Transistor base current compensation circuitry
FIELD OF THE INVENTION 
     This invention relates to a system which compensates for load current drawn from a source to limit loading of the source and in particular to a system which effectively cancels the loading effects of bipolar transistors of a voltage reference circuit so as to increase the accuracy thereof. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,263,519, in which there is common inventorship and a common assignee with the present patent application, is directed to a plurality of voltage reference circuits that each use the parasitic bipolar transistors formed by the drain regions, p-wells and the monolithic substrate of a Complementary Metal-Oxide Silicon (CMOS) integrated circuit. Reproduced herein and denoted as FIG. 1 is the circuitry of FIG. 5 of the U.S. Pat. No. 4,263,519. The E REF  voltage appearing at output terminal 70 is a reference voltage that is relatively accurate. The bandgap voltage (E BG ) appears between terminals 69 and 30. If resistor 61 is equal to resistor 62, then E REF  =2 E BF . The operation of the circuit of FIG. 1 herein is well known and is described in U.S. Pat. No. 4,263,519 which is incorporated herein by reference. Some applications require greater accuracy than this circuit is capable of. One limiting factor on the accuracy of this circuit is that base current is drawn from node 69 to drive transistors 31 and 32. This base current, even through it is typically only a small fraction of the current flow through resistor 61, limits the accuracy of the voltage appearing at output terminal 70. In some applications, the needed accuracy of a reference voltage is greater than can be achieved by the circuitry of U.S. Pat. No. 4,263,519. 
     U.S. Pat. No. 3,551,832 (J. G. Graeme) is directed to complementary bipolar circuitry which generates a current equal to load base current it draws from a source. The generated current is fed back to the source such that effectively the circuitry draws essentially no current from the source. Accordingly, there is effectively no loading of the source and the output voltage thereof can stay within a highly accurate range. One requirement of the Graeme circuitry is that the collectors of the transistors be separate. A silicon chip in which there are fabricated complementary metal-oxide silicon (CMOS) transistors inherently contains parasitic bipolar transistors in which all of the collectors are common, typically being part of the substrate of the chip. Thus, the Graeme circuitry is not easily fabricated in such a chip since it requires bipolar transistors with separate collectors. A chip, which includes CMOS and bipolar transistors in which the collectors are separate, is more complex to fabricate and therefore generally more expensive than one which uses the inherent parasitic bipolar transistors. 
     It is thus desirable to have circuitry which can be formed using the parasitic bipolar transistors and conventional Field Effect Transistors (FETs) of a Complementary Metal-Oxide-Silicon (CMOS) integrated circuit to compensate for the needed base drive of a voltage reference circuit as described. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment the present invention is directed to current compensation circuitry which is connectible to a voltage generator (e.g., the previously discussed bandgap reference voltage generator) which comprises or drives a load element, such as bipolar transistors, whose base current requirements limit the accuracy of the voltage generator. The compensation circuitry is adapted to supply the needed base current and thus improves the accuracy of the voltage generator. It preferably is fabricated on a CMOS integrated circuit chip using parasitic bipolar transistors. 
     In one embodiment the compensation circuitry comprises an operational amplifier having two inputs and an output, a current mirror having an input and two outputs and a load element. The output of the operational amplifier is coupled to the input of the current mirror. The first output of the current mirror is coupled to the second input of the operational amplifier and to the load element. The second output of the current mirror is coupled to the first input of the operational amplifier. The output of the operational amplifier is coupled to the input of the current mirror. 
     In another embodiment the compensation circuitry comprises a first load element, first circuit means having first and second inputs and an output with the output being coupled to the second input thereof for generating at the second input thereof a potential level which is essentially the same as one applied to the first input thereof, and second circuit means coupled to the first and second inputs of the first circuit means for sensing current drawn by the first load element and for generating an essentially identical current flow into a node coupled to the first input of the first circuit means. 
     The invention will be better understood from the following detailed description taken in connection the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic of a prior art reference voltage generator; 
     FIG. 2 shows a reference voltage generator with current compensation circuitry in accordance with the present invention; and 
     FIG. 3 shows a preferred embodiment of the current compensation circuitry of FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 2, there is shown a reference voltage generator circuit with current compensation 100 comprising within a first dashed line rectangle a reference voltage generator 102 and within a second dashed line rectangle current compensation circuitry 104 in accordance with the present invention. Reference voltage generator 102 is the same as the prior art reference voltage circuitry shown in FIG. 1 herein. The reference numbers used for the components and terminals of circuitry 102 are the same as those used for the corresponding components and terminals of the prior art circuitry sown in FIG. 1 with a &#34;0&#34; added there after. One limitation of the accuracy of reference voltage generator 102 is that base current needed to bias n-p-n transistors 310 and 320 is drawn from resistor 610 via node 690. The base current for transistors 310 and 320 varies with the betas of the transistors and with temperature. Transistor 310 and resistors 340 and 360 and transistor 320 and resistor 350 may be denoted as load elements. As will become clear from the following description, current compensation circuitry 104 generates a current which flows into node 690 and is essentially identical to the base current which flows from node 690 and into the bases of transistors 310 and 320. The base current normally drawn through node 690 from resistor 610 is replaced by current compensation circuitry 104 and thus essentially all of the current flow through resistor 610 flows through resistor 620. This improves the accuracy of the output voltage E REFO  appearing at terminal 700 of reference generator circuitry 102 by typically an order of magnitude or better. 
     Current compensation circuitry 104 comprises a two input operational amplifier 112, a current mirror circuit 118, an n-p-n transistor 120 and a resistor 124. Operational amplifier 112 may be denoted as a first circuit means; current mirror circuit 118 may be denoted as a second circuit means; and transistor 120 and resistor 124 may be denoted as a load element or as a dummy load element. Node 690 is coupled to a positive input terminal of operational amplifier 112 and to a second (slave) output terminal of current mirror circuit 118. A negative input terminal of operational amplifier 112 is coupled to a first (master) output terminal of current mirror circuit 118, to the base of transistor 120 and to a node 116. An output terminal of operational amplifier 112 is coupled to an input (generally denoted in the art as a common terminal) of current mirror circuit 118 and to a node 114. The emitter of transistor 120 is coupled to a first terminal of resistor 124 and to a node 122. The collector of transistor 120 is coupled to a terminal 200 and to a positive voltage +VO. A second terminal of resistor 124 is coupled to a terminal 300 and a reference voltage which is shown as ground. 
     The electrical path from the output of operational amplifier 112, through current mirror circuit 118 and to the negative input of operational amplifier 112 and the base of transistor 120, effectively causes the potential of node 116 to be essentially the same as the potential of node 690. Transistor 120 is designed to be the equivalent of transistors 310 and 320 and resistor 124 is designed to be equal to the equivalent of resistors 340, 360 and 350. If the same power supplies and base voltages are applied to transistors 310, 320 and 120, then the same total base current that flows into both transistors 310 and 320 flows into the base of transistor 120. Current mirror 118 acts to generate a flow of current into the bases of transistors 310 and 320 (node 690) which is identical to that flowing into the base of transistor 120 (node 116). Thus the current flow from node 690 to provide base current for transistors 310 and 320 is supplied into node 690 by circuitry 104 instead of having to be supplied from resistor 610. Accordingly, circuitry 104 supplies all of the base current for transistors 310 and 320 and thus all the current which flows through resistor 610 also flows through resistor 620. This improves the accuracy of the voltage E REFO  appearing at the output terminal 700 of reference voltage generator 102 by typically an order of magnitude or better. 
     Referring now to FIG. 3, there is illustrated a preferred embodiment of current compensation circuitry 104 with circuitry of operational amplifier 112 shown within a dashed line rectangle 112a and circuitry of the current mirror circuit 118 shown within a dashed line rectangle 118a. 
     Operational amplifier 112 comprises Field Effect Transistors (FETs) 124, 126, 128 and 130, an n-p-n bipolar transistor 132 and a resistor 138. Current mirror circuit 118 comprises FETs 134 and 136. In a preferred embodiment FETs 124 and 126 are both n-channel Metal-Oxide-Silicon (MOS) FETs and FETs 128, 130, 134 and 136 are all p-channel MOS FETs. The gate of transistor 124 is coupled to the source of FET 136 and to node 690. The sources of transistors 124 and 126 are coupled to a first terminal of resistor 138 and to a node 144. Second terminals of resistors 138 and 124 are coupled to terminal 300 and to ground potential. The sources of transistors 128 and 130 and the collectors of transistors 120 and 132 are coupled together to terminal 200 and to positive voltage +VO. The drain of transistor 124 is coupled to the gates of transistors 128 and 130, to the drain of transistor 128 and to a node 140. The drain of transistor 126 is coupled to the drain of transistor 130, to the base of transistor 132 and to a node 142. The emitter of transistor 132 is coupled to the sources of transistors 134 and 136 and to node 114. The gates of transistors 126, 134 and 136 are coupled to the drain of transistor 134, to the base of transistor 120 and to node 116. The emitter of transistor 120 is coupled to one terminal of resistor 124 and to a node 122. 
     Transistors 134 and 136 serve as the master and slave legs, respectively, of the current mirror 118. The current that flows through transistor 134 is duplicated and flows through transistor 136. Thus the current that flows into the base of transistor 120 is essentially the same as flows into node 690 from transistor 136. The gates of transistors 124 and 126 draw essentially no current out of nodes 690 and 116, respectively, since the input impedances of transistors 124 and 126 is high as they are both FETs. Since transistor 120 and resistor 124 are the equivalent of transistors 310 and 320 and resistors 340, 360 and 350, and the supply voltages, +VO and ground, used for power are identical, the current flowing into the base of transistor 120 is essentially equal to the sum of the currents flowing into the bases of transistors 310 and 320. In view of this it is clear that the current needed to bias transistors 310 and 320 is supplied by compensation circuitry 104. Thus the current which flows through resistor 610 is the same as flows through resistor 620 and accordingly the accuracy of voltage generator circuitry 102 is improved. 
     It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications are possible within the scope of the invention. For example, the circuit configurations used for operational amplifier 112 and current mirror circuit 118 may be modified or completely changed so long as the basic functions performed by these elements are maintained. Still further, the combination of transistor 120 and resistor 124 would be modified if the load elements of the voltage generator circuit are modified.