Abstract:
An integrated current reference circuit uses two current mirror circuits, in which one of the transistors of one of the current mirrors has a back gate connection to the power rail, the drain-source path being connected to the power rail via a voltage offset element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an integrated current reference circuit. 
     BACKGROUND OF THE INVENTION 
     It is known to provide a constant current generating circuit using two interconnected current mirrors, of which one current mirror is of p FETs and the other is of n FETs. Such circuits have traditionally required one of the branches of the current generator to contain a resistor. 
     Use of resistors in integrated circuits is not desirable for a number of reasons, for instance because of the temperature dependence thereof, because of the area occupied by a resistor and the difficulty of manufacture. 
     The present invention therefore aims to at least partly mitigate the difficulties of the prior art. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an integrated current reference circuit comprising a first current mirror and a second current mirror, each current mirror having a respective controlling node and a respective controlled node, the controlling node of the first current mirror being connected to the controlled node of the second current mirror and vice-versa, wherein the first current mirror comprises a first FET and a second FET, said first and second FETs each having a respective source, gate and drain terminal, said second FET further having a substrate terminal, the first FET having its gate and drain electrode connected together in common and forming the controlling node of the first current mirror and the second FET having its gate connected in common with the commoned gate and drain of the first FET, and further comprising voltage offset circuitry connecting the source electrodes of the first and second FETs to a supply terminal, the substrate of the first FET being connected to its source and the substrate terminal of the second FET being connected to the supply terminal. 
     Preferably the second current mirror comprises a first FET and a second FET, the first FET of the second current mirror having a gate and a drain electrode connected together in common and the second FET of the second current mirror having a gate connected to the commoned gate and drain of the first FET of the second current mirror and further comprising an output FET having a gate connected in common to the gate of the second FET of the second current mirror. 
     Advantageously the first FET of the second current mirror has a smaller current carrying capacity than the second FET of the second current mirror. 
     Advantageously said first and second FETs of the first current mirror are p FETs and said first and second FETs of the second current mirror are n FETs. 
     Conveniently said voltage offset circuitry comprises a first offset element connected between the source electrode of the first FET of the first current mirror and said supply terminal and a second offset element connected between the source electrode of the second FET of the first current mirror and said supply terminal. 
     Preferably said first and second offset elements comprise diode-connected p FETs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 shows a prior art constant current generating apparatus and; 
     FIG. 2 shows a preferred embodiment of a current reference circuit in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the various figures like reference numerals refer to like parts. 
     Referring to FIG. 1, a current reference circuit according to the prior art consists of a first current mirror comprising a first p FET  11  having a gate connected in common with its drain and a source connected to a positive supply terminal  1 , and a second p FET  10  having a source connected to the positive supply terminal  1  and a gate connected to the common gate/drain electrodes of the first transistor  11 . 
     The circuit further comprises a second current mirror which consists of a first n FET  12  having a gate electrode connected in common with its drain electrode, and a source electrode connected to a negative supply terminal  2 . The second current mirror has a second n FET  13  whose gate is connected to the common gate and drain electrodes of the first n FET  12 . The source of the second n FET  13  of the second current mirror is connected via a resistor  17  to the negative supply terminal  2 . 
     The gate electrode of the second n FET  13  is also connected to the gate electrode of an output transistor  14 , which has a source electrode connected to the negative supply terminal  2 , the drain  15  of the output transistor  14  providing a circuit output. 
     The common gate and drain electrodes of the first transistor  11  of the first current mirror constitutes a controlling node of that current mirror and the drain of the second transistor  10  of the first current mirror constitutes a controlled node of that current mirror. As is known to those skilled in the art, as the parameters of the transistors  10  and  11  are matched by virtue of their being formed on an integrated circuit, application of a current to the controlling node causes a corresponding current at the controlled node, depending on the relative sizes of the transistors. 
     Similarly, the common gate and drain electrodes of the first transistor  12  of the second current mirror constitutes a controlling node of the second current mirror whereas the drain of the second transistor  13  of the second current mirror constitutes the controlled node of that transistor. 
     Further reference to FIG. 1 shows that the controlled node of the first current mirror is connected to the controlling node of the second current mirror and the controlling node of the first current mirror is connected to the controlled node of the second current mirror. 
     In the arrangement described, the second transistor  13  of the second current mirror is “stronger” than the first transistor  12  of the second current mirror. It will be clear to those skilled in the art that the arrangement shown in FIG. 1 has in fact two stable operating conditions, namely one in which no current flows through either current mirror and a second state in which a non-zero current is sunk by the output terminal  15 . 
     Considering the second stable state, with second n FET  13  having a conductivity which is n times that of the first n FET  12 . Naming the current through the controlling transistor  11  of the first current mirror and the controlled transistor  13  of the second current mirror as I 2 , and the current through the controlled transistor  10  of the first current mirror and the controlling transistor  12  of the second current mirror as I 1 , the following arise: 
     The first current mirror constrains the two currents such that 
     
       
         I 1 =I 2   
       
     
     The second current mirror constrains the two currents such that 
     
       
         I 2 =n×I 1 . 
       
     
     Clearly these two constraints alone cannot be satisfied. However, the source potential of the transistor  13  is increased by the current flow through the resistor  17 . This reduces the gate-source potential, and thus the ability of transistor  13  to conduct current under the bias conditions provided by the transistor  12 . 
     The result is that the two currents I 1  and I 2  reach an equilibrium condition at which the two currents become equal and independent of the voltage applied to the circuit. 
     Referring now to FIG. 2, the current reference circuit shown has no resistor in either branch. Thus, the source electrodes of the first transistor  12  and the second transistor  13  of the second current mirror are connected directly to the negative supply terminal  2 . The first current mirror comprises a first p FET  31  having its gate connected in common with its drain and a second p FET  30  having a gate connected to the commoned gate and drain terminal of he first p FET  31 . The source of the first p FET  31  is connected to the positive supply terminal via a diode-connected p FET  21  and the source of the second p FET  30  of the first current mirror is connected to the positive supply terminal  1  via a second diode-connected p FET  20 . The substrate of the first p FET  31  is connected to the source of the first p FET  31  as is conventional; however the substrate of the second p FET  30  is connected to the positive supply terminal  1  so as to provide a so-called “back gate” connection. 
     As is known to those skilled in the art the provision of a back gate connection to a relatively high potential—here provided by the voltage offset circuitry  20 —modifies the threshold voltage of the associated transistor due to the so-called “body effect”. 
     The first p FET  31  of the first current mirror is a relatively small device, whereas the second p FET  30  of the first current mirror is a relatively large device. 
     As is known to those skilled in the art, the back gate connection of the second p FET  30  requires an additional voltage to be applied to the front (conventional) gate to achieve the same value of current as would be achieved by a similar transistor having a back gate connection to the source. Thus, the threshold voltage of the second p FET  30  is increased. 
     In operation, the current provided by the first transistor  31  (the smaller transistor) is constrained to be the same as that provided by the second (larger) transistor  30  by the second current mirror comprising transistors  12  and  13 . This stabilization occurs because the gate-to-source voltage of the first transistor  31  is effectively opposed by the back gate voltage on the first transistor  30 .