GB2130451A - Current splitter - Google Patents

Abstract

A current splitter is formed by two transistors (Q1 and Q2) whose bases are coupled, and whose emitters are commoned to a current source (CS). One transistor (Q1) also has its base and collector coupled. Assuming identical transistors, the current IIN due to the current source (CS) is split equally between the two collector circuits, which thus produce output currents whose values are substantially independent of the load impedances and voltages when the circuit remains linear. With a number n of transistors the current can be split equally between them, or split according to ratios dependent on transistor characteristics or collector connections. <IMAGE>

Description

SPECIFICATION Current splitter This invention relates to a current splitting circuit, i.e. a circuit arranged to split the current from a current source into two or more ratioed output currents.

According to one aspect of the present invention, there is provided a current splitting circuit, which includes first and second transistors having their bases connected together, a connection between the collector and the base of the first of said two transistors, and a current source connected between a reference potential and the commoned emitters of the two transistors, the arrangement being such that the current generated by the current source splits between the collector circuits of the first and the second transistors, the ratio of the currents in the first and second transistors being dependent on the characteristics of those transistors assuming that the circuit remains substantially linear in its operation.

The main feature of the invention is that the choice of the parameters of the components in the output circuits defines the ratio of the output currents. In such circuits, the output currents operate into different independent voltages and load impedances without affecting the current splitting ratio. The outputs are in phase with the input and with each other. With such a circuit, no power supply voltage is needed, except for the load supplies and for the supply rails for any operational amplifiers used. Such a circuit can be floating or grounded.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 illustrates the basic idea for a 50:50 splitting of the current.

Figure 2 shows the complementary circuit of Figure 1.

Figure 3 shows the application of the invention to ann-way current splitter.

Figure 4 shows a 67:33 current splitter embodying the invention.

Figure 5 shows a practical application of a simple current splitter embodying the invention.

Figure 6 is a circuit which is generally similar to the circuit of Figure 1, but with emitter resistor for the transistors.

Figure 7 is the complementary version of Figure 6.

Figure 8 - 16 are further circuits embodying the invention, based on improved versions of the current mirror.

In Figure 1, the transistors Q1 and Q2 are identical, and it will be seen that their emitters are connected together and to one terminal of a current source CS, the other side of which is connected to a reference potential, in this case earth. In this circuit the current source CS, which could be the output of a long-tailed pair, produces an input current 11N and for the two output currents 10i = la2 = IIN/2- These output currents occur at terminals 1 and 2, and their values are substantially independent of the voltages and impedances present in the circuits connected to the terminals 1 and 2.

It will be seen that this circuit can be regarded as being derived from the well-known current mirror. The main differences are that while terminal 2 remains an output as in the current mirror, terminal 1 which is an input in the current mirror is now an output, and finally the common point of the two emitters is now the input to the circuit.

The circuit of Figure 2 is the complement of that of Figure 1, in that it uses PNP transistors instead of NPN transistors.

In both Figures 1 and 2 the circuit can be floating or grounded, dependent on the load connected to the output terminal 1.

Figure 3 shows an n-way current splitter, where the transistors all have their bases connected together. In addition the transistors have their emitters connected together and to the current source. In this case, assuming that the transistors are identical, the current is split equally between all of the transistors so that each delivers an output of IINZn. When the transistors are not identical the current splitting ratios depend on the relations between the parameters of the transistors.

Figure 4 shows how a 67-33 current splitter is produced. Here we have three transistors Q3, Q4 and 05, connected as in Figure 3, except that the collectors of Q4 and Q5 are connected together and to output 2.

Thus the output current at terminal 1 is llN/3 while the output current at terminal 2 is 2INS3. Other ratios can be achieved by various connections of the transistors.

Another way to vary the ratio of the output currents is to use transistors of different emitter areas or by the addition of series emitter resistors to Figures 1, 2,3 and 4.

Figure 5 shows a typical use of a simple current splitter 07-08-CS, where the output 2 drives into a low load impedance formed by the long-tailed pair Q9-Q10, at a voltage level of -0.7 volts determined by the load. Output 1 drives into a load resistance R4 at a different voltage level close to +50 volts. The two output currents are determined by the current splitter, and are substantially unaffected by the output load impedances or voltages.

Such current splitters are useful wherever balanced, equal current operation is needed, e.g. where the currents in a telephone subscriber's line circuit have to be controlled.

We have indicated above that the present invention is derived from the well-known current mirror; hence the improvements that have recently been applied to current mirrors are in general applicable to the circuits embodying the present invention. These include the use of series resistors as mentioned above, as shown in Figures 6 and 7.

In the circuit of Figure 6, where resistors R6 and R7 are cnnected in the emitter circuits of the transistors 01 and 02, the circuit gives a 50:50 current split when R6 = R7. If R6 and R7 are unequal, the currents i and I in 01 and 02 are given by the following relations:

These resistors reduce the effect of VBE mismatch, and also allows the attainment of different splitting ratios.

Figure 7 is the complementary version of Figure 6.

Figure 8 shows a three transistor version of the arrangement of Figure 1: the addition of the third transistor 09 helps to isolate 02 from output voltage changes.

Figure 9 is the complementary version of the circuit of Figure 8.

The emitter resistors as used in Figures 6 and 7 can also be used in the circuits of Figures 8 and 9: Figure 10 in fact shows a version of Figure 8 with emitter resistors. The complemetary version of Figure 10 is not shown as its derivation from Figure 9 should be clear to one skilled in the art.

Another three transistor arrangement is shown in Figure 11, in which the transistor 010 has its base-emitter path connected across the collector-base of 01. This, which in effect means that 01 is replaced by Q1-010 in a Darlington connection, improves the accuracy of operation of the circuit. Emitter resistors can also be used in Figure 11, and complementary versions of these circuits can be used.

Figure 12 is a four-transistor version of our basis circuit: the use of the fourth transistor equalises the input and output collector voltages, further reducing the effects of any influences tending to cause unbalance.

Figure 13 is the complementary version of the circuit of Figure 12.

Emitter resistors can also be used in Figures 12 and 13.

Figures 14 and 15 show two versions of the three-transistor arrangements in which multi-emitter arrangements are used. In Figure 14 the multi-emitter transistor Q11 is in effect equivalent to four similar parallel-connected transistors so we have the result: i=1/sljn and l=4/sljn Similarly, in Figure 15,012 is equivalent to two parallel-connected transistors, so we have =1/3 In and l=2/3Iin Complementary versions of these circuits, and versions using emitter resistors are also possible.

In these arrangements the transistors Q11 -Q1 and Q1-012 have similar collector areas, but Q11 has four similar emitter areas and Q12 has two similar collector areas.

Figure 16 shows a multi-collector arrangement in which 013 has two separate and similar collector regions, while Q13 and Q1 have equal emitter areas. With this arrangement we have: i3 = lin i ~ lin I ~ lin 4 4 2 Different emitter areas can be used for 01 and 013 to give different sealing factors.

Complementary versions of the circuit of Figure 16 and its derivations are possible, as are versions thereof using emitter resistors.

Claims (9)

1. A current splitting circuit, which includes first and second transistors having their bases connected together, a connection between the collector and the base of the first of said two transistors, and a current source connected between a reference potential and the commoned emitters of the two transistors, the arrangement being such that the current generated by the current source splits between the collector circuits of the first and the second transistors, the ratio of the currents in the first and second transistors being dependent on the characteristics of those transistors, assuming that the circuit remains substantially linear in its operation.

2. A circuit as claimed in claim 1, in which the first and the second transistors are identical, so that the current in the two collector circuits are substantially identical.

3. A circuit as claimed in claim 1, in which the second transistor has one or more further transistors connected in parallel with it, the current which would, in the absence of the one or more further transistors, would flow in the second transistor being split between the second transistor and the one or more further transistors.

4. A circuit as claimed in claim 1 or 3, in which the transistors have different emitter areas, the ratio in which the current is split between the transistors being dependent on the ratio's of the emitter areas.

5. A current as claimed in claim 1 or 2, and in which a further transistor has its collector-emitter path connected in series with the collector-emitter path of the first transistor and its base connected to the collector of the second transistor.

6. A current as claimed in claim 5, modified in that there is yet another transistor with its collector-emitter path in series with the collector-emitter path of the second transistor, the base of said another transistor being connected to the base of said further transistor.

7. A current as claimed in claim 1, 2, 5 or 6, and in which the first transistor and or the second transistor is a multi-emitter transistor.

8. A circuit as claimed in claim 1,2,5 or 6, and in which the second transistor is a multi-collector transistor.

9. A current splitting circuit substantially as described with reference to any one of Figures 1 to 16 of the accompanying drawings.