Abstract:
A current source using a bandgap voltage circuit includes a current gain circuit between the output of the bandgap circuit and the current output transistor. On-off control is provided by a switchable bias circuit providing an ON potential to start the bandgap and a clamping circuit opening the feedback loop.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a current source circuit using a bandgap voltage circuit.  
           [0003]    2. Description of the Related Art  
           [0004]    Current sources using bandgap voltage circuit are known in the art for example from U.S. Pat. No. 5,581,174.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention provides embodiments which have improved power supply rejection and which allow for turning on and off the current generator in a simple manner. Further embodiments allow for multiple current outputs.  
           [0006]    According to an embodiment of the present invention there is provided a current source having a sensing transistor and a bandgap circuit having first and second control transistors and a current mirror, the sensing transistor having a control electrode and a main current path, the main current path being connected to a feedback resistance at a first node, the other end of the feedback resistance being at a reference potential, each of the first and second control transistors having respective control electrodes, respective emitters and respective collectors, the first node being connected to the control electrodes of the first and second control transistors, the emitter of the first control transistor coupled to the reference potential via a first resistance and the emitter of the second control transistor coupled to the emitter of the first control transistor via a second resistance, the current mirror having a diode-connected transistor and a controlled transistor, the diode connected transistor connecting the collector of the first control transistor to a power rail and the controlled transistor connecting the collector of the second control to the power rail, the bandgap circuit being dimensioned to provide a first potential across said feedback resistance, characterised by a current amplifier having an input and an output, the input being connected to the collector of the second control transistor and the output being connected to the control electrode of the sensing transistor.  
           [0007]    Preferably the first and second control transistors are of a first conductivity and the current mirror transistors are of a second opposite conductivity and wherein the current amplifier has a first amplifying transistor of said second conductivity having a control electrode connected to the collector of the second control transistor and a collector connected to the input of a second current mirror, said second current mirror comprising transistors having said first conductivity coupled to said reference potential.  
           [0008]    Advantageously said second current mirror has an output connected to a diode-connected transistor of said second conductivity type, said output being further connected to the control electrode of said sensing transistor.  
           [0009]    Conveniently the controlled transistor of the said current mirror has a first width and the amplifying transistor has a greater width.  
           [0010]    Preferably the current source further comprises a start up circuit for the bandgap, the stare up circuit having a pull-up transistor for pulling said first node up to a second potential having a lesser magnitude than the first potential.  
           [0011]    Advantageously said pull-up transistor is an emitter follower of said first conductivity and has a base connected to a voltage source comprising plural series diodes.  
           [0012]    Conveniently said base is further connected to a switch for selectively shorting said diodes in response to a control signal.  
           [0013]    Conveniently again said switch is an n FET.  
           [0014]    Preferably said start up circuit further comprises a clamping transistor connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal.  
           [0015]    Advantageously said clamping transistor is a p FET.  
           [0016]    Preferably said switch is an n FET, and said start up circuit further comprises a p FET connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal, the current source having a control terminal for receiving a first voltage level operable to turn off said current source and a second voltage level operable to start said current source, said control terminal being connected to a control electrode of the p FET and to the gate of the n FET via an inverter.  
           [0017]    Preferably again the current source has a plurality of second conductivity type output transistors, each having an emitter connected to said power supply rail, a base connected to the control electrode of the sensing transistor, wherein each of said output transistors has a collector providing a respective current output.  
           [0018]    Advantageously at least one of said output transistors has greater width than another of said output transistors whereby said at least one output transistor provides a higher output current. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0019]    A preferred but exemplary embodiment of the invention will now be described with reference to the accompanying FIGURE which shows a schematic diagram of a current source in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The current source of the embodiment consists of a bandgap circuit  1  which has a first NPN bipolar transistor  2  having a base connected in common to that of a second NPN bipolar transistor  7 . The first bipolar transistor  2  has a greater effective width that the second transistor  7 , for example five times greater. The effect is that for a similar base-emitter potential the first transistor  2  will conduct more current than the second transistor  7 . The emitter of the first transistor  2  is connected to the emitter of the second transistor  7  via a resistance  19  and the emitter of the second transistor  7  is connected to a reference potential VEE via a resistance  15 .  
         [0021]    The collectors of the first  2  and second  7  NPN transistors are connected to a positive supply rail Vcc via a current mirror  6  composed of PNP transistors  9 , 28 . The second NPN transistor  7  has its collector connected to the positive supply rail Vcc via a diode-connected PNP transistor  9  which has its base connection in common with a controlled PNP transistor  28  serving to connect the collector of the first NPN transistor  2  to the positive supply rail Vcc. The collector of the first NPN transistor  2  is further connected to the base of a first amplifying PNP transistor  44  which has an emitter connected to the positive supply rail and a collector connected to a second current mirror  3 . The second current mirror  3  has a first NPN transistor  46  which is diode-connected, and which has an emitter connected to the reference rail VEE. The base of transistor  46  is connected in common to the base of a controlled NPN transistor  45 , with emitter connected to the reference rail VEE and with a collector connected to a diode-connected PNP transistor  47  and the emitter of transistor  47  connected to the positive supply rail. Together the transistors  44 - 47  form a current amplification circuit  48 . To provide current gain the first amplifying transistor  44  is wider than the controlled transistor  28  of the first current mirror  6 , for example twice as wide. In the preferred embodiment transistors  45 ,  46  and  47  are of the same size as transistor  28 .  
         [0022]    The collector of transistor  45  is also connected to the base of a sensing transistor  10 , being a PNP transistor having its emitter connected to the positive supply rail Vcc. The collector of transistor  10  is connected to the reference rail VEE via a feedback resistor  60 , the node  50  between the transistor  10  and the resistor  60  being connected to the commoned bases of the first and second NPN transistors  2 ,  7 .  
         [0023]    In operation the bandgap circuit, being connected in a loop including the current amplifier and the feedback resistor, provides a constant potential at the node  50 . The constant potential at node  50  is produced by virtue of a constant current through the sensing transistor  10  and the base potential of the sensing transistor  10  is thus such as to give rise to this constant current. The base potential is fed to three output PNP transistors  11 ,  12 ,  13 , each of which has a respective emitter connected to the positive supply rail and a respective collector forming an output node  101 ,  102 ,  103 . In the embodiments shown transistor  11  and  12  are each twice the width of transistor  10  and transistor  13  is four times the width of transistor  10 . As a result output terminals  101  and  102  each produce a magnitude of current double that of the current through transistor  10  whereas the node  103  produces a current four times the magnitude of the current through transistor  10 .  
         [0024]    The current source circuit has a high power supply rejection, defined as the amount of variation of power supply voltage which appears in the output current. The power supply rejection at the output, which depends upon the power supply rejection at node  50  is the ratio of the output resistance of the sensing transistor  10  to the feedback resistance  60  divided by the loop gain of the circuit. Given that the node  50  is in the feedback loop and given the gain of the loop including the current amplifying circuit a theoretical value of power supply rejection of minus 78 dB may be achieved in embodiments of the invention.  
         [0025]    The circuit so far defined is unlikely to be self-starting. To achieve self-starting it is necessary to cause the bandgap circuit I to start to conduct. To achieve this an NPN emitter follower transistor  26  has its emitter connected to the commoned bases of the first and second NPN transistors  2  and  7 . The collector of the emitter follower  26  is connected to the positive supply rail Vcc and the base is connected to the positive supply rail Vcc via a resistor  61 . The base is further connected to the reference rail VEE via the series connection of two diode-connected NPN transistors  4 A and  4 B. A switch in the form of an N-FET  35  has its main current path connected between the base of emitter follower transistor  26  and the reference supply rail VEE and the FET has a gate connection to the output of a CMOS inverter having a P-type pull-up transistor  36  and an N-type pull-down transistor  37 . The gates of the transistors  36  and  37  are connected in common to a control terminal  40  which is also connected to a P-type transistor  41  having its main current path between the positive supply rail Vcc and the collector of the second NPN transistor  2 .  
         [0026]    The operation of the start-up circuitry will now be described:  
         [0027]    When the control terminal  40  is at a high potential the gate of the switch  35  is at a low potential and therefore the switch  35  remains non-conducting. In this situation current flows through the resistor  61  to the series connection of the diodes  4 A and  4 B causing a base potential on the emitter follower  26  of two diode voltages above the reference potential. Hence the emitter of the emitter follower  26  will have a potential of one diode voltage above the reference potential and this value is fed to the commoned gates of the first and second NPN transistors of the bandgap circuit  1 , this potential being sufficient to start the bandgap. Once the bandgap loop is operational the potential at the node  50  is higher than one diode potential above the reference rail and as a result the emitter follower  26  plays no part in the normal operating mode.  
         [0028]    The P-type transistor  41  constitutes a control for turning off the current source.  
         [0029]    During the start-up condition the high potential at the control terminal  40  maintains the P-transistor  41  off, therefore not affecting operation of the bandgap. When however the potential at the control terminal  40  falls towards the reference level, the P-type transistor  41  turns on and pulls the collector of the second NPN transistor  2  of the bandgap towards the positive supply potential. This in turn causes the current amplifying transistor  44  to turn off and turns off the bandgap loop. At the same time the low potential at control terminal  40  is supplied to the inverter  36 ,  37  and the N-type switch  35  turns on shorting out the diodes  4 A and  4 B and reducing the base voltage of the emitter follower  26  to substantially zero.  
         [0030]    The constant current circuit described produces a constant current output over temperature and supply voltage. It is turned on and off easily and the control circuitry for starting and stopping operation has no substantial effect on operation.  
         [0031]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, U.S. Pat. No. 5,581,174 are incorporated herein by reference, in their entirety.  
         [0032]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.