A CURRENT TO VOLTAGE CONVERTER CIRCUIT

Abstract: A conversion circuit (40) for providing an output voltage that is proportional to an applied current input. The conversion circuit (40) comprises a current mirror (l0) which sinks a current at an output (22) the magnitude of which varies directly with the current input that is applied thereto and a feedback amplifier (72, 74, 76, 78) coupled with the output (22) of the current mirror (l0) which provides a feedback current thereto the magnitude of which varies as a function of the current sank at the is established at an output (38) of the feedback amplifier.

Description: Background of the Invention The present invention relates to converter circuits and, more particularly, to a circuit for producing an output voltage that is representative of an applied current input. There are a myriad of uses for a current to voltage conversion circuit wherein an output voltage is produced that has a known relationship to a current input signal. For example, a digital to analog converter (DAC) may be realized utilizing such a conversion circuit. Thus, a digital input code consisting of a plurality of discrete current inputs can be converted into a representative analog output voltage. Such a conversion circuit requires an accurate current mirror in conjunction with other circuitry for accurately converting the applied current input into the output voltage. Hence, a need exists for a current to voltage integrated conversion circuit including a precision current mirror in conjunction with feedback circuitry for producing an output voltage having a known relationship to an applied current input signal. The following patents are noted as disclosing subject matter related to the present invention: US-A-4,485,352 discloses a current mirror with buffering and similar feedback topology, and US-A-4,525,683 features a base current error cancellation circuit. Summary of the Invention In accordance with the present invention there is provided a current to voltage conversion circuit, comprising: a current mirror circuit including first and second transistors each having a collector, a base and an emitter, said bases being coupled together; amplifier means having an input coupled to said collector of said first transistor and having an output coupled to the output of the circuit for buffering the voltage developed at said collector of said first transistor; a feedback network for sampling the voltage at the output and returning a proportional signal to said emitter of said second transistor; and current supply means for providing substantially equal currents to said collectors of said first and second transistors, the conversion circuit having an input coupled to said emitter of said first transistor via a resistor network characterized in that the balancing of the voltages developed at said collectors of said first and second transistors is achieved by coupling the inverting and noninverting inputs of a differential amplifier to said collectors of said first and second transistors, respectively, and by coupling the output of said differential amplifier to said bases of said first and second transistors. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial block and schematic diagram illustrating a current mirror circuit utilized in the conversion circuit of the present invention; FIG. 2 is a detailed schematic diagram illustrating the current mirror of FIG. 1; and FIG 3 is a schematic diagram of the current to voltage converter circuit of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning to FIG. 1 there is shown a block diagram of a precision current mirror 10 that is utilized in the conversion circuit of the present invention. Current mirror 10 is suited to be manufactured in integrated circuit form and can be fabricated using present day low voltage integrated circuit fabrication processes. Current mirror 10 includes a pair of matched, i.e. equal emitter area transistors 12 and 14 which have their base or control electrodes coupled together. The emitters or first electrodes of the transistors are returned to ground reference. The collector or second electrode of transistor 12 is coupled both to reference current source 16 and the non-inverting input of differential amplifier l8 at node 20. Reference current source l6, which is coupled to power supply conductor 24 at which is supplied a source of DC operating potential, sources a reference current I R to the collector of transistor l2. The inverting input of differential amplifier l8 is coupled to the collector of transistor l4 at node 22 at which an output current I O is sunk. The output of amplifier l8 is coupled to the bases of transistors l2 and l4. The output of current mirror l0 is taken at output terminal 26. In operation, output 26 is coupled to some load circuitry (not shown) such that the current I o is sourced from node 22 which establishes the voltage V O thereat. Differential or operational amplifier l8 forces the voltage developed at node 20 to be substantially equal in value to the voltage V O while providing base current drive at the output thereof to transistors l2 and l4. A quiescent operating balanced state is established when transistor l2 is supplied sufficient base drive to enable it to sink substantially all of the current supplied from current reference l6. Since transistors l2 and l4 are matched devices, they will have the same base-emitter voltage drop thereacross whereby the current I O will be substantially equal to the current I R . Moreover, because the voltage at node 20 is forced to be substantially equal to the voltage established at node 22 the collector-base voltage drops of the two transistors l2 and l4 will also be equal and will track one another. Thus, the effects of the "Early" voltage errors as well as beta process variations, can be neglected. Referring now to FIG. 2 current mirror l0 is shown in more detail. It is to be understood that the components in FIG. 2 which correspond to components in FIG. 1 are designated by the same reference numerals. Differential amplifier l8 is illustrated in FIG. 2 comprises a pair of PNP transistors 28 and 30 the emitters of which are differentially connected to current supply 32. The bases of these transistors are coupled respectively to nodes 20 and 22 which correspond to the two inputs of amplifier l8. The collectors of transistors 28 and 30 are coupled to a differential-to-single ended output load comprising diode connected transistor 34 and transistor 36. The differential-to-single ended load circuit is conventional in operation and is well known to those skilled in the art. Current supply 32 provides the "tail" current to the differential amplifier. In operation, transistor 30 will be rendered conductive to supply the base currents to transistors l2 and l4 thereby turning these devices on until the balanced condition is reached at which transistor l4 sinks the current I O from the load circuitry coupled to output 26. In the balanced condition transistor l2 is supplied sufficient base current drive from transistor 30 to sink all of the current from supply l6. Transistor 28 is sufficiently turned on by transistor l2 being rendered conductive to, in turn, render diode connected transistor 34 conductive. This turns on transistor 36 such that transistor 30 provides the required base current drive to transistors l2 and l4 as previously described. Any variations in the voltage V O established at node 22 is forced onto node 20 as aforementioned. Hence, the collector-base voltage drops across transistor l2 and l4 track each other whereby the operation of the current mirror l0 functions in the manner described above with reference to FIG. 1. Turning now to FIG. 3 there is illustrated current to voltage converter circuit 40 of the present invention which includes current mirror l0 as described above. Converter 40 produces a voltage at output 38 that is representative of the current input supplied at input 42 to the current mirror. It is again to be understood that components of FIG. 3 corresponding to like components shown in FIGS. 1 and 2 are designated by the same reference numerals. Further, current to voltage converter circuit 40 is suited to be fabricated in integrated circuit form using conventional bipolar fabrication processes well known to those skilled in the art of manufacturing integrated circuits. Current mirror l0 is realized by differential amplifier l8 which comprises transistors l2, l4, 28, 30, 34, and 36 as described above. The reference current supply circuit includes a pair of matched PNP transistors 44 and 46 the bases of which are coupled together with the emitter of transistor 48. The emitters of transistors 44 and 46 are returned to power supply conductor 24 via resistors 50 and 52 respectively. A current source 54 is coupled with the collector of transistor 44 as well as to the base of transistor 48 which sources a predetermined and substantially constant current I R to ground reference via power supply conductor 56. Multiple collector transistor 46 has two of its collectors coupled via diodes 60 and 62 to nodes 20 and 22, the inputs of differential amplifier l8. A third collector of transistor 46 is connected via lead 58 to supply the tail current required by differential amplifier l8 as described above. The emitter of transistor l2 is coupled to ground reference through series connected resistors 64 and 66. Similarly, the emitter of transistor l4 is coupled to ground reference through series connected resistors 68 and 70 with the interconnection therebetween being connected to input 42. An amplifier comprising NPN transistor 72 and quasi-Darlington connected NPN transistors 74 and 76 provides both the voltage output and a current feedback signal via resistor 78 to the interconnection between resistors 64 and 66, at node 80, of current mirror l0. Resistor 82 provides biasing between transistor 74 and 76 as is well known. The base of transistor 72 is coupled to the anode of diode 60 at which is established a bias potential for the transistor. The input to the amplifier is coupled to the output of current mirror l0 at node 22 and corresponds to the base of transistor 74. Capacitor 84 stabilizes the loop formed between current mirror l0 and the amplifier by placing a pole in the transfer characteristics of converter 40 to prevent oscillations. In operation, with no current input supplied at input terminal 42, converter 40 will seek a balanced operating state or condition that forces the voltage developed across resistor 66 to be equal to the voltage established across resistor 70 as will now be described. Transistors 44 and 46 are turned on by base current drive sourced through transistor 48 whereby current supply 54 sources a current through transistor 44 substantially equal to the value I R . This current is mirrored through transistor 46 such that bias currents are sourced from the multiple collectors of the transistor to render differential amplifier l8 operative. A bias voltage is therefore developed across diode 60 which enables transistor 72 to be turned on which, in turn, enables transistor 74 and 76 to be rendered conductive. Diode 62, it should be noted, is provided to ensure that current mirror l0 has a balanced configuration. As long as an unbalanced state exists, i.e. the voltage across resistor 66 being less than the voltage established at node 42, transistors 30 and 36 are rendered less conductive than transistors l2 and 28 whereby excess current drive is available to the base of transistor 74. This causes transistor 74 and 76 to conduct which supply a current feedback via resistor 78 to node 80 which raises the voltage developed across resistor 66 until this voltage equals the voltage established across resistor 70. Thereafter, transistors l2 and l4 conduct equally and the current sourced to output 22 of the current mirror l0 is equal to the current sank through transistor l4 plus the base current drive to transistor 74. The operation of the current to voltage converter circuit 40 is then at a quiescent balanced operating condition. As a current input is supplied to input terminal 42 the voltage developed across resistor 70 and hence the voltage at the emitter of transistor l4 increases. The current mirror will seek a new balanced operating state which will force the voltage developed at node 80 to again equal the voltage established at terminal 42 due to the current input. The current which is required to establish the voltage at node 80 is provided by transistor 76 which flows through resistor 78. Hence, the output voltage developed at output 38 is the sum of the voltages developed across resistors 66 and 78. Thus, the output voltage is proportional to the ratio of resistors 66 and 78 and is a function of the current signal applied at input 42. Current to voltage converter 40 may be utilized to provide a digital analog conversion. If, for example, multiple current inputs are supplied to input 42 that correspond to individual bits of a digital coded input signal, the analog output voltage produced at output 38 is representative of the digital signal. Hence, what has been described above, is a novel current to voltage conversion circuit suitable for producing an output voltage that is related to a current input and which is proportional to a resistor ratio.

Claims: A current to voltage conversion circuit (40), comprising: a current mirror circuit including first and second transistors (14, 12) each having a collector, a base and an emitter, said bases being coupled together; amplifier means ( 72, 74, 76) having an input coupled to said collector of said first transistor (14) and having an output coupled to the output (38) of the circuit (40) for buffering the voltage developed at said collector of said first transistor (14); a feedback network (78) for sampling the voltage at the output (38) and returning a proportional signal to said emitter of said second transistor (12); and current supply means (46) for providing substantially equal currents to said collectors of said first and second transistors, the conversion circuit (40) having an input (42) coupled to said emitter of said first transistor (14) via a resistor network (68, 70) characterized in that the balancing of the voltages developed at said collectors of said first and second transistors is achieved by coupling the inverting and non-inverting inputs of a differential amplifier (18) to said collectors of said first (14) and second (12) transistors, respectively, and by coupling the output of said differential amplifier to said bases of said first and second transistors. The circuit of claim 1 wherein said current mirror circuit includes: first (68) and second (70) resistors series coupled between said emitter of said first transistor and a ground reference potential, the input (42) of the conversion circuit (40) being coupled to the interconnection between said first and second resistors; and third (64) and fourth resistors (66) series coupled between said emitter of said second transistor and said ground reference potential, the interconnection between said third and fourth resistors being coupled to said output of said amplifier means (72, 74, 76). The circuit of claim 2 wherein said feedback network (78) includes a resistor coupled between the output (38) and said interconnection of said third and fourth resistors. The circuit of claim 3 wherein said amplifier means includes: a third transistor (74) having a base, a collector and an emitter, said collector being coupled to a source of operating potential, said base being coupled to the output of said current mirror circuit; and a fourth transistor (76) having a base, a collector and an emitter, said emitter being coupled to said output of said amplifier means, said collector being coupled to said source of operating potential, said base being coupled to said emitter of said third transistor. The circuit of claim 4 wherein said current supply means includes: a fifth transistor (44) having a base, a collector and an emitter, said emitter being coupled to said source of operating potential via an optional resistor; a sixth transistor (46) having a base, an emitter and first and second collectors, said emitter being coupled to said source of operating potential via an optional resistor, said first and second collectors being respectively coupled to said inverting and non-inverting inputs of said differential amplifier, said base being coupled to said base of said fifth transistor; and current source means (54) between said collector of said fifth transistor and said ground reference potential for sourcing a reference current therefrom. The circuit of claim 5 wherein said current source means further includes a seventh transistor (48) having a base, a collector and an emitter, said emitter being coupled to said base of said fifth transistor, said collector being coupled to said ground reference potential, said base being coupled to said collector of said fifth transistor. The circuit of claim 6 wherein said current mirror means further includes first and second diodes (60, 62) respectively coupled between said first and second collectors of said sixth transistor and said inverting and non-inverting inputs of said differential amplifier. The circuit of claim 7 wherein said amplifier means includes an eighth transistor (72) having a base, a collector and an emitter, said emitter being coupled to said collector of said third transistor, said collector being coupled to said source of operating potential, said base being coupled to said second collector of said sixth transistor.

Inventors: MILLER, IRA
Applicants: MOTOROLA, INC.
Application year: 1986
Main classification: H03F3
IPC codes: H03F3, G05F1, G05F3