Constant-current digital-to-analog converter

Abstract

A digital-to-analog converter the output circuit of which comprises a set of switching transistors arranged as current generators. The currents through the switching transistors are maintained constant by means of a supply voltage adjusting circuit comprising a separate reference transistor matched to one of the switching transistors and energized by the same voltage supply lines as the switching transistors. The supply voltage adjusting circuit includes an operational amplifier which senses the collector current of the reference transistor, and adjusts the supply voltage so as to maintain that collector current constant. This automatic adjustment of the supply voltage also maintains the current through the switching transistors constant.

I claim: .[.1. A digital-to-analog converter, the combination of: 
     
        between said first and second common lines..]. .[.7.  Apparatus as in claim 4, wherein the collector currents of said switching transistors are directed to said output junction; said sensing means being responsive to the collector current of said reference transistor..]. .[.8. Apparatus as in claim 7, wherein said sensing means includes means to compare the reference transistor collector current with a reference current of fixed magnitude..]. .[.9. Apparatus as in claim 8, wherein said voltage varying means comprises an operational amplifier the output of which controls the voltage applied between the base of said reference transistor and said other end of said reference   
     
     
        resistor..]. 10. For use in signal-processing apparatus, such as a digital-to-analog converter, incorporating switchable current source means comprising a transistor having a collector, an emitter, and a base, and wherein the magnitude of current flow produced by such source is set by a resistor connected at one end to said emitter to carry the emitter current of such transistor, there being DC power supply means having a pair of output terminals connected respectively to the base of such transistor and to the remote end of said resistor to maintain a DC voltage differential therebetween, whereby to produce from the collector of such transistor an output current which is relatively unaffected by changes in load at the output of the current source; that improvement for maintaining the output current of such current source .Iadd.means .Iaddend.constant in the face of changes in operating characteristics of such current-source transistor, comprising:   a reference transistor having a collector, an emitter and a base, said reference transistor being located adjacent said current source transistor and matched thereto;   a reference resistor connected at one end to the emitter of said reference transistor;   circuit means connecting said power supply means output terminals respectively to the base of said reference transistor and to the remote end of said reference resistor;   sensing means responsive to changes in magnitude of the collector current of said reference transistor; and   feedback voltage adjustment means under the control of said sensing means for automatically altering the DC voltage produced at said DC power supply output terminals as required to maintain constant the collector current from said reference transistor, thereby serving to continuously adjust the DC voltage applied to said current source transistor so as to maintain constant the collector current thereof in the face of changes in operating   
     
     
        characteristics of the current-source transistor. 11. Apparatus as claimed in claim 10, wherein said voltage adjustment means comprises an operational amplifier having its output serving as one of said power 
     
     
        supply output terminals. 12. Apparatus as claimed in claim 11, including a source of reference current connected to the input of said operational amplifier in opposition to said collector current of said reference transistor, said operational amplifier continuously comparing said reference current with the collector current of said reference transistor and automatically adjusting its output voltage to maintain those two 
     
     
        currents equal. 13. In a digital-to-analog converter and the like, comprising individually-activatable switching transistors each having a collector, an emitter and a base, said transistors being arranged to produce output currents for summing at an output junction; a plurality of resistors each connected between a first common line and the emitter of a respective switching transistor;   a second common line connected to the bases of said switching transistors;   a reference transistor having a collector, an emitter and a base;   a reference resistor connected between the emitter of said reference transistor and said first common line;   means connecting the base of said reference transistor to said second common line;   an operational amplifier having its output connected to one of said common lines to control the potential thereof relative to the other of said common lines;   first circuit means connecting the collector of said reference transistor to the input of said operational amplifier to supply to said input a first current corresponding to the collector current to the reference transistor; and   second circuit means connected to the input of said operational amplifier to supply to said input a second current of fixed magnitude opposing said first current, said operational amplifier automatically adjusting the potential difference between said first and second common lines to maintain said first current equal to said second current, maintaining the collector current of said reference transistor constant, and thereby   
     
     
        holding the collector current of said switching transistors constant. 14. For use in a signal-processing apparatus, such as a digital-to-analog converter, incorporating constant current generator means comprising a transistor current source having a collector, an emitter, and a base, and wherein the magnitude of current flow produced by such source is set by a circuit connected between the base and the emitter and including a resistor and a DC power supply means, said resistor and the DC voltage of said power supply means being connected in series between said base and said emitter, whereby to produce from the collector of said current source transistor an output current which is relatively unaffected by changes in load at the output of the current source; that improvement for maintaining the output current of such current source constant in the face of changes in operating characteristics of such current-source transistor, comprising:   a reference transistor having a collector, an emitter and a base, said reference transistor being located adjacent said current source transistor and matched thereto;   a reference resistor;   circuit means connecting the DC voltage of said power supply means and said reference resistor in series between the base and the emitter of said reference transistor;   sensing means responsive to changes in magnitude of the current passing through said reference transistor; and   feedback voltage adjustment means under the control of said sensing means for automatically altering the DC voltage produced by said DC power .Iadd.supply .Iaddend.means as required to maintain constant the sensed current passing through said reference transistor, thereby serving to continuously adjust the DC voltage applied to said current source transistor so as to maintain constant the current passing therethrough in the face of changes in operating characteristics of the current source transistor. .Iadd. 15. For use in signal-processing apparatus incorporating a transistor current source energized by a supply voltage provided by a DC power supply means so as to produce an output current proportional to the magnitude of said supply voltage;   the improvement for fixing the output current of such current source at a particular value and for maintaining the current at such value irrespective of changes in operating characteristics of the current-source transistor resulting, for example, from changes in ambient temperature; said improvement comprising:   a control transistor located adjacent said current source transistor and having operating characteristics matched thereto;   circuit means connecting said supply voltage of said DC power supply means to said control transistor to produce current therethrough proportional to the magnitude of said supply voltage;   means to produce a controlled reference current; and   a negative feedback circuit responsive to a comparison of said reference current and the current flow through said control transistor for adjusting the supply voltage of said DC power supply means to tend to maintain the current through said control transistor proportional to said reference current, thereby to maintain the output current from said current-source transistor proportional to said reference current and to minimize changes of said output current resulting from changes in operating characteristics of the current-source transistor. .Iaddend. .Iadd. 16. Apparatus as claimed in claim 15, wherein said reference current means comprises:   a source of reference voltage; and   impedance means connected to said source of reference voltage to develop a reference current proportioned to the voltage magnitude. .Iaddend..Iadd. 17. Apparatus as claimed in claim 15, wherein said transistor current source comprises a first collector, a first emitter, and a first base;   a first resistor connected at one end to said first emitter;   means connecting said DC power supply means between said first base and the other end of said resistor, whereby the transistor current flow through the said first emitter is proportional to the magnitudes of said first resistor and the voltage developed by said DC power supply means,   said control transistor comprising a second collector, a second emitter and a second base;   a second resistor connected at one end to said second emitter; and   means connecting said DC power supply means between said second base and the other end of said second resistor, whereby the transistor current flow through said second emitter is proportional to the magnitudes of said second resistor and the voltage developed by said DC power supply means. .Iaddend. .Iadd. 18. Apparatus as claimed in claim 17, wherein said negative feedback circuit comprises an operational amplifier;   means for directing to the input of said operational amplifier currents corresponding to said reference current and the current through said control transistor; and   means connected to the output of said operational amplifier for automatically adjusting the magnitude of voltage developed by said DC power supply means so as to maintain said control transistor current proportional to said reference current. .Iaddend..Iadd. 19. Apparatus as claimed in claim 17, wherein said negative feedback circuit includes means for comparing said reference current with the current through said second collector and for adjusting said supply voltage to provide proportionally therebetween. .Iadd. 20. Signal-processing apparatus comprising a first transistor arranged as a current source energized by a DC power supply means so as to produce an output current; said current source having a pair of terminals to which a control voltage is to be applied to control said output current to a level proportional to the magnitude of said control voltage;   control means for fixing the output current of said current source at a particular value and for maintaining the current substantially at such value in the face of changes in operating characteristics of the first current-source transistor resulting, for example, from changes in ambient temperature; said control means comprising:   a second transistor current source adjacent said first transistor current source and matched thereto;   said second current source having a pair of terminals to which a control voltage is to be applied to control the output current of said second current source to a level proportional to the applied control voltage;   said DC power supply means including means to produce a common control voltage for both of said current sources;   voltage-adjusting means responsive to a control signal for varying said control voltage;   means to produce a reference current;   means for comparing said reference current and the current flow through said second current-source transistor; and   negative feedback means coupled to said comparing means for developing and applying a control signal to said voltage-adjusting means to automatically adjust said control voltage to tend to maintain the current through said second current-source transistor proportional to said reference current, thereby to maintain the output current from said first current-source transistor proportional to said reference current and minimize changes of said output current resulting from changes in operating characteristics of   
     
     
        said first current-source transistor. .Iaddend..Iadd. 21.  In a digital-to-analog converter, the combination of: a plurality of current sources comprising respective transistors;   each of said current source transistors including a base, an emitter, and a collector;   a plurality of current-setting resistors each connected at a first end thereof to the emitter of a respective current source transistor;   output circuit means coupled to all of said current sources to combine the currents contributed thereby in accordance with a digital input signal;   a reference current source comprising a reference transistor matching one of said plurality of current source transistors, and including a base, an emitter, and a collector;   a reference resistor connected at a first end thereof to said reference transistor emitter;   power supply means for developing supply voltages for said transistors;   means coupling a controllable supply voltage of said power supply means between (1) the bases of said transistors and (2) the second ends of said resistors, thereby to control the current through said transistors in accordance with the magnitude of said controllable supply voltage; and   sensing means responsive to the magnitude of current flow through said reference transistor, said sensing means including means operable to adjust said controllable supply voltage to tend to maintain said current through said reference transistor constant, the adjustments to said controllable supply voltage serving to tend to maintain the currents through said plurality of current source transistors constant.   
     
     
        .Iaddend..Iadd. 22.  Apparatus as claimed in claim 21, wherein said plurality of current source transistors have output currents providing equal current densities within all of those transistors. .Iaddend.