Q₂₈. Further, a control terminal T₁₂ is connected to the respective bases of the transistors Q₁₇, Q₂₁, Q₂₅ and Q₂₉, while a control terminal T₁₃ is connected to the respective bases of the transistors Q₁₈, Q₂₂, Q₂₆ and Q₃₀. The collectors of the transistors Q₁₅, Q₂₀, Q₂₅ and Q arranged in the switching circuits 11 to 14 are connected in common to a first output terminal T₁₆. A capacitor C₁ forming a low pass filter is interposed between the first output terminal T₁₆ and the ground. In the same manner, the collectors of the transistors Q₁₆, Q₂₁, Q₂₆, Q₂₇, and Q₃₀ are connected in common to the output terminal T₁₆. Further, the collectors of the transistors Q₁₇, Q₂₂, Q₂₃ and Q₂₈ are connected in common to a second output terminal T₁₇. A capacitor C₂ forming a low pass filter is interposed between the second output terminal T₁₇ and the ground. Further in the same manner, the collectors of the transistors Q₁₈, Q₁₉, Q₂₄ and Q₂₉ are connected in common to the outputter terminal T₁₄ which leads to the current generating circuit for the lower digit. A capacitor C₃ forming a low pass filter is interposed between the output terminal T₁₄ and the ground.

0017In the construction shown in fig 3, the control terminals T₁₀ to T₁₃ are respectively supplied with two-­phase control pulses A₁ to A₄ as shown in fig 4. For example, when the control terminal T₁₀ is supplied with the pulse A₁, the transistors Q₁₅, Q₁₉, Q₂₃ and Q₂₇ are turned on and maintain the "on" condition as long as the pulse A₁ is at the high level, so that a current flows in the direction indicated by the arrow. The same operation is carried out with the pulses A₁ to A₄. When the current I is fed to the input terminal T₁₅, the current I/2 is derived at the first output terminal T₁₆ and the current I/4 is derived at the second output terminal T₁₇ and the terminal T₁₄.

0018With the circuit construction of fig 3 as described above, while the control terminals T₁₀ to T₁₃ are respectively applied with the pulses A₁ to A₄ for one cyclic period thereof, the switching circuit 11 has the transistors Q₁₅ and Q₁₆, connected to the output terminal T₁₆ in common, are turned on to supply the current to the transistor Q₁₁ arranged in the current dividing circuit 3. The switching circuit 12, in the same manner, has the transistors Q₂₀ and Q₂₁ turned on to supply the current to the transistor Q₁₂ in the current dividing circuit 3, the switching circuit 13 also has the transistors Q₂₅ and Q₂₆ turned on to supply the current to the transistor Q₁₃ in the current dividing circuit 3, and the switching circuit 14 has the transistors Q₂₇ and Q₃₀ turned on to supply the current to the transistor Q₁₄ of the current dividing circuit 3. This, if the number of the transistors arranged in the current dividing circuit 3 (the current source) are increased to 2, 4, 8, 16 ... 2ⁿ, or if the number of the processed bits (output derived at the output terminals T₁₆ and T₁₇) taken out from each of the current generating circuits 4a, 4b ... connected in the cascading manner as shown in fig 2 are increased to 1, 2, 3 ... n, the number of transistors in the switching circuits 11 to 14 for turning on and off each of the transistors in the current dividing circuit 3 is correspondingly increased to 2, 4, 8 ... 2ⁿ. In particular, for realising a highly accurate D/A converter the number of necessary transistors becomes unacceptably large.

0019Further, the current change-over circuit 2 formed of the differential switches 1a and 1b shown in fig 1 presents another disadvantage that the emitter currents of the respective transistors Q₃ to Q₆ thereof do not wholly flow to their collectors but the collector current is only β/(1+β) portion of the emitter current and the rest 1/(1+β) of the same flows to the base. In other words, the output currents delivered to the output terminals T₃ and T₄ are reduced by 1/(1+β) from (I₁+I₂)/2. Accordingly, if the current generating circuits of fig 1 are connected in multiple stages as shown in fig 2, the error in the output current becomes larger in further succeeding stages, so that if it is used as the aforementioned current source type DA converter, there will be produced a large conversion error.

0020This problem can be solved by constructing the transistors Q₃ to Q₆ of the differential switching circuits 1a and 1b in the current change-over circuit 2 shown in fig 1 in a Darlington configuration to thereby make the β factor substantially large.

0021Fig 5 illustrates a circuit in which the differential switching circuit 1a, for example, of the current change-­over circuit 2 in fig 1 is formed in a Darlington configuration. In this example, the transistor circuit Q₃ is formed of a pair of Darlington-connected transistors Q_3a and Q_3b, and the transistor circuit Q₄ is formed of a pair of Darlington-connected transistors Q_4a and Q_4b. Specifically, the emitter of the transistor Q₃b is connected to the base of the transistor Q₃a, while the collector of the transistor Q_3b. The emitter of the transistor Q_4b is connected to the base of the transistor Q_4a, while the collector of the transistor Q_4a is connected to the collector of the transistor Q_4b.

0022Assuming that the factors β of the transistors Q_3a to Q_4b are equal, the current flowing to the bases of the respective transistors Q_3b and Q_4b is represented by I₁/(1+β)². It is therefore understood that the accuracy of the output current of the input current is improved.

0023However, such a configuration requires the minimum operating voltage at the output terminals T₃ and T₄, on the basis of the common emitter of the transistors Q_3a and Q_4a, to be twice the base-emitter voltage V_BE of the transistors Q_3a to Q_4b, that is 2V_BE. Therefore, if the current generating circuits are connected in multiple stages as shown in fig 2, the minimum operating voltage for the current generating circuit at the highest stage will be four times the value 2V_BE. Thus, as the number of stages is increased, the minimum operating voltage thereof is greatly elevated.

0024In view of the inconvenience mentioned above, it is an object of the present invention to provide a current generating circuit which is capable of reducing the number of transistors arranged in switching circuits thereof.

0025It is another object of the present invention to provide a current generating circuit which is capable of reducing the error occurring in the output currents with respect to the input current as well as preventing elevation of the minimum operating voltage when the current generating circuits are connected in multiple stages.

0026According to a first aspect of the present invention, there is provided a current generating circuit for generating a plurality of currents accurately in a predetermined integer ratio, comprising: <ul id="ul0001" list-style="none"><li>a) a current dividing circuit for dividing an input current into 2ⁿ (n is a positive integer); and characterised by</li><li>b) a plurality of current change-over circuits for dividing respective output currents from said current dividing circuit in a predetermined ratio by supplying to input terminals thereof a predetermined number of change-­over clock signals in one cyclic period; and</li><li>c) a current synthesising circuit for synthesising the output currents from said respective current change-over circuits and generating currents in a predetermined ratio.</li></ul>

0027According to a second aspect of the present invention, there is provided a current generating circuit for generating a plurality of currents accurately in a predetermined integer ratio, comprising: <ul id="ul0002" list-style="none"><li>a) at least two sets of first and second current dividing devices each comprising a current dividing circuit for dividing an input current into 2ⁿ (n is a positive integer), a plurality of current change-over circuits for dividing respective output currents from said current dividing circuits in a predetermined ratio and a current synthesising circuit for synthesising the output currents from said respective current change-over circuits and generating currents in a predetermined ratio; and</li><li>b) a voltage compensating circuit connected between an output terminal of said second current directing circuit and an output terminal of said first current directing circuit for shifting the voltage level of a first input voltage.</li></ul>

0028These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, throughout which like reference numbers designate like elements and parts. In the drawings:- <ul id="ul0003" list-style="none"><li>Fig 1 is a schematic circuit diagram showing a previously proposed current generating circuit;</li><li>Fig 2 is a block circuit diagram showing a previously proposed current generating circuit;</li><li>Fig 3 is a schematic diagram showing another previously proposed current generating circuit;</li><li>Fig 4 is timing charts showing the waveforms of control pulses which are applied to the current generating circuit of fig 3;</li><li>Fig 5 is a schematic circuit diagram showing a previously proposed differential switching circuit;</li><li>Fig 6 is a schematic circuit diagram showing an embodiment of a current generating circuit according to the present invention;</li><li>Fig 7 is timing charts showing the waveforms of control pulses which are applied to the current generating circuit of fig 6;</li><li>Fig 8 is a schematic circuit diagram showing an embodiment of a differential switching circuit according to the present invention;</li><li>Fig 9 is a schematic circuit diagram showing another embodiment of a differential switching circuit according to the present invention; and</li><li>Fig 10 is a schematic circuit diagram showing a further embodiment of the present invention.</li></ul>

0029Now, a first embodiment of a current generating circuit according to the present invention will hereinafter be described with reference to figs 6 and 7.

0030Fig 6 is a circuit diagram showing the first embodiment of the current generating circuit 4 according to the present invention, and fig 7 is waveform charts of respective control signals supplied to control terminals shown in fig 6, wherein the parts corresponding to those in fig 3 are designated the same reference numerals and a repeated explanation thereof will be omitted.

0031In fig 6, the transistors Q₁₁, Q₁₂, Q₁₃ and Q₁₄ arranged in the current dividing circuit 3 of the current generating circuit 4 are constructed in the same manner as those in fig 3. Switching circuits 15, 16, 17 and 18 are respectively formed of three switching transistors Q₁₈, Q₁₇, and Q₃₁; Q₁₉, Q₂₂ and Q₃₂; Q₂₄, Q₂₃ and Q₃₃; and Q₂₉, Q₂₈ and Q₃₄, among which the transistors Q₁, Q₃₂, Q₃₃ and Q₃₄ are switching transistors performing also the functions of the transistors Q₁₅ and Q₁₆; Q₂₀ and Q₂₁; Q₂₅ and Q₂₆; and Q₂₇ and Q₃₉, as shown in fig 3, respectively.

0032The respective sets of the three transistors Q₁₈, Q₁₇, and Q₃₁; Q₁₉, Q22 and Q₃₂; Q₂₄, Q₂₃ and Q₃₃; and Q₂₉, Q₂₈ and Q₃₄ respectively constituting the four switching circuits 15, 16, 17 and 18 have their emitters connected in common to the respective collectors of the transistors Q8₁, Q8₂, Q₁₃ and Q₁₄ in the current dividing circuit 3. The collectors of the respective transistors Q₃₁, Q₃₂, Q₃₃ and Q₃₄ in the switching circuits 15 to 18 are connected in common to the first output terminal T₁₆ for the upper digit, while the bases of the transistors Q₃₁ and Q₃₃ are connected together to a control terminal T₁₉ which is supplied with a control pulse B₁ shown in fig 7. The bases of the transistors Q₃₂ and Q₃₄ are connected together to a control terminal T₁₈ which is supplied to a control pulse B₂ (with the opposite-phased pulse with respect to the control pulse B₁) as shown in fig 7.

0033The collectors of the transistors Q₁₇, Q₂₂, Q₂₃ and Q₂₈ are connected in common to form a current synthesising circuit, the output terminal of which is connected to a second output terminal T₁₇ for the second most significant bit. The base of the transistor Q₁₇ is connected to a control terminal T₂₂ which is supplied with the control pulse A₃ shown in fig 7, while the base of the transistor Q₂₂ is connected to a control pulse A₄ shown in fig 7. Further, the base of the transistor Q₂₃ is connected to a control terminal T₂₀ which is supplied with the control pulse A₁ shown in fig 7, while the base of the transistor Q₂₈ is connected to a control terminal T₂₁ which is supplied with the control pulse A₂ shown in fig 7. The control pulses A₁ to A₄ are four-phase pulses synchronised with the control pulses B₁ and B₂.

0034The collectors of the transistors Q₁₈, Q₁₉, Q₂₄ and Q₂₉ form a current synthesising circuit the output terminal of which is connected to the output terminal T₁₄ which leads to the current generating circuit for the lower digit (not shown). The base of the transistor Q₁₈ is connected to the terminal T₂₀ which is supplied with the control pulse A₁ shown in fig 7. Likewise, the base of the transistor Q₁₉ is connected to the terminal T₂₁, the base of the transistor Q₂₄ to the control terminal T₂₂, and the base of the transistor Q₂₉ to the control terminal T₂₃, while these control terminals T₂₀, T₂₁, T₂₂ and T₂₃ are supplied with the control pulses A₁ A₂, A₃ and A₄ shown in fig 7, respectively.

0035The transistors Q₁₈, Q₁₉, Q₂₄ and Q₂₉ in the switching circuits 15 to 18 or the current synthesising circuit formed by connecting their collectors makes the output current, obtained by dividing the input current I by four, pass therethrough for a quarter on one cyclic period T when the respective control pulses A₁ to A₄ are at the high level and delivers the same to the output of the current synthesising circuit. The transistors Q₁₇, Q₂₂, Q₂₃ and Q₂₈ in the switching circuits 15 to 18 or the current synthesising circuit formed by connecting their collectors makes the output current I/4, obtained by dividing the input current by four, pass therethrough for a quarter of one cyclic period T when the respective control pulses A₃, A₄, A₁ and A₂ are at the high level and delivers the same to the output of the current synthesising circuit. Further, the two transistors Q₃₁ and Q₃₃ among the transistors Q₃₁ to Q₃₄, each of which also serve as two switching transistors, in the switching circuits 11-14 in fig 3 make the output current I/2 obtained by dividing the input current I by two pass therethrough for half the period T when the control pulse B₁ is at the high level. The high level period of the control pulse B₁ is coincident with the on-period or high level period of the control pulses A₂ and A₄.

0036In the same manner, the transistors Q₃₂ and Q₃₄ make the output current I/2 obtained by dividing the input current I by two pass therethrough for half the cyclic period T in which the control pulse B₂ is at the high level. The control pulse B₂ goes high in level in synchronism with the high level periods of the control pulses A₁ and A₃. Thus, the switching circuits 15 to 18 can constitute current synthesising circuits which generate a first output current obtained by dividing the input current I by 2ⁿ and a second output current obtained by dividing the input current I by 2^m (m=1, 2, 3 ... m&lt;n).

0037The above-mentioned embodiment relates given to the case of the 2-bit current generating circuit 4 in which the switching transistors Q₃₁ to Q₃₄ are turned on twice by the control pulses B₁ and B₂ for one cyclic period T in which the control pulses A to A₄ sequentially become the high level. If a 3-bit current generating circuit is concerned, for example, the second bit is applied with a control pulse which turns on the switching transistors twice within one cyclic period of the control pulse for the lower digit bit corresponding to the control pulses A₁ to A₄ and the most significant bit is applied with a control pulse which turns on the switching transistors four times within the cyclic period T. The same principle can be adapted to the case of an n-bit current generating circuit.

0038In the above embodiment, the time period is weighted by the number of the control pulses A₁ to A₄. Alternatively, the weighting of the time may be effected by changing the pulse width of the control pulses.

0039Next, an embodiment of a current change-over circuit according to the present invention will hereinafter be explained with reference to fig 8 in which the present invention is adapted to the current dividing circuit 3 to form the current generating circuit 4. The parts in fig 8 corresponding to those in fig 6 are designated the same reference numerals and a repeated explanation thereof will be omitted. Also, capacitors constituting low pass filters are not illustrated in fig 8.

0040The transistor circuits Q₃, Q₄, Q and Q₆ of the differential switching circuits 1a and 1b in the current change-over circuit 2 are respectively formed of transistors Q_3a and Q_3b; Q_4a and Q_4b; Q_5a and Q_5b; and Q_6a and Q_6b, each being connected in a Darlington configuration.

0041The emitters of the transistors Q_3b, Q_4b, Q_5b and Q_6b are respectively connected to the bases of the transistor Q_3a, Q_4a, Q_5a and Q_6a. The emitters of the transistors Q_3a and Q_4a are connected in common to a current source IS₁, while the emitters of the transistors Q_5a and Q_6a are connected in common to a current source IS₂. Further, the input terminal T₁ which is applied with the switching signal (refer to fig 1) is connected to the bases of the transistors Q_3b and Q_6b in common, while the input terminal T₂ which is supplied with another switching signal is connected to the bases of the transistors Q_4b and Q_5b in common.

0042The collectors of the transistors Q_3a and Q_5a are connected with each other, and the potential V_BE at the connecting point therebetween becomes the minimum operating voltage for the emitter voltage (hereinafter referred to as "reference potential") of the transistors Q_3a and Q_5a. The collectors of the transistors Q_4a and Q_6a are connected to each other, and the minimum operating voltage at the connecting point therebetween with respect to the reference potential is V_BE. Therefore, if these connecting points are connected to the current dividing circuit in the next stage, the elevation of the minimum operating voltage due to the multi-stage connection can be suppressed.

0043Since the minimum operating voltage with respect to the reference potential of each collector of the transistors Q_3b to Q_6b is 2V_BE, these collectors are connected to a high voltage level (2V_BE) in the next stage.

0044Next, an example of connecting the current generating circuit 4 formed of the current dividing circuit 3 and the current change-over circuit 2 shown in fig 8 in two stages is explained with reference to fig 9. In this example, reference numeral 4 designates a current generating circuit in the first stage to which a current dividing circuit 4′ in the next stage is connected. In the next stage, only a current dividing circuit 3′ is illustrated.

0045The collectors of the transistors Q_3a and Q_5a arranged in the current change-over circuit 2 in the first stage current generating circuit 4 is connected to the output terminal T₃, and further to the emitters of the transistors Q′₁ and Q′₂ of the current dividing circuit 3′ in the next stage in common. The collectors of the transistors Q_4a, Q_4b, Q_6a and Q_6b are connected in common to the output terminal T₄ as well as to the ground through a capacitor Cy constituting a low pass filter. Further, the collector of the transistor Q_3b is connected to the base of the transistor Q′₁ through the output terminal T₅, while the collector of the transistor Q_5b is connected to the collector of the transistor Q′₂ through the output terminal T₆.

0046Since the minimum operating voltage of the collectors of the transistors Q_3a and Q_5a with respect to the reference voltage is V_BE, these collectors are respectively connected to the emitters of the transistors Q′₁ and Q′₂ in the current dividing circuit 3′. Meanwhile, since the minimum operating voltage of the collectors of the transistors Q_3b and Q_5b with respect to the reference potential is 2V_BE, these collectors are respectively connected to the collectors of the transistors Q′₁ and Q′₂.

0047In this construction, the output current delivered to the output terminal T₃ (called "the main current ") is represented by I₀′, the output current delivered to the output terminal T₄ by I₀₁. The collector current of the transistor Q_3b (called "the correcting current") by I₉. The collector currents of the transistors Q′₁ and Q′₂ of the current dividing circuit 3′ are represented by I₁′ and I₂′, respectively.

0048Now, if the difference between the currents I₁, I₂ and I₀/2 (I₀ is the input current) is represented by ±Δ₁, and the difference between the currents I′₁, I′₂ and I₀′/2 by ± Δ₂, the currents I₁, I₂, I′₁ and I′₂ are expressed by the following equations:     I₁ = I₀/2 + Δ₁ ... (1)     I₂ = I₀/2 - Δ₁ ... (2)     I′₁ = I₀/4 + Δ₂ - (I₀ + 2Δ₁) . ((1/4)(1 + β_3a) . (1 + β_3b))      (3)     I′₂ = I₀/4 - Δ₂ - (I₀ + 2Δ₁) . ((1/4)(1 + β_5a) . (I + β_5b))      (4) where β_3a, β_3b, β_5a and β_5b.

0049It is understood from the equations (1) to (4) that even without connecting the collectors of the transistors Q_3a and Q_3b, and those of the transistors Q_5a and Q_5b connected in Darlington configuration for forming the transistor circuits Q₃ and Q₅, the respective emitters, bases and collectors thereof have the same current relationship as in the case where the collectors are connected to each other, and accordingly the factors β thereof can be made larger.

0050Next, a further embodiment of the present invention will be explained with reference to fig 10. This embodiment is such that the current generating circuit 4 at the first stage is connected to a current generating circuit 4′ at the next stage, wherein transistor circuits Q₃ to Q₆ and Q′₃ to Q′₆ of their current change-over circuits 2 and 2′ are respectively formed of Darlington-connected three-stage transistors Q_3a, Q_3b and Q_3c to Q_6a, Q_6b, and Q_6c, and Q′_3a, Q′_3b, and Q′_3c to Q′_6a, Q′_6b, and Q′_6c.

0051Since there are provided the Darlington-connected three-stage transistors, the accuracy of the output current with respect to the input current can be correspondingly improved. However, the three stage configuration causes the minimum operating voltage of the collector to be increased to 3V_BE with respect to the reference potential, so that a voltage compensating circuit 46 is required between the current change-over circuits 2 and 2′.

0052This voltage compensating circuit 46 will hereinafter be explained.

0053The voltage compensating circuit 46 comprises a pair of differential switching circuits 35 and 36 which are formed of transistors 37 and 38, and 39 and 40, respectively. The emitters of the transistors 37 and 38 are connected in common, and the connecting point thereof is grounded through a capacitor 42 constituting a low pass filter. In the same manner, the emitters of the transistors 39 and 40 are connected in common, and the connecting point thereof is ground through a capacitor 43 constituting a low pass filter.

0054Input terminals 44 and 45, which correspond to the input terminals T₁ and T₂ through which the switching signals E₁ and E₂ are supplied to the current generating circuit 4 at the first stage, are fed with the same switching signals E₁ and E₂, respectively. The switching signal input terminal 44 is connected in common to the bases of the transistors 37 and 40, while the switching signal input terminal 45 is connected in common to the bases of the transistors 38 and 39.

0055In the current change-over circuit 2 in the current generating circuit 4 at the first stage, the collectors of the transistors Q_3a and Q_5a are connected in common, and the connecting point thereof is connected through the output terminal T₃ to the emitters of the transistors Q′₁ and Q′₂ of the current dividing circuit 3′ in the current generating circuit 4′ at the next stage. The collectors of the transistors Q_4a, Q_4b, Q_4c, Q_6a, Q_6b and Q_6c are connected in common to the output terminal T₄. The collector of the transistor Q_3b is connected through the output terminal T₅ to the collector of the transistor Q′₁ of the current dividing circuit 3′. The collector of the transistor Q_5b is connected through the output terminal T₆ to the collector of the transistor Q′₂ of the current dividing circuit 3′. The collector of the transistors Q_3c is connected to the emitters of the respective transistors 37 and 38 of the voltage compensating circuit 46, while the collector of the transistor Q_5c is connected to the emitters of the respective transistors 39 and 40 thereof.

0056The current change-over circuit 2′ in the current generating circuit 4′ at the next stage has the collectors of the transistors Q′_3a and Q′_5a connected with each other, and the connecting point thereof is connected, through an output terminal T′₃, commonly to the emitters of the transistors of a current dividing circuit in a current generating circuit (not shown) at the third stage. The collectors of the transistors Q′_4a, Q′_4b, Q′_4c, Q′_6a, Q′_6b and Q′_6c are connected in common to an output terminal T′₄ as well as to the collectors of the transistors 38 and 40 of the voltage compensating circuit 46 through the anode-­cathode path of a diode 41. The collector of the transistor Q′_3b is connected to the base of the transistor of the current dividing circuit in the current generating circuit at the third stage through an output terminal T′₅ as well as to the collector of the transistor 37 of the current compensating circuit 46. The collector of the transistor Q′_5b is connected through an output terminal T′₆ to the collector of the transistor of the current dividing circuit in the current generating circuit at the third stage as well as to the collector of the transistor 39 of the voltage compensating circuit 46.

0057Incidentally, the voltage compensating circuit 46 also has a function of averaging currents Ia and Ib which are applied respectively to the collectors of the transistors Q_3c and Q_5c of the current change-over circuit 2 in the current generating circuit 4 at the first stage.

0058Particularly in the present embodiment, the collector of the transistor Q_3c, the minimum operating voltage of which is 3V_BE with respect to the reference potential, in the current change-over circuit 2 in the current generating circuit 4 at the first stage, is connected to the collector of the transistor Q′_3b of the current change-over circuit 2′ in the current generating circuit 4′ at the next stage through the emitter-to-collector path of the transistor 37 as well as to the collector of the transistor Q′_4a of the current change-over circuit 2′ through the emitter-to-­collector path of the transistor 38 and the diode 41. Similarly, the collector of the transistor Q_5c, the minimum operating voltage of which is 3V_BE with respect to the reference potential, constituting the current change-over circuit 2 in the current generating circuit 4 at the first stage, is connected to the collector of the transistor Q′_5b of the current change-over circuit 2′ in the current generating circuit 4′ at the next stage through the emitter-­to-collector path of the transistor 39 in the voltage compensating circuit 46 as well as to the collector of the transistor Q′_4a of the current change-over circuit 2′ through the emitter-to-collector path of the transistor 40 and the diode 41.

0059The rest of the present embodiment is constructed in the same manner as the construction shown in fig 9, so that the repeated explanation thereof will be omitted.

0060According to the construction of the change-over circuit of the present invention as set forth above, as the number of processing bits is increased, the number of transistors which are used to turn on the single current dividing circuit can be further reduced. Accordingly, assuming that the number of processing bits is n, the number of transistors necessary to turn on each of the transistors constituting the current dividing circuit can be reduced from 2ⁿ to n+1, which is highly effective in constructing a current generating circuit for a highly accurate D/A converter or the like.

0061Also, since the current generating circuit of the present invention is provided with output terminals arranged in parallel corresponding to the number of the processing bits by connecting a plurality of current generating circuits in a cascading manner, the number of the processing bits can be increased in a predetermined stage, thereby making it possible to largely reduce the number of cascaded stages, the number of elements in the change-over circuit and the operating source voltage.

0062Particularly, since the current dividing circuit in Darlington configuration embodiment according to the present invention can suppress errors in the output current with respect to the input current as well as divide the input current with high accuracy, it is possible to prevent the elevation of the minimum operating voltage when the current dividing circuits are connected in multiple stages.