High resolution monolithic digital-to-analog converter

Claim 2

In an IC digital-to-analog converter, wherein a plurality of individual switchable current generators are connected to a common current supply terminal tO furnish weighted current contributions thereto when selectively activated by a digital input signal, each of said current generators having a control terminal to which a switch signal is supplied to activate the associated current generator; the improvement for providing a current generator which develops a very small current without requiring the use of an excessively large resistor, comprising: first and second transistors; means connecting the bases of said transistors together; means connecting the emitters of said transistors together; a resistor connected between said emitters and a supply voltage line to produce a flow of current through both of said transistors and said resistor when the two transistors are switched on; switch means responsive to one bit of a digital input signal and including means to activate both of said transistors together; and means connecting the output of said first transistor to said current supply terminal to furnish thereto a fractional portion of the total current flowing through both of said transistors.

Claim 3

Apparatus as in claim 1, wherein the areas of said emitters are equal, whereby the total current divides equally between the two transistors.

Claim 4

Apparatus as in claim 1, wherein the areas of said emitters are proportioned in an integral multiple relationship.

Claim 5

Apparatus as in claim 3, wherein the area of the emitter of said second transistor is three times the area of the emitter of said first transistor.

Claim 6

A digital-to-analog converter having a reference current source the output current of which is maintained substantially independent of changes in temperature, said reference current source comprising: first transistor means comprising at least two transistors; a current generator having an output terminal connected to said first transistor means to produce current flow through said two transistors; first circuit means interconnected with said first transistor means and said current generator to produce through the second of said transistors a current proportional to the difference in the base-to-emitter voltages of said two transistors; a first resistor connected between said second transistor and said current generator output terminal to conduct the current passing through said second transistor and to produce at said generator output terminal a reference voltage having a first component proportional to the difference in base-to-emitter voltages of said two transistors; a third transistor; means supplying current flow through said third transistor; second circuit means interconnecting said first resistor and said third transistor to add to said reference voltage a second component proportional to the base-to-emitter voltage of said third transistor, the thermally-produced changes in said first and second components being in opposite direction whereby said reference voltage is compensated for temperature changes; the further improvement wherein said current generator comprises a fourth transistor having its emitter connected to said output terminal; a reference current output transistor; supply voltage means; an output resistor connected between the emitter of said output transistor and said supply voltage means to set the level of current flow through said output transistor; means connecting the base of said output transistor to the base of said fourth transistor, whereby the regulated voltage at the emitter of said third transistor produces a correspondingly regulated voltage at the emitter of said output transistor and a correspondingly regulated output current through said output resistor; and a current load connected in series with said output transistor and said supply voltage to receive said regulated output current.

Claim 7

A digital-to-analog converter having a reference current source the output current of which is maintained substantially independent of changes in temperature, said reference current source comprising: first transistor means comprising at least two transistors; a current generator having an output terminal connected to said first transistor means to produce current flow through said two transistors; first circuit means interconnected with said first transistor means and said current generator to produce through the second of said transistors a current proportional to the difference in the base-to-emitter voltages of said two transistors; a first resistor connected between said second transistor and said current generator output terminal to conduct the current passing through said second transistor and to produce at said generator output terminal a reference voltage having a first component proportional to the difference in base-to-emitter voltages of said two transistors; a third transistor; means supplying current flow through said third transistor; second circuit means interconnecting said first resistor and said third transistor to add to said reference voltage a second component proportional to the base-to-emitter voltage of said third transistor, the thermally-produced changes in said first and second components being in opposite direction whereby said reference voltage is compensated for temperature changes; the further improvement wherein said current generator comprises a fourth transistor having its emitter connected to said output terminal; a reference current output transistor; supply voltage means; an output resistor connected between the emitter of said output transistor and said supply voltage means to set the level of current flow through said output transistor; means connecting the base of said output transistor to the base of said fourth transistor, whereby the regulated voltage at the emitter of said third transistor produces a correspondingly regulated voltage at the emitter of said output transistor and a correspondingly regulated output current through said output resistor; and a current load connected in series with said output transistor and said supply voltage to receive said regulated output current.

Claim 8

Apparatus as claimed in claim 5 wherein said load is connected to the collector of said output transistor.

Claim 9

Apparatus as claimed in claim 6, wherein said load comprises a regulaTing circuit arranged to produce constant current output from the converter.

Claim 10

Apparatus as claimed in claim 6, wherein said load comprises means to produce a bipolar offset current in the output of the converter.

Claim 11

A reference current source for use in digital-to-analog converters and the like comprising: an output transistor; an output resistor connected between the emitter of said output transistor and a supply line, to fix the magnitude of current flow through that transistor in proportion to the voltage between the supply line and the transistor base; an output circuit connected to the collector of said output transistor to supply to a load the current flowing through said output transistor and resistor; a second transistor the emitter of which serves as a reference voltage terminal; said output transistor and said second transistor being arranged to provide equal current densities therein; means connecting the bases of said two transistors together, whereby the base voltage of said output transistor will track the base voltage of said second transistor; a reference voltage circuit connected to the emitter of said second transistor to receive current therefrom and to develop at that emitter a reference voltage closely corresponding to the voltage to be developed at the emitter of said output transistor; said reference voltage circuit including transistor means and resistor means interconnected between said second transistor emitter and said supply line and arranged to cooperate in regulating said reference voltage with respect to changes in temperature to provide substantially constant current through said load.

Claim 12

A reference current source for use in digital-to-analog converters and the like comprising: an output transistor; an output resistor connected between the emitter of said output transistor and a supply line, to fix the magnitude of current flow through that transistor in proportion to the voltage between the supply line and the transistor base; an output circuit connected to the collector of said output transistor to supply to a load the current flowing through said output transistor and resistor; a second transistor the emitter of which serves as a reference voltage terminal; said output transistor and said second transistor being arranged to provide equal current densities therein; means connecting the bases of said two transistors together, whereby the base voltage of said output transistor will track the base voltage of said second transistor; a reference voltage circuit connected to the emitter of said second transistor to receive current therefrom and to develop at that emitter a reference voltage closely corresponding to the voltage to be developed at the emitter of said output transistor; said reference voltage circuit including transistor means and resistor means interconnected between said second transistor emitter and said supply line and arranged to cooperate in regulating said reference voltage with respect to changes in temperature to provide substantially constant current through said load.

Claim 13

Apparatus as claimed in claim 9, wherein said transistor means comprises at least one transistor connected to simulate more than one diode and to develop a voltage corresponding to the base-to-emitter voltage of such diodes, said reference voltage being set at a value equal to an integral multiple of 1.205 volts.

Claim 14

Apparatus as claimed in claim 10, including a voltage-dividing network having first, second and third series-connected resistors; the emitter of said one transistor being connected to the remote end of said third resistor; the base of said one transistor being connected to the junction between said second and third transistors; the collector of said one transistor being connected to the junction between said first and second resistors; the remote ends of said first and third resistors being connected to said voltage terminal and a power supply line respectively.

Claim 15

Apparatus as claimed in claim 11, wherein said series-connected resistors are in the ratio of 1:9:10; and said reference voltage is set at 2.410 volts.

Claim 16

Apparatus as claimed in claim 9, wherein said transistor means includes means to produce a non-linear variation with temperature in the value of said reference voltage, to compensate for the non-linear variation in base-to-emitter voltage of said output resistor.

Claim 17

Apparatus as claimed in claim 13, wherein said transistor means includes one transistor with its base connected to said voltage terminal to produce at said terminal a voltage component corresponding to the base-to-emitter voltage of said one transistor; and current-generator means arranged to flow through said one transistor a current which varies non-linearly with respect to temperature, whereby the base-to-emitter voltage of said one transistor will vary non-linearly to develop the required compensation.

Claim 18

Apparatus as claimed in claim 14, wherein said current-generator means comprises a supply transistor having an emitter resistor connected in series with its emitter; and voltage means to maintain the voltage between the base of said supply transistor and the remote end of said emitter resistor at a value slightly different from 1.205 volts.

Claim 19

Apparatus as claimed in claim 15, wherein said voltage means fixes said voltage at aboUt 1.45 volts, to provide the desired non-linearity of current with temperature.

Claim 20

Apparatus as claimed in claim 9, wherein said transistor means includes one transistor; current-generator means supplying a flow of current through said one transistor to develop a base-to-emitter voltage therein; a base resistor connected between the base of said one transistor and said voltage terminal; the ohmic resistances of said base resistor and said output resistor being proportioned to provide, in response to a change in temperature, a change in voltage across said base resistor, due to the change in base current of said one transistor, equal to the change in voltage across said output resistor that would be caused by the change in base current in said output transistor, whereby to provide for constant collector current in said output transistor in the face of changes in base current thereof.

Claim 21

Apparatus as claimed in claim 17, wherein said ohmic resistances are proportioned in the ratio of the collector currents of said output transistor and said one transistor.

Claim 22

For use in a digital-to-analog converter and the like, a reference voltage circuit comprising: transistor means including first and second transistors for developing a first voltage component proportioned to the difference in base-to-emitter voltages thereof, and a third transistor developing a second voltage component proportional to the base-to-emitter voltage thereof to be combined at an output terminal with said first component to provide a thermally-compensated composite reference voltage; means to set said reference voltage at an integral multiple of 1.205 volts with respect to a power supply ground point; said integral multiple being greater than one; circuit means connecting said first transistor between said output terminal and said power supply ground point; said circuit means including means to simulate the equivalent effect of more than one diode-connected transistor.

Claim 23

For use in a digital-to-analog converter and the like, a reference voltage circuit comprising: transistor means including first and second transistors for developing a first voltage component proportioned to the difference in base-to-emitter voltages thereof, and a third transistor developing a second voltage component proportional to the base-to-emitter voltage thereof to be combined at an output terminal with said first component to provide a thermally-compensated composite reference voltage; means to set said reference voltage at an integral multiple of 1.205 volts with respect to a power supply ground point; said integral multiple being greater than one; circuit means connecting said first transistor between said output terminal and said power supply ground point; said circuit means including means to simulate the equivalent effect of more than one diode-connected transistor.

Claim 24

Apparatus as claimed in claim 19, wherein said circuit means comprises first, second and third resistors series-connected between said terminal and said ground point; the emitter of said first transistor being connected to said ground point; the base of said first transistor being connected to the junction between said second and third resistors; and the collector of said first transistor being connected to the junction of said first and second transistors.

Claim 25

Apparatus as claimed in claim 20, wherein the ohmic resistors of said series-connected resistors are in the ratio of 1:9:10.

Claim 26

A regulated reference current source comprising: first circuit means having an input terminal arranged to receive an input current to produce at said input terminal a reference voltage; said first circuit means including temperature-compensating circuit means operable to alter selected parameters of said first circuit means to regulate said reference voltage so as to maintain its magnitude at least approximately constant with changes in temperature; a current generator comprising a first transistor with its emitter connected to said input terminal to supply said input thereto; a reference current output transistor with its emitter connected to a resistor to form a current generator producing an output current proportional to the base voltage; said first transistor and said output transistor being arranged to provide equal current densities therein; means connecting the bases of said first transistor and said output transistor together, whereby the emitter voltage of said output transistor tracks said regulated voltage at the emitter of said first transistor and thereby regulates said output current; and an output circuit connected between the collector of said output transistor and the remote end of said resistor, to supply regulated current to a load.

Claim 27

A regulated reference current source comprising: first circuit means having an input terminal arranged to receive an input current to produce at said input terminal a reference voltage; said first circuit means including temperature-compensating circuit means operable to alter selected parameters of said first circuit means to regulate said reference voltage so as to maintain its magnitude at least approximately constant with changes in temperature; a current generator comprising a first transistor with its emitter connected to said input terminal to supply said input thereto; a reference current output transistor with its emitter connected to a resistor to form a current generator producing an output current proportional to the base voltage; said first transistor and said output transistor being arranged to provide equal current densities therein; means connecting the bases of said first transistor and said output transistor together, whereby the emitter voltage of said output transistor tracks said regulated voltage at the emitter of said first transistor and thereby regulates said output current; and an output circuit connected between the collector of said output transistor and the remote end of said resistor, to supply regulated current to a load.

Claim 28

A digital to-analog converter comprising a group of current sources aRranged to be selectively activated in a pattern corresponding to a digital input; means to sum the current contributions of said current sources to produce an analog output signal; regulating means operable to control the magnitude of currents produced by said current sources, said regulating means including means to compare a current proportional to the currents of said sources with a reference current and to automatically adjust a parameter of said current sources to cause the currents produced thereby to conform to said reference current; a source of reference current for said regulating means, said source of reference current having a control terminal to which a control voltage is supplied to regulate the magnitude of said reference current and maintain it substantially constant; a source of bipolar offset current for said converter to supply to the output thereof a current which is a fractional portion of the full-scale output current of said converter, said source of bipolar offset current having a control terminal to which a control voltage is supplied to regulate the magnitude of said offset current and maintain it substantially constant; and a voltage control source having its output voltage connected to said control terminals of said source of reference current and said source of offset current, said voltage control source including means for adjusting said output voltage to maintain said reference current and said offset current at substsantially constant magnitudes, thereby to assure precise tracking between the offset output current and the converter output current produced by said group of current sources.

Claim 29

A digital to-analog converter comprising a group of current sources aRranged to be selectively activated in a pattern corresponding to a digital input; means to sum the current contributions of said current sources to produce an analog output signal; regulating means operable to control the magnitude of currents produced by said current sources, said regulating means including means to compare a current proportional to the currents of said sources with a reference current and to automatically adjust a parameter of said current sources to cause the currents produced thereby to conform to said reference current; a source of reference current for said regulating means, said source of reference current having a control terminal to which a control voltage is supplied to regulate the magnitude of said reference current and maintain it substantially constant; a source of bipolar offset current for said converter to supply to the output thereof a current which is a fractional portion of the full-scale output current of said converter, said source of bipolar offset current having a control terminal to which a control voltage is supplied to regulate the magnitude of said offset current and maintain it substantially constant; and a voltage control source having its output voltage connected to said control terminals of said source of reference current and said source of offset current, said voltage control source including means for adjusting said output voltage to maintain said reference current and said offset current at substsantially constant magnitudes, thereby to assure precise tracking between the offset output current and the converter output current produced by said group of current sources.

Claim 30

An integrated-circuit monolithic digital-to-analog converter comprising: a first group of switch transistors each having emitter, base and collector electrodes; a first set of resistors for said first group of switch transistors respectively, each such resistor being connected in the emitter circuit of the corresponding switch transistor to set the level of current therethrough; said first set of resistors having ohmic resistances related in accordance with a binary pattern to binarily weight the individual switch currents correspondingly; the emitter areas of said switch transistors being proportioned to the magnitude of current flowing through the respective transistor; means connecting the bases of all of said switch transistors together; a second group of switch transistors identical to said first group of switch transistors; a second set of resistors identical to said first set of resistors and interconnected with said second group of switch transistors in the same manner as said first set of resistors is interconnected with said first group of switch transistors; means connecting together the bases of all of said second group of switch transistors; power supply means coupled to said bases to set the potentials thereof at predetermined controlled levels with respect to the emitter resistors; first and second current-divider transistors each having emitter, base and collector electrodes; means connecting the bases of said current-divider transistors together; first and second resistors each connected at one end to the emitter of a respective one of said current-divider transistors; means connecting said first and second resistors together at the other ends thereof; said first and second resistors having ohmic resistances proportioned in a ratio corresponding binarily to the number of switch transistors in said first group of transistors, whereby to provide that the current-divider transistors separately conduct current according to that ratio; the areas of the emitters of said two current-divider transistors also being proportioned in accordance with said ratio, to provide for equal current densities in said two current-divider transistors; a current-summing terminal; means coupling the collectors of all of said first group of switcH transistors to said current summing terminal; means coupling the common connection of said first and second resistors to the collectors of all of said second group of switch transistors; means coupling the collector of one of said current-dividing transistors to said current-summing terminal to deliver thereto a predetermined fraction of the collector currents of all of said second group of switch transistors, in accordance with said ratio; and means coupling the collector of the other of said current-dividing transistors to said power supply means, to provide for current flow through said other current-divider transistor without the current flowing into said current summing terminal.

Claim 31

An integrated-circuit monolithic digital-to-analog converter comprising: a first group of switch transistors each having emitter, base and collector electrodes; a first set of resistors for said first group of switch transistors respectively, each such resistor being connected in the emitter circuit of the corresponding switch transistor to set the level of current therethrough; said first set of resistors having ohmic resistances related in accordance with a binary pattern to binarily weight the individual switch currents correspondingly; the emitter areas of said switch transistors being proportioned to the magnitude of current flowing through the respective transistor; means connecting the bases of all of said switch transistors together; a second group of switch transistors identical to said first group of switch transistors; a second set of resistors identical to said first set of resistors and interconnected with said second group of switch transistors in the same manner as said first set of resistors is interconnected with said first group of switch transistors; means connecting together the bases of all of said second group of switch transistors; power supply means coupled to said bases to set the potentials thereof at predetermined controlled levels with respect to the emitter resistors; first and second current-divider transistors each having emitter, base and collector electrodes; means connecting the bases of said current-divider transistors together; first and second resistors each connected at one end to the emitter of a respective one of said current-divider transistors; means connecting said first and second resistors together at the other ends thereof; said first and second resistors having ohmic resistances proportioned in a ratio corresponding binarily to the number of switch transistors in said first group of transistors, whereby to provide that the current-divider transistors separately conduct current according to that ratio; the areas of the emitters of said two current-divider transistors also being proportioned in accordance with said ratio, to provide for equal current densities in said two current-divider transistors; a current-summing terminal; means coupling the collectors of all of said first group of switcH transistors to said current summing terminal; means coupling the common connection of said first and second resistors to the collectors of all of said second group of switch transistors; means coupling the collector of one of said current-dividing transistors to said current-summing terminal to deliver thereto a predetermined fraction of the collector currents of all of said second group of switch transistors, in accordance with said ratio; and means coupling the collector of the other of said current-dividing transistors to said power supply means, to provide for current flow through said other current-divider transistor without the current flowing into said current summing terminal.

Claim 32

Apparatus as in claim 24, including means coupling said power supply means to the bases of said two current-dividing transistors and comprising impedance means to establish the base voltages thereof at a level offset with respect to the base voltage of said second group of switch transistors; and means connecting together all of the bases of said first and second groups of switch transistors.