Abstract:
Three-transistor, four-terminal precision current source is switchable. Current is passed between two of the terminals in the presence of a switching signal on another terminal. While switched on, the current is regulated according to a reference voltage supplied to the remaining terminal.

Description:
TECHNICAL FIELD 
     This invention relates to constant current supplies, and particulary to switchable precision current supply circuits that can be easily integrated in parallel. 
     Although constant current supply sources are well known, switchable current sources present difficult problems, especially when it is desired to be able to integrate the circuits on a substrate. Usually, precision current sources that are switchable require a large number of stages or active elements, such as transistors. The size and complexity of the circuits also increase as the required current increases. 
     The circuits become more complicated when there are additional constraints. The latter may include special load requirements such as common-cathode light-emitting diodes (LED&#39;s). It may be desirable to drive up to 3000 such LED&#39;s each requiring 10 to 40 milliamperes current which must be essentially equal. 
     BACKGROUND ART 
     U.S. Pat. No. 4,118,712 discloses the use of switchable current sources for digital-to-analog conversion of a counter value. The switched currents are proportional to the binary values of the signals used for switching. The switched currents are summed to produce a voltage that is proportional to the logarithm of the binary value being converted. 
     U.S. Pat. No. Re. 29,619 also shows digital-to-analog conversion using switchable current sources and current summing. Another constant current switching circuit is shown in U.S. Pat. No. 3,787,734 which also uses a constant voltage reference source. 
     U.S. Pat. No. 4,282,478 shows the sensing of a current through the load using a voltage drop resistor in conjunction with a threshold detector for controlling the current. 
     U.S. Pat. No. 4,277,696 shows a three-transistor, three-terminal switching circuit using an inverted Darlington pair to provide thyristor switching action. 
     These references are typical of the current sources and switching circuits that are found in the prior art, but none of the references found in the prior art show or suggest the invention as claimed herein. 
     DISCLOSURE OF THE INVENTION 
     In accordance with the present invention, a switchable current source is coupled between a power supply and a load and utilizes three transistors. A first transistor is coupled from the power supply to the load. Two other complementary transistors have their emitters coupled together and their collectors coupled between the base and collector of the first transistor. One of the two complementary transistors has its base coupled to a switching signal source and the other has its base coupled to a reference level voltage. The latter regulates the current through the first transistor and the other switches the current on or off. 
     This circuit arrangement allows several stages to be coupled in parallel to provide more current switching capacity. It can be easily integrated on a substrate because it contains few components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing FIGURE is a schematic showing the use of the invention to provide current to a light-emitting diode using four circuits of the invention in parallel to increase the current supplied to the load. 
    
    
     DETAILED DESCRIPTION 
     As shown in the FIGURE, four parallel circuits are coupled between a power source 20 and a load 17. The operation of each of the circuits is the same and therefore, only the first one need be explained in detail. 
     The current to the load is carried by a transistor 10 which is connected at its collector end to the power source 20 by a resistor 15 which acts to limit the maximum current through the transistor 10 and to insure that all parallel branches carry an equal amount of the load current. 
     The main drive transistor 10 has its base coupled to its collector through two complementary transistors 11 and 12. The base of the transistor 11 is coupled to a reference voltage source 21. The action of the transistor 11 is to regulate the current through the driver transistor 10. To regulate is herein used to mean to determine the amount of current, i.e., to provide a constant current through the driver transistor 10. 
     The base of the other complementary transistor is coupled to a switching signal source which may include, as shown in the FIGURE, a level shifting circuit comprising a transistor 19 and a voltage divider comprising the resistors 16 and 18. The action of the transistor 12 is to control the current through the driver transistor 10. To control is herein used to mean to turn off or to turn on the current flow through the transistor 10. 
     When the transistor 12 is turned on by a suitable switching signal, it is driven into the saturation region so that the voltage drop from its collector to its emitter is a small and constant value. Since the resistance 15 is assumed to be very small compared to the emitter resistance and the base spreading resistance of the transistor 11, the base-to-emitter voltage drop of the latter is maintained substantially constant and consequently, the current through the transistor 11 is constant. Since the current passed by the driver transistor 10 is proportional to its base current, which is the current through the transistor 11, the current supplied to the load through the driver transistor 10 remains constant while the transistor 12 is turned on. 
     Turning off the transistor 12 by a suitable change in the switching signal cuts off the current through the transistor 11 which, in turn, cuts off the current through the driver transistor 10. 
     Therefore, due to the action described above, the current through the drive transistor 10 is turned on and off via the action of the applied switching signal. While turned on, the current is regulated to a precise value. 
     Although the transistors 11 and 12 are shown and described as complementary transistors, they can also be of the same type as long as they are coupled in series so as to conduct in the same direction. That is, if they are transistors of the same polarity, preferably PNP types, the collector and emitter of the transistor 12 would be reversed from the orientation shown in the FIGURE. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.