GB2135846A

GB2135846A - Current splitter

Info

Publication number: GB2135846A
Application number: GB08303130A
Authority: GB
Inventors: Peter Stuart Bridger
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-02-04
Filing date: 1983-02-04
Publication date: 1984-09-05
Also published as: GB2135846B; GB8303130D0; BE898832A

Abstract

A current splitter is formed by a current source (CS), e.g. the output of a long-tailed pair, which feeds two resistors (R1, R2), one (R1) of which is connected to one output of the circuit, while the other (R2) is connected to the other output via the emitter-collector path of a transistor (Q1). A differential amplifier, such as an operational amplifier (OA1) has one input connected to the first output of the circuit while its other input is connected to the junction of the second resistor (R2) and the transistor. The output of the amplifier drives the base of the transistor. With this arrangement the current from the source (CS) splits between the resistors in a ratio dependent on the values of the resistors, and the current values are substantially independent of the output loads and voltages - as long as the circuit remains linear. In another arrangement, the amplifier is a pair of transistors (Q5, Q6) in long-tailed pair-like configuration connected to a current mirror (Q3, Q4). Also described are a number of circuits in which two current splitters such as referred to above are connected together in parallel. <IMAGE>

Description

SPECIFICATION Current splitter The present invention relates to current splitters, i.e.

to electronic circuits which are fed by a current source and which split the current therefrom in accordance with preset ratios, i.e. to so-called static splitters.

The main feature of the invention is a current splitting circuit, which includes a current source connected between a reference potential and a common point, a first output terminal connected via a first resistive impedance to the common point, a second output terminal connected to the other end of the collector-emitter path, a transistor whose collector-emitter path is connected via a second resistive impedance to the common point and a differential amplifier having its output connected to the base of the transistor, one of its inputs connected to the first output terminal and the other of its inputs connected to the emitter of the transistor, the arrangement being such that the current generated by the current source is split between the output circuits connected to the two output terminals in a ratio dependent on the ratio of the values of the two resistive impedances.

One application for such a circuit is the supply of balanced currents to the wires of a telephone subscriber's line circuit.

Embodiments of the invention will now be described with reference to the accompanying Figures 1 to 16.

The circuit of Figure 1 is indicative of the basic principles on which all of the circuits to be described herein are founded. Here an output terminal 1 is connected via a resistor R1 to a current source CS, which latter may be a constant current source. In some areas it is not a constant current source; in fact in a telephone system the source CS could be DC modulated by speech. It can also be an AC source, or a pulsed current source. Another output terminal 2 is connected via the collector-emitter terminal of a transistor Q1 and a resistor R2 to the current source CS. This current source could be an output from a long-tailed pair. Terminal 1 is also connected to the non-inverting input of an operational amplifier OA1, the inverting input of which is connected to the emitter of Q1.The output of the operational amplifier OA1 controls the electrical condition of the base of Q1. The ratio of the two output currents Ii and 12 is defined by the ratio of the resistances of the two resistors R1 and R2, due to the "comparator-type" action of the operational amplifier OAl. This ensures that the correct relationship is maintained between these currents, thus eliminating the effects of the VBE drop of the transistors 01.

In this circuit, the splitting ratio is controlled precisely by the ratio of the two resistors R1 and R2, and the outputs are in-phase with each other and with the input. The splitting ratio is substantially independent of the load impedences and voltages, as long as the circuit is operating in the linear region.

The effect of VBE variation with junction temperature is also eliminated, as indicated above, by the negative feedback loop from the emitter of Q1 to the input of OA1. Note that in this circuit the phase inversion introduced by the negative feedback loop is provided by the operational amplifier OA1.

In this circuit the transistor Q1 can be of the bipolar, JFET or MOSFET type.

Figure 2 is the complementary version of Figure 1 in which, in the bipolar case, 01 is a PNP transistor, whereas in the Figure 1, Q1 is an NPN transistor.

Figure 3 is similar in principle to Figure 1, but differs therefrom in the following respects: (a) the phase inversion for the negative feedback loop is provided by the transistor 02, in this case of the PNP type, rather than by the operational amplifier, (b) the transistor 02 is connected as an emitter follower rather than as a grounded emitter circuit, (c) if the voltage present at output 2 is a high voltage relative to the input voltage of the operational amplifier OA2, then OA2 is a higher voltage device than that used in Figure 1 because its output has to drive the base of the emitter follower.

Figure 4 is the complementary version of Figure 3, which uses an NPN transistor instead of a PNP transistor.

Figure 5 is a discrete component version of the circuit of Figure 3, in which OA2 is replaced by the current mirror 03-Q4 plus the two transistors Q5 and Q6. Q5 and Q6 form a long-tailed pair with a current source CSA connected to the commoned emitters of Q5 and Q6. This long-tailed pair is the key element of the equivalent of OA2, so that 03 to Q6 are in effect an operational amplifier.

Figure 6 is the complementary version of Figure 5, i.e. the discrete component version of Figure 4.

Figure 7 is an n-way current splitter derived from the two way circuit of Figure 1. This circuit, if the resistors R1, R2, R3... Rn are equal, splits the current so that each of then outputs carries a current 11N!n If one needs to generate a current which is an integral multiplex of IINlnr thenx of the outputs are connected together.

The complementary version of this circuit is not shown as it can readily be derived from Figure 7.

Figure 8 is a circuit similar to Figure 7, but derived from Figure 3: here also a complementary version can easily be derived.

Figure 9 shows how the circuit ideas of Figures 1 and 3 can be combined in one circuit. Thus in this circuit OA3-Q3 follows the principles of Figure 1, while OA4-O4follows the principles of Figure 3. As usual, a complementary version of Figure 9 can be derived, but this is not shown.

The next group of circuits to be described are complementary current splitters, with in most cases, antiphse outputs. In Figure 10, 05-OA5 follows the principles of Figure 1,while Q6-OA6 follows the principles of Figure 2, so that we effectively have two circuits in series, one as Figure 1 and the other as Figure 2. All four resistors are 1K resistors. With an input current 11N of2 mA, 11 =12=1 mA, but if the input current is pulsed, it is found that the pulses forming 12 are inverted as compared with those of 11.

With the current values quoted, there is 0.3 volt at the base of Q5 and -1 volt at its emitter. The voltage at the lower end of the resistor R5 is -2 volts, which gives -1 volt at the non-inverting input of OA5. The voltage at the base of 06 is then -1.7 volts, with -1 volt at its emitter. Note that the emitters of Q5 and 06 are at the same potential, i.e. VE5 = VE6. Thus the second portion of the circuit is in effect an inverter.

In this circuit, as in the earlier circuits, the current source can be DC and/or AC, and can be modulated with speech or other variable control signals. It could also be a pulsed current source.

Another circuit similar in some respects to that of Figure 10 will be described below with reference to Figure 16.

Figure 11 also shows two circuits such as those of Figures 1 and 2 connected in series with that of Figure 2 "above" that of Figure 1, whereas in the case of Figure 10, it is Figure 1 which is above Figure 2. In this circuit the two outputs are effectively in antiphase. Here the resistors R6 and R7 are each of 1 Kohm, while R8 and R9 are of 0.9Kohm. Thus with 11N - 2 mA, we have Ii =12 = 1 mA, and 2mA flows in the series ci rcuit formed by R8, R9 and C3. Hence we have -1 volt at the junction of OA8. The voltage on the base of Q8 is -2.7 volts, and the voltage on the emitter of Q8 is -2 volts.

Whereas in Figures 10 and 11 the two portions are effectively in series, in the next three circuits they are effectively in parallel.

Figure 12 is in effect a parallel connection of two circuits, one as Figure 1 and the other as Figure 2, with 1, = 12 = 1 mA, 11N being 2 mA. The resistors are all of 1 Kohm value, so we have +0.7 volt on the base of Q9, with 0 volts on its emitter. The voltage at the non-inverting input of OA10 is -1 volt, as is the voltage on the emitter of Q10, so the voltage on the base of 010 is -1.7 volts.

In Figure 13, -OA11 and Q12-OA12 are derived respectively from Figures 2 and 3, so we have the parallel connection of circuits such as those two figures. The resistors are all of 1 Kohm value, so at the base of 011 we have 0.7 volts less than the voltage on output 1,0 volts at the collector of Q11, and -1 volt at the inverting input of OA12 and hence also on the emitter dof Q12.

Figure 14 is a parallel connection of circuits such as Figures 3 and 4, from which respectively are derived Q13-OA13 and Q14-OA14. II = 12 = 1 mA, 11N being 2 mA, the resistors being of 1 Kohm value.

Figure 15 is similar in many respects to the earlier circuits but with the current source CS differently connected. It shows that the current source can be connected to the "top" of the circuit as drawn. A similar re-arrangement of the connection of the current source can be effected for the circuits of Figures lotto 14. As an example ofthis, see Figure 16, which is derived from Figure 10.

In Figure 16, lo1 is out of phase with VIN while 102 is in phase with IIN- Also it should be notedthatthe emitters of 015 and 016 are at the same potential.

Note that in the circuits of Figures 1 to 9, the outputs are in-phase with each other and with the input, whereas Figures 10 to 16 each have two outputs which are in anti-phase. For all of the circuits described, the splitting ratios are independent of the load impedances and load voltages in the linear region.

Claims

1. Acurrentsplitting circuit, which includes a current source connected between a reference potential and a common point, a first output terminal connected via a first resistive impedance to the common point, a transistor whose collector-emitter path is connected via a second resistive impedance to the common point, a second output terminal connected to the other end of the collector-emitter path, and a differential amplifier having its output connected to the base of the transistor, one of its inputs connected to the first output terminal and the other of its inputs connected to the emitter of the transistor, the arrangement being such that the current generated by the current source is split between the output circuits connected to the two output terminals in a ratio dependent on the ratio of the values of the two resistive impedances.

2. A circuit as claimed in claim 1, in which the differential amplifier is an operational amplifier whose non-inverting input is connected to said first output terminal and whose inverting input is connected to the emitter of said transistor, and in which the transistor has its collector connected to the second output terminal.

3. A current splitting circuit, which includes a current source connected between a reference potential and a common point, a first output terminal connected via a first resistive impedance to the common point, a transistor whose collector is connected to a second output terminal and whose emitter is connected via a second resistive impedance to the common point, so that the connection from the second output terminal to the common point is via the collector-emitter path of the transistor, and an operational amplifier having its output connected to the base of the transistor; its noninverting input connected to the first output terminal and its inverting input connected to the emitter of the transistor, the arrangement being such that the current generated by the current source is split between the output circuits connected to the two output terminals in a ratio dependent on the ratio of the values of the two resistive impedances.

4. A circuit as claimed in Figure 3, in which the differential amplifier is an operational amplifier whose inverting input is connected to the first output terminal and whose non-inverting input is connected to the collector of the transistor, and in which the transistor has its emitter connected to the second output terminal.

5. A circuit as claimed in claim 1, in which the differential amplifier includes a long-tailed pair formed by two transistors whose emitters are connected together and to another current source and a further two transistors connected as a current mirror between a reference potential and the other two transistors, each current mirror collector being connected to a long-tailed pair collector, in which the first output terminal is connected to the base of one of the long-tailed pair transistors the collector of which is connected to the base of the first-mentioned transistor, and in which the base of the other long-tailed pair transistor is connected to the collector of the first-mentioned transistor.

6. A current splitting circuit which includes a circuit as claimed in claim 1 plus further differential amplifier-transistor combinations all of whose amplifiers are connected to the first output terminal.

7. A current splitting circuit which includes a current source connected between a reference potential and a common point, a first output terminal connected via a first resistive impedance to the common point, a first transistor one end of whose emitter-collector path is connected via a second resistive impedance to the commonpoint, a second output terminal connected to the other end of the transistor's emitter-collector path, and a differential amplifier formed by second, third, fourth and fifth transistors, wherein the emitters of the second and third transistors are connected together and to a second current source, wherein the base of the third transistor is connected to the first output terminal while the base of the fourth transistor is connected to the junction between the second resistive impedance and the first transistor, wherein the collector of the second transistor is connected to the collector of the fourth transistor and to the base of the first transistor, the collector of the third transistor being connected to the collector of the fifth transistor, wherein the emitters of the fourth and fifth transistors are connected together and to another reference potential, wherein the bases of the fourth and fifth transistors are connected together while the base of the fifth transistor is also connected to the collector of the fifth transistor and wherein the arrangement is such that the current from the first current source is split between the two output terminals in a ratio dependent on the ratio of the values of the two resistive impedances.

8. A current splitting circuit, substantially as described with reference to Figures 1, 2, 3, 4, 5, 6, 7, B,9, 10,11,12,13,14, iSor 16 of the accompanying drawings.