Patent Publication Number: US-7723975-B2

Title: Current detecting circuit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a U.S. national stage of application No. PCT/JP2006/300444, filed on 16 Jan. 2006. Priority under 35 U.S.C. 119(a) and 35 U.S.C. 365(b) is claimed from Japanese Application No. 2005-009654, filed 17 Jan. 2005, the disclosures of which are also incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a current detection circuit which uses a current mirror circuit to indirectly detect a current. 
   2. Description of the Related Art 
   Current detection circuits are used in various applications such as motor drivers, power supplies, and the like. In a current detection circuit, in general, a resistor is inserted in a current supply path to a load, and current is directly detected from the two end-voltages thereof, but a method of indirect detection using a current mirror circuit is also being attempted. FIG. 1 of Patent Document 1 discloses a current mirror circuit 1 formed of transistors 3 and 2. When a size ratio of the transistors 3 and 2 is n:1, current flowing in the transistors 3 and 2 also has a ratio of n:1. When a predetermined voltage is given to a control terminal 4, the transistor 3 supplies a current IL1 to a load 13, and the transistor 2 supplies a detection current IS1=IL1+n to a resistor 11 via a transistor 9. Consequently, if the resistance value of the resistor 11 is R, a detection voltage R×IS1=R×IL1÷n is outputted at a detection terminal 12. 
   As described in Patent Document 2, this type of circuit can detect a load current with good accuracy and with no loss, without interposing a sense resistor in a load current path, and when detecting the load current, it is possible to easily switch gain in accordance with the size of the load current. 
   Patent Document 1: Japanese Patent Application, Laid-Open No. 2001-264365 
   Patent Document 2: Japanese Patent Application, Laid Open No. H7-113826 
   In  FIG. 1  of the abovementioned Patent Document 1, with the transistors 3 and 2, which make up the current mirror circuit 1, in an unsaturated state, a current IDS between source-drain terminals has a dependency with a voltage VDS across the source-drain terminals. Consequently, a feedback circuit 5 formed of an operational amplifier 6 and a transistor 7 performs control so that voltages VDS across the source-drain terminals of the transistors 3 and 2 become equal. 
   However, since capacity for phase compensation is necessary inside the operational amplifier 6 included in the feedback circuit 5, when this type of feedback circuit is provided, the circuit size becomes large. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in light of these conditions, and a general purpose thereof is the provision of a current detection circuit which, when indirectly detecting current using a current mirror circuit, can perform with good accuracy with a small sized circuit. 
   A current detection circuit in an embodiment of the present invention comprises: a current detection current mirror circuit including an output current transistor which supplies an output current, and a current detection transistor which supplies a detection current proportional to the output current; a compensation current mirror circuit which is connected to a current supply side of the current detection current mirror circuit, so that a terminal voltage of the output current transistor and a terminal voltage of the current detection transistor have a predetermined relationship; and a current detector which indirectly detects the output current by detecting the detection current. The “predetermined relationship” may be a relationship in which the terminal voltage of the output current transistor and the terminal voltage of the current detection transistor are equal. 
   According to this embodiment, by connecting the compensation current mirror circuit to the current detection current mirror circuit, it is possible to raise accuracy, with a small sized circuit, of potential of an electrode, such as, for example, a collector electrode or a drain electrode, of the output current transistor and the current detection transistor. 
   The current detection current mirror circuit may further include an input current transistor in order to generate, in the output current transistor, output current proportional to an input current from outside, and an input current supply current mirror circuit may also be provided, including a first input current supply transistor which receives the input current, a second input current supply transistor, connected to the input current transistor, which generates a current corresponding to the input current in the input current transistor, and a third input current supply transistor which supplies a bias current to the compensation current mirror circuit. 
   According to this embodiment, by supplying the bias current coupled with the input current, to the compensation current mirror, it is possible to adjust potential of electrodes, such as, for example, the collector electrode or the drain electrode, of the output current transistor and the current detection transistor, according to the input current. 
   The compensation current mirror circuit may include a first compensation transistor which is connected to the current detection transistor and which supplies, as the detection current, a current proportional to the bias current; and a second compensation transistor which is connected to the output current transistor and which receives the bias current at an opposite electrode thereof. 
   According to this embodiment, by making the output current of the first compensation transistor the detection current, it is possible to realize a detection current in which an effect of deviation in mirror ratio on the current detection current mirror circuit is reduced. 
   A ratio of current drive capability of the input current transistor and the current detection transistor, a ratio of current drive capability of the second compensation transistor and the first compensation transistor, and a ratio of current drive capability of the second input current supply transistor and the third input current supply transistor may be made to be corresponding, and the current drive capability of the second compensation transistor may be different to that of the input current transistor. 
   According to this embodiment, by adjusting the current drive capability of the second compensation transistor, within the collector current or the drain current of the output current transistor, a portion, flowing in the second compensation transistor, that does not become an output current Io, can be reduced, and it is possible to raise detection accuracy. 
   At least the current detection current mirror circuit and the compensation current mirror circuit may be integrated on one semiconductor substrate. The integration may also include a current detector and the integration may additionally include the input current supply current mirror circuit. 
   A motor apparatus of another embodiment of the present invention is provided with a motor, the current detection circuit of the above-described embodiment, and a motor drive circuit which drives the motor, referring to a current detected by the current detection circuit. 
   An electronic device of another embodiment of the present invention is provided with a recording disk, and the motor apparatus of the above described embodiment, which controls rotation of the recording disk. 
   In addition, arbitrary combinations of the above mentioned component elements, and changes of the representation of the present invention among devices, methods, systems, and the like, are also valid embodiments of the present invention. 
   Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
       FIG. 1  shows a configuration of a current detection circuit according to a first embodiment; 
       FIG. 2  shows a configuration of the current detection circuit according to a second embodiment; and 
       FIG. 3  shows a configuration of an electronic device in which the current detection circuit of  FIG. 1  or  FIG. 2  is preferably used. 
   

   DESCRIPTION OF THE INVENTION 
   The invention will now be described based on preferred embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention. 
   First Embodiment 
   A first embodiment illustrates a current detection circuit which detects current indirectly, using a current mirror circuit that uses a bipolar transistor. Furthermore, a configuration is illustrated in which collector potential between transistors that make up the current mirror circuit is kept the same as far as possible. 
     FIG. 1  shows a configuration of the current detection circuit  100   a  of the first embodiment. With this current detection circuit  100   a , an explanation is given of an example using a PNP bipolar transistor. An input current transistor Q 2 , a current detection transistor Q 4 , and an output current transistor Q 6  make up the first current detection current mirror circuit CM 2 . Emitter electrodes of the input current transistor Q 2 , the current detection transistor Q 4 , and the output current transistor Q 6  are connected to a high potential side reference potential Vcc, such as a power supply voltage, or the like. Base electrodes thereof are wire-connected to each other, and a current path thereof is connected to a current path of a collector side of the input current transistor Q 2 , via a first base current compensation transistor Q 8 . 
   A collector current IC of a bipolar transistor is given by Equation 1 as follows.
 
 IC= ( qDn/WB ) nP ·exp( qVBE/kT )  (Equation 1)
 
   Here, q represents charge, Dn represents electronic diffusion coefficient, WB represents base width, nP represents donor concentration, VBE represents inter base-emitter voltage, k represents Boltzmann coefficient, and T represents absolute temperature. 
   Base current IB of the bipolar transistor is given by Equation 2 as follows.
 
 IB= ( qDq/Lp ) pN {exp( qVBE/kT )−1}  (Equation 2)
 
   Here, Dq represents hole diffusion coefficient, Lp represents hole diffusion length, and pN represents base acceptor concentration. 
   Accordingly, a current amplification factor hFE of the bipolar transistor is given by Equation 3 as follows.
 
 hFE= ( qDn/WB ) nP ·exp(− qVBE/kT )/( qDn/WB ) pN {exp( qVBE/kT )−1}=( Dn/Dp )( nP/pN )( Lp/WB )  (Equation 3)
 
   As may be understood from Equation 3 described above, the current amplification factor hFE of the bipolar transistor is determined by transistor structure. For example, if the base width WB is halved, the current amplification factor hFE can be doubled. 
   In the present embodiment, the input current transistor Q 2 , the current detection transistor Q 4 , and the output current transistor Q 6  have different structures. Consequently, the current amplification factors hFE of each of the transistors Q 2 , Q 4 , and Q 6 , that is the current drive capabilities, are set to be different. In the present embodiment, a ratio of the current drive capabilities thereof, from the input current transistor Q 2 , in order, is set as 1:n:mn. Accordingly, in the first current detection current mirror circuit CM 2 , in order that the voltage across the base emitters is equal, a collector current flows in accordance with current drive capability. Specifically, a collector current of 1:n:mn, in order, from the input current transistor Q 2  is flowing. 
   An emitter electrode of a first base current compensation transistor Q 8  is commonly connected to base electrodes of the three transistors Q 2 , Q 4 , and Q 6 , and that make up the first current detection current mirror circuit CM 2 ; the base electrodes are connected to a current path on a collector side of the input current transistor Q 2 ; and the emitter electrodes are connected to a low potential side reference potential, such as a ground potential or the like. The first base current compensation transistor Q 8  uses its current drive capability to act to reduce current flowing into a current path on the collector side of the input current transistor Q 2 . In this way, accuracy is improved for a mirror ratio of current flowing in a current path on the collector side of the input current transistor Q 2 , and current flowing in a current path on a collector side of the output current transistor Q 6 . 
   In the present embodiment, supply of an input current to the current detection circuit  100   a  is carried out by an input current supply current mirror circuit CM 4  that includes a first input current supply transistor Q 10 , a second input current supply transistor Q 12 , and a third input current supply transistor Q 14 . Input current from outside is represented in  FIG. 1  by a symbol of a variable current source  10 . The first input current supply transistor Q 10 , the second input current supply transistor Q 12 , and the third input current supply transistor Q 14  have an NPN configuration, and use a similar structure. 
   An emitter electrode of the first input current supply transistor Q 10  is grounded via a first resistor R 2 ; a base electrode thereof is commonly connected to base electrodes of the transistors Q 12  and Q 14  making up an input current supply current mirror circuit CM 4 ; and collector electrodes thereof receive input current from outside. Furthermore, in order to configure the input current supply current mirror circuit CM 4 , a current path on a collector side of the first input current supply transistor Q 10 , and a current path in which base electrodes of the transistors Q 10 , Q 12 , and Q 14 , that make up the input current supply current mirror circuit CM 4 , are connected. In addition, similar to the first base current compensation transistor Q 8 , a transistor maybe inserted therebetween. 
   An emitter electrode of the second input current supply transistor Q 12  is grounded via a second resistor R 4 , and a collector electrode thereof is connected to a collector electrode of the input current transistor Q 2 . An emitter electrode of the third input current supply transistor Q 14  is grounded via a third resistor R 6 , and a collector electrode thereof is connected to a collector electrode of a second collector potential compensation transistor Q 18 , described later. 
   Consequently, a current equal or approximately equal to an input current from outside flows in a current path on a collector side of the input current transistor Q 2 , and at the same time, flows as a bias current in a current path on a collector side of the second collector potential compensation transistor Q 18 , described later. 
   A first collector potential compensation transistor Q 16  and a second collector potential compensation transistor Q 18  make up a first compensation mirror circuit CM 6 . The ratio of current drive capabilities thereof is set at n:1. This ratio corresponds to a ratio of current drive capabilities of the first current detection current mirror circuit CM 2 ; the first collector potential compensation transistor Q 16  and the current detection transistor Q 4  are set to a same current drive capability; and the second collector potential compensation transistor Q 18  and the input current transistor Q 2  are also set to a same current drive capability. 
   An emitter electrode of the first collector potential compensation transistor Q 16  is connected to a collector electrode of the current detection transistor Q 4 , and the collector electrodes thereof are connected to a current detector  20 , described later. The emitter electrode of the second collector potential compensation transistor Q 18  is connected to a collector electrode of the output current transistor QG, and the collector electrode thereof is connected to a collector electrode of the third input current supply transistor Q 14 . 
   Furthermore, in order to configure the first compensation current mirror circuit CM 6 , base electrodes of the first collector potential compensation transistor Q 16  and the second collector potential compensation transistor Q 18  are wire-connected; and a current path thereof and a current path on the collector side of the second collector potential compensation transistor Q 18  are connected via a second base current compensation transistor Q 20 . The second base current compensation transistor Q 20  functions similarly to the first base current compensation transistor Q 8 . 
   In the current detection circuit  100   a  of the present embodiment, an output current path branches from a current path connecting the collector electrode of the output current transistor Q 6  and the emitter electrode of the second collector potential compensation transistor Q 18 ; and, from this current path, a collector current of the output current transistor Q 6  is outputted to the outside, as an output current Io. A little of this collector current also flows in the emitter electrode of the second collector potential compensation transistor Q 18 , but is of a level that can be ignored. This output current Io has a value for which the input current is amplified, corresponding to a ratio of the current drive capabilities of the input current transistor Q 2  and the output current transistor Q 6 . In the present embodiment, by detecting the collector current of the first collector potential compensation transistor Q 16 , but not by directly detecting this output current Io, the output current Io is indirectly detected, and compensation is done for the collector potential of the current detection transistor Q 4  and the output current transistor Q 6 . 
   The current drive capability of the current detection transistor Q 4  is set at 1/M times the current drive capability of the output current transistor Q 6 . For example, this may be set at 1/1000. Consequently, the collector current of the current detection transistor Q 4  has a value 1/M times the output current Io and n times the input current. 
   Furthermore, in the present embodiment, the current drive capabilities of the second input current supply transistor Q 12 , the third input current supply transistor Q 14 , the input current transistor Q 2 , and the second collector potential compensation transistor Q 18 , are set to be the same; and the current drive capabilities of the first base current compensation transistor Q 8  and the second base current compensation transistor Q 20  are set to be the same; and since the ratio of the current drive capabilities of the first collector potential compensation transistor Q 16  and the second collector potential compensation transistor Q 18  is set to n:1, the collector current of the first collector potential compensation transistor Q 16  has a value n times the input current, and, at the same time, is 1/M times the output current Io. 
   The current detector  20  detects the collector current of the first collector potential compensation transistor Q 16 . For example, a sense resistor, not shown in the figures, may be inserted between ground and a collector electrode thereof, and voltage on the collector electrode side of this sense resistor may be detected, to detect the collector current. By amplifying this current corresponding to a ratio of the current drive capabilities of the current detection transistor Q 4  and the output current transistor Q 6 , it is possible to indirectly detect the output current Io of a location where detection is to be carried out. With this type of detection method, it is possible to curtail voltage losses, in comparison to a method in which the output current Io is directly detected. 
   Since the first collector potential compensation transistor Q 16  and the second collector potential compensation transistor Q 18  make up the first compensation current mirror circuit CM 6 , a current corresponding to a ratio of the drive capabilities thereof flows in both transistors Q 16  and Q 18 , and since the base voltages become equal, the emitter voltages also become equal. In this way, when variations occur in potential of the two collector electrodes of the current detection transistor Q 4  and the output current transistor Q 6 , compensation is done so that the potential of the two collector electrodes becomes equal. For example, when the output current transistor Q 6  transits to a linear region, a deviation occurs in a mirror ratio with the current detection transistor Q 4 , but by connecting to the first compensation current mirror circuit CM 6 , compensation is done so that the potential of the collector electrodes becomes equal. 
   According to the present embodiment as explained above, also in cases in which the output current Io changes, since the two inter base emitter voltages of the first collector potential compensation transistor Q 16  and the second collector potential compensation transistor Q 18  are equal, compensation is done so that the potential of the two collector electrodes of the current detection transistor Q 4  and the output current transistor Q 6  become equal. Consequently, it is possible to perform current detection with good accuracy. On such occasions, since a differential amplifier circuit is not used, the number of circuit components can be reduced, and it is possible to simplify the overall circuit. In particular, when the present current detection circuit is formed as a monosilic IC, since differential amplifier circuit capacity can be saved, it is possible to make chip size quite small. Consequently, it is possible to detect current with good accuracy, with a small sized circuit. 
   Second Embodiment 
   The second embodiment illustrates a current detection circuit which detects current indirectly, using a current mirror circuit that uses a FET (field-effect transistor). Furthermore, a configuration is illustrated in which drain potential between FETs that make up the current mirror circuit is kept the same as far as possible. 
     FIG. 2  shows a configuration of the current detection circuit  100   b  according to the second embodiment. With this current detection circuit  100   b , an explanation is given of an example using a P-channel MOSFET (metal oxide semiconductor FET). An input current FET (M 2 ), a current detection FET (M 4 ), and an output current FET (M 6 ) make up a second current detection current mirror circuit CM 8 . Source electrodes of the input current FET (M 2 ), the current detection FET (M 4 ), and the output current FET (M 6 ) are connected to a high potential side reference potential Vdd, such as a power supply voltage, or the like. Gate electrodes thereof are wire-connected, and are connected to a current path of a drain side of the input current FET (M 2 ). A fourth resistor R 8  is inserted between a connection point of these current paths and a drain electrode of the input current FET (M 2 ). 
   A drain-source current IDS of the FET is given by Equation 4 as follows.
 
 IDS=uC ( W/L ){( VGS−VT ) VDS−VDS   2 /2}}  Equation 4
 
Here, u represents hole mobility, C represents capacity of a gate oxide film, W represents channel width, L. represents channel length, VGS represents gate-source voltage, VT represents threshold voltage, and VDS represents source-drain voltage.
 
   As will be understood from Equation 4 described above, the drain-source current IDS of the FET has a proportional relationship with the structure thereof, for example channel size (W/L). For example, if the channel width W is doubled, the drain-source current IDS can also be doubled. 
   In the present embodiment, a ratio of the current drive capabilities determined by each structure of the above-mentioned three FETs (M 2 , M 4 , and M 6 ), is set to 1:n:mn, in order, from the input current FET (M 2 ). In the second current detection current mirror circuit CM 8 , since the gate-source voltages are equal, a drain-source current corresponding to the current drive capability flows. 
   In the present embodiment, supply of the input current to the current detection circuit  100   b  is similar to that of the first embodiment and so an explanation is omitted. 
   A first drain potential compensation FET (M 8 ) and a second drain potential compensation FET (M 10 ) make up a second compensation current mirror circuit CM 10 . The ratio of the current drive capabilities thereof is set at n:1. This ratio corresponds to a ratio of the current drive capabilities of the second current detection current mirror circuit CM 8 ; the first drain potential compensation FET (M 8 ) and the current detection FET (M 4 ) are set to a same current drive capability; and the second drain potential compensation FET (M 1 ) and the input current FET (M 2 ) are also set to a same current drive capability. 
   A source electrode of the first drain potential compensation FET (M 8 ) is connected to a drain electrode of the current detection FET (M 4 ), and this drain electrode is connected to the current detector  20 , similar to the first embodiment. A source electrode of the second drain potential compensation FET (M 10 ) is connected to a drain electrode of an output current FET (M 6 ), and this drain electrode is connected to a collector electrode of a third input current supply transistor Q 14 . 
   Furthermore, in order to configure the second compensation current mirror circuit CM 10 , gate electrodes of the first drain potential compensation FET (M 8 ) and the second drain potential compensation FET (M 10 ) are wire-connected, and are connected to a current path on a drain side of the second drain potential compensation FET (M 10 ). Between a connection point with the current path thereof and a drain electrode of the second drain potential compensation FET (M 10 ), a fifth resister R 10  is inserted. 
   In the current detection circuit of the present embodiment, an output current path branches from a current path connecting a drain electrode of the output current FET (M 6 ) and a drain electrode of the second drain potential compensation FET (M 10 ), and from this current path, a drain current of the output current FET (M 6 ) is outputted to the outside, as an output current Io. This output current Io has a value for which the input current is amplified, corresponding to a ratio of the current drive capabilities of the input current FET (M 2 ) and the output current FET (M 6 ). In the present embodiment, by detecting the drain current of the first drain potential compensation FET (M 8 ), and not by directly detecting this output current Io, the output current Io is indirectly detected. 
   Since the first drain potential compensation FET (M 8 ) and the second drain potential compensation FET (M 10 ) make up the second compensation current mirror circuit CM 10 , a current corresponding to a ratio of the drive capabilities thereof flows, and since gate voltages become equal, source voltages also become equal. In this way, when variations occur in potential of the two drain electrodes of the current detection FET (M 4 ) and the output current FET (M 6 ), compensation is done so that the potential of the two drain electrodes becomes equal. For example, when the output current FET (M 6 ) is saturated, a deviation occurs in a mirror ratio with the current detection FET (M 4 ), but by connecting the second compensation current mirror circuit, compensation is done so that the potential of the drain electrodes becomes equal. 
   According to the present embodiment as explained above, also in cases in which the output current Io changes, since the two gate-source voltages of the first drain potential compensation FET (M 8 ) and the second drain potential compensation FET (M 10 ) are equal, compensation is done so that the potential of the two drain electrodes of the current detection FET (M 4 ) and the output current FET (M 6 ) becomes equal. Consequently, effects similar to those of the first embodiment are realized. Furthermore, in the second embodiment, normal operation is possible even when the output voltage is a comparatively low voltage, for example, 1 volt. 
     FIG. 3  is a circuit diagram showing a configuration of an electronic device  1000  in which the current detection circuit  100 , according to the above described embodiment, is preferably used. This electronic device  1000  is provided with a motor apparatus  200  and a recording disk  300 . The electronic device  1000  is a laptop or desktop personal computer, a DVD player, a game machine, or the like, and the recording disk  300  is a hard disk, a DVD disk, or the like. The motor apparatus  200  controls rotation of the recording disk  300 . The motor apparatus  200  includes a motor drive circuit  210 , a current detection circuit  100 , and a motor  220 . The motor drive circuit  210  drives the motor  220  via the current detection circuit  100 . At this time, reference is made to a detection current of the current detection circuit  100 , to control current that is to be supplied to the motor  220 . For example, control is carried out so that overcurrent is not supplied to the motor  220 , and control is appropriately performed so that a constant current is supplied to the motor  220 . As described above, it is possible to realize a motor apparatus of a small size with high accuracy, and an electrical device in which the motor is installed. 
   The present invention has been explained based on the embodiments above. The embodiments are examples; various modified examples in combinations of various component elements and various processes thereof are possible, and a person skilled in the art will understand that such modified examples are within the scope of the present invention. 
   In the first embodiment, the ratio of the current drive capabilities of the current detection transistor Q 4  and the first collector potential compensation transistor Q 16  need not be in an equal ratio but may be set at n:kn; the ratio of the current drive capabilities of the input current supply transistor Q 2  and the second collector potential compensation transistor Q 18  may be set to 1:k; and the collector current of the first collector potential compensation transistor Q 16  can have a value kn times the input current, and k/M times the output current Io. By setting this k to be less than 1, it is possible to reduce reactive current that does not contribute to the output current Io, within the collector current of the output current transistor Q 6 , a little of which flows in the second collector potential compensation transistor Q 18 . The current detector  20  can detect current in a desired range. Furthermore, at this occasion, it is necessary to set the ratio of the current drive capabilities of the first base current compensation transistor Q 8  and the second base current compensation transistor Q 20 , and a similar ratio for the second input current supply transistor Q 12  and the third input current supply transistor Q 14 , to be 1:k. Furthermore, also in the second embodiment, if the current drive capabilities of circuit elements corresponding to the first embodiment are set as described above, and a ratio of resistance values of the fourth resistor R 8  and the fifth resistor R 10  is set at k:1, a similar relationship holds true. 
   Furthermore, the current detector  20  may detect a current flowing from the collector electrode of the current detection transistor Q 4 , or the drain electrode of the current detection FET (M 4 ). 
   Furthermore, the current mirror circuits shown in the first and second embodiments are not limited to the above described types, and various types of circuit, such as a Wilson-type or the like, may be used. In addition, there is no limitation to the transistors used in the above described embodiments, and other transistors may be used, such as an NPN bipolar transistor, a P-channel MOSFET, or the like. Moreover, in the second embodiment, the fourth resistor R 8  and the fifth resistor R 10  need not be inserted. 
   Furthermore, when the present current detection circuit is a monosilic IC, the input current transistor Q 2 , the current detection transistor Q 4 , and the output current transistor Q 6  may be designed to be in pairs, in a layout. The first collector potential compensation transistor Q 16  and the second collector potential compensation transistor Q 18  may similarly be designed to be in pairs. Moreover, corresponding FETs in the second embodiment may also be designed in the same way. 
   While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.