IMPROVED EXPONENTIAL CURRENT GENERATOR AND METHOD

Abstract: An apparatus and method of generating a current pair Ip, Im where the ratio of the pair is exponentially related to a control signal, and where either Ip or Im is greater than or less than a minimum or maximum value. The apparatus includes a feedback correction circuit used to sense the value of Im or Ip. The correction circuit supplies a boost current Iboost when the sensed value of Ip or Im is less than or greater than the minimum or maximum value. Iboost is preferably maintained proportional to the difference of the desired value and Ip or Im.

Description: IMPROVED EXPONENTIAL CURRENT GENERATOR AND METHOD BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to exponential current generators and methods, and more particularly to generators used in variable gain amplifiers (VGA). II. Description of the Related Art As noted in the co-pending and related application cited above, variable gain amplifiers (VGA) may be employed to amplify received signals and signals to be transmitted. The gain of the VGA is determined by a control signal where the gain of the VGA in some applications is ideally exponentially (or, "linear in dB") related to the control signal. The VGA may have an input stage and one or more current amplifier stages coupled to the input stage. The input stage receives a voltage signal, converts the voltage signal to current signals, and amplifies the current signals. The current amplifier stage further amplifies the current signals generated by the input stage. Ideally, the gain of each current amplifier is exponentially related to the control signal. The control signal may be any type of control signal including a control current, digital control, or voltage signal, for example. In the co-pending and related application a voltage control signal, V control is employed to determine or control the gain of each current amplifier. As also described in the co-pending application, each current amplifier stage employs an exponential current generator that receives V control and generates a pair of current signals, 1 ^ and I p , where the ratio of I p /I π , is exponentially related to the control signal V control . The pair J^, L is used to control the gain of the current amplifier as described in the co-pending application. As noted above, a current amplifier stage is coupled to the input stage of the VGA. Due to certain circuit dynamics associated with coupling the input stage to a current amplifier stage, it is desirable that I m not fall below some minimum current value ("I bleed "). As described in the related and incorporated co-pending application, in order to prevent I m from falling below a certain minimum current value, a current source is included to produce a fixed current I bleed in parallel with I m . Adding I bleed to I m ensures a minimum tail current in the input stage of the translinear loop of the current amplifier as described in the related application. Disadvantageously, the addition of I bleed to I m changes the ratio of the current pair produced by the exponential current generator to be proportional to I p /O^ + I b ι eed ). This ratio is disadvantageously not exponentially related to the control signal V control . This is particularly true as the ratio becomes large and I m becomes correspondingly small, i.e., as J^ approaches I bleed . Disadvantageously, this distortion causes the gain of a current amplifier employing the generator to become more linearly related than exponentially related to V control . This distortion can create power control problems when the amplifier is used as part of a VGA that is used in a transmitter, in particular in a transmitter that generates a Code Division Multiple Access (CDMA) signal. Thus, a need exists for an improved exponential current generator that generates a current pair whose ratio is exponentially related to a control signal even when I m is less than I bleed . The present invention provides such an improved exponential current generator. SUMMARY OF THE INVENTION The invention includes an apparatus for generating a current pair, I p , I m where the ratio of the pair is exponentially related to a control signal. In one embodiment of the present invention, a boost current, I boost is added to a fixed current I fixed to control the minimum or maximum value of either I p or I m while keeping the ratio of I p /I m exponentially related to the control signal. In a particular embodiment, I boost is added so that I m is equal to or greater than a minimum value I bleed . The apparatus includes a differential amplifier and a correction feedback circuit. In one embodiment, the differential amplifier generates the current pair, L ^ , I p , based on the control signal, the fixed current 1^^, and the boost current I boost . The correction feedback circuit is coupled to the differential amplifier and senses the level of I m . In accordance with this embodiment, the correction feedback circuit generates a boost current I boost when !„, is less than I bleed so that I m will be greater than I bleed and the ratio of I p /1 ^ still exponentially related to the control signal. In a preferred embodiment, I b00st is proportional to a difference between I bleed and !„, when I m is less than I bleed , otherwise I boost is zero. The differential amplifier may include a current source that generates the fixed current 1^^. In addition, the differential amplifier may further include a pair of BJTs, where the control signal is coupled to the bases of the BJTs to generate the current pair I m , I p . The apparatus may also include a pair of FET current mirrors that are coupled to the differential amplifier. In the preferred embodiment, the feedback circuit may include a current source that generates a current I bIeed . The feedback circuit may also include at least one FET coupled to the pair of FET current mirrors and the current source. In this embodiment, the at least one FET of the feedback circuit may sense I m . The present invention also includes a method of generating a current pair, I p , 1 ^ where the ratio of the current pair is exponentially related to a control signal, and where I m or L p is ideally greater than or less than a minimum or maximum value. The method may include the steps of inputting the control signal and generating the current pair I m , I p as a function of at least the control signal and a boost current 1^^. The method may also include the step of sensing the level of 1 ^ or I p and generating a boost current I b00st when I m or L p is less than or greater than the minimum or maximum value. In a preferred embodiment, I boost may be proportional to a difference between I bleed and ^ when 1 ^ is less than I bleed , otherwise I boost is zero. Further, the step of generating a boost current may include the steps of: 1) comparing to I leed 2 ) setting I boost to zero when I m is greater than or equal to I bleed ; and 3) setting I ^t to a proportional difference of I bleed and I m when I m is less than • ^blee - BRIEF DESCRIPTION OF THE DRAWINGS The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: FIG. 1 is a schematic of an exponential function generator described in the related and co-pending application. FIG. 2 is a flowchart of a method for generating a current pair in accordance with the invention. FIG. 3 is a block diagram of an improved exponential function generator made in accordance with the present invention. FIG. 4 is a schematic of an improved exponential function generator according to the present invention. FIG. 5 is a block diagram of a current amplifier adapted for use with the exponential function generator shown in FIG. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an exponential function generator described in the related and co-pending application and is referred to hereafter as the exponential function generator 360. As shown in FIG. 1 and as described in the related application, the generator 360 primarily comprises a differential amplifier 465. The amplifier 465 generates a current pair 1 ^ , L p at an output port 358 as a function of the differential value of a control signal, in this case a voltage control signal V control provided at an input port 130. As shown in FIG. 1, V control is coupled to the bases of Bipolar Junction Transistors (BJT) 461 and 462. The ratio of the collector currents of the BJTs 461 and 462 is exponentially related to the differential base voltage (V control ) due to the known exponential input voltage-to-output current relationship of BJTs. A pair of FET current mirrors 474 are used to copy the collector currents that are generated by the pair of BJTs 461, 462. In particular, the collector currents I p and 1 ^ are provided to devices coupled to the generator 360 at the output ports 358. In the embodiment shown in FIG. 1, the current mirrors 474 include four field effect transistors (FET) 464, 466, 468, and 470. As shown in FIG. 1, the generator 360 also includes a current source 472. The current source 472 generates a fixed current I fixed . It is noted that the sum of L p and I m is equal to I fixed . When the control signal is made large (for a large gain in a current amplifier wherein the generator 360 is employed), L p becomes large and close to I fixed while I m becomes small and close to zero. Likewise, as the control signal is made small (for a small gain in the current amplifier), I m becomes large and close to 1^^ while I p becomes small and close to zero. As noted above, in some applications of generator 360 it is desirable to control the maximum or minimal value(s) of I or I m . For example, it may be desirable to prevent I m from falling below some minimum value when the control signal is large (for a large gain of the current amplifier). Accordingly, I f i xed ma y De set to a large value. This solution, however, would increase the energy consumption of the generator 360. Ideally, 1^^ should be selected to be as small as possible to reduce energy consumption of the generator 360 while providing minimum or maximum value(s) of p and L ^ , for a conservative range of control signal values (or current amplifier gains). The selected value of 1^^ in the application presented the co-pending and related application is about 10 mA after scaling. As noted above in the related application, it is desirable that I m never fall below a minimum value I bleed . To prevent ^ from falling below I bleed while keeping 1^^ as small as possible, the value of I bleed was added to ^ after the generation of I m by generator 360. This disadvantageously changes the ratio of the current pair provided to the current amplifier. In particular, the addition of I bleed to 1^ disadvantageously changes the ratio of to L p /^ + I bleed ). If I m is much greater than L bleed the effect is negligible. As ^ approaches I bleed , however, the effect of the addition of I bleed is more pronounced and the ratio of is no longer exponentially related to the control signal (V control ). This is especially true as the ratio approaches a maximum value and 1 ^ becomes small. The distortion or deviation may be significant in certain applications where the generator 360 can be utilized. For example, this distortion becomes significant when the generator 360 is used as part of a variable gain amplifier (VGA) of a transmitter used to generate CDMA signals. As known to those skilled in the art, power control of CDMA signals is critical in preventing interference between CDMA signals. The invention provides an improved apparatus and method of generating the current pair, L p , 1 ^ as a function of a control signal where either L p or L ^ is ideally greater than or less than some minimum or maximum value and the sum of L p and L ^ is equal to a bias current I bias . The bias current is equal to the sum of the fixed current L-^ and a boost current I oost - One embodiment of an improved method according to the present invention is described with reference to FIG. 2. The method is a continuous process including at least four steps. As shown in FIG. 2, at a step 205, the value of either L ^ , or L p is sensed or determined. The method then proceeds to a step 210, where either 1 ^ or L p is compared with a desired minimum or maximum value. When the value of I m or I p is greater than or less than the desired minimum or maximum value, no change to I bais is necessary. In this case, the method proceeds to a step 214 wherein I boost is set equal to zero. When at step 210, 1,,, or I p is determined to be less than or greater than some minimum or maximum value, then the overall current must be increased or decreased so that ^ or L p is made greater than or less than the desired minimum or maximum value. Note: the addition or subtraction of current does not effect the ratio of p /I m - Therefore, when at step 210 it is determined that L ^ or L p is less than or greater than some minimum or maximum value, then the boost current I boost is set equal to a difference between value of L ^ or p (differential value) and the desired minimum or maximum value times a gain, k. In a preferred embodiment, k is approximately equal to 100. Then, the method proceeds to step 216, whereat I bias is set equal to the value of L^ + I boost . As noted above L-^ is a fixed current whose value is selected for nominal ratios and values of I p and I m . The method then proceeds to a step 218, wherein I p and ^ are generated as a function of the control signal and I blas . Using this technique, 1^^ may be set to a low value to preserve energy and then boosted in when necessary. An example of an application of the method of the present invention is described with reference to the system presented in the co-pending and related application. As noted above, in the related application it is desirable that I m never fall below a minimum value, I bleed (I bleed is approximately equal to 1 mA in one embodiment). Using the above method, at step 205, the value of I m would be sensed or determined. Then at step 210, I m would be compared to I bleed . When I m is determined to be greater than or equal to I bleed , no current would be added and the method would proceed to step 214 whereat I boost would be set to zero. When I m is determined to be less than I bleed at step 210, the method would proceed to step 212 whereat I boost would be set equal to k*(I bleed - I m ) (the differential value). Then at step 216, I bιas would be set to is approximately 10 mA after scaling in this embodiment). The above method, thus could be used to modify the generator 360 to produce the current pair L p , I m where I m is always greater than leed while the exponential ratio of the current pair to control signal (V control in this embodiment) is maintained. A block diagram of a preferred apparatus 400 of the present invention capable of implementing the method shown in FIG. 2 is described below with reference to FIG. 3. As shown in FIG. 3, the apparatus 400 includes a current mirror circuit 474, a differential amplifier 465, and a correction feedback circuit 500. The current mirror circuit 474 and the differential amplifier 465 are similar to those described above with reference to FIG. 1. As noted above, the apparatus 400 includes a correction feedback circuit 500 that is coupled to the differential amplifier 465 via a link 520. In addition, the correction feedback circuit 500 is coupled to the current mirror circuit 474 to sense or sample the value of I m or L p . The correction feedback circuit 500 generates a current I b00st as described above and provides I boost to the differential amplifier 465 via the link 520. The differential amplifier 465 generates the current pair, L p , L ^ as a function of the control signal, 1^^, and I b00st as described above. FIG. 4 presents an embodiment of a generator 600 according the present invention for use as generator 360 in the co-pending and related application. As shown in FIG. 4, the current mirror circuit 474 and the differential amplifier 465 are identical to that described above with reference to FIG. 1. In this exemplary embodiment, the correction feedback circuit 610 includes four FETs 512, 508, 502, 504, and a current source 506. The current source 506 generates a current I bleed . Similar to the feedback correction circuit 500 as described above, the feedback circuit 610 generates the current I boost 520 that is effectively added to I flxed to generate the current I bias . As one of ordinary skill in the electronics art will note, in this feedback circuit 610 when ^ is sensed to be greater than I bleed , I boost is shunted to zero. When I m is sensed to be less than I bleed , then the feedback circuit 610 generates a current I boost that is equal to k*(I bleed - I j J, where k is the gain of the feedback circuit 610 and is also a function of the FETs 502, 504. As noted above, in a preferred embodiment, k is approximately equal to 100. Thus, generator 600 may be used as a substitute for generator 360 or 861 shown in the co-pending and related application. In particular, the exponential function generator 600 may be part of the current amplifier, such as an amplifier 160A or 160B shown and described in the co-pending related application. The function of the exponential function generator 600 in this embodiment is briefly described with reference to FIG. 5. As shown in FIG. 5, the current amplifier 160A includes a darlington differential amplifier 510, a cascode differential amplifier 520, a pair of bipolar current mirrors 860 and the exponential function generator 600. The darlington differential amplifier 510, cascode differential amplifier 520, and pair of bipolar current mirrors 860 are described in detail in the co-pending related application. Briefly, signal currents 190 are amplified by the current amplifier 160A to generate amplified, signal currents 180 as a function of a control signal, in particular, a voltage control signal, V control 130. The exponential function generator 600 generates the current pair I p , I m at an output 358. As noted, the ratio of I p /I m is ideally exponentially related to V control . The current pair p , 1 ^ is used to control the gain of the amplifiers 510, 520 as described in the co-pending and related application. Note that the current amplifier 160A may be part of a VGA. Further, the VGA may be used in transmit or receive circuitry of a mobile transceiver unit. In addition, the exponential function generator 400 may used in many other applications where it is desirable or necessary to provide linear in dB gain control. For example, the exponential function generator 400 may be used to replace the generator 360 described in the co-pending related application. The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WHAT IS CLAIMED IS:

Claims: CLAIMS 1. An apparatus for generating a current pair, L, I m wherein the ratio of the pair is exponentially related to a control signal, and wherein one of I p and L ^ is one of greater than a minimum value and less than a maximum value, the apparatus comprising: a differential amplifier, the differential amplifier generating the current pair L ^ , L as a function of the control signal and a bias current, I bιas , where I bιas is a function of a boost current I boost and the sum of L p and I m equals I bιas ; and a correction feedback circuit, coupled to the differential amplifier, the feedback circuit sensing one of I m and I p and generating the boost current ^ boost hen one of I m and L p is one of less than the minimum value and greater than the maximum value. 2. The apparatus of claim 1, wherein I boost is proportional to a difference between one of the minimum value and the maximum value and one of I m and L p . 3. The apparatus of claim 1, wherein I boost is proportional to a difference between the minimum value and I m . 4. The apparatus of claim 2, wherein I bιas is equal to the sum of I boost and a fixed current, I flxed . 5. The apparatus of claim 4, wherein the differential amplifier includes a current source that generates the current I flxed . 6. The apparatus of claim 5, wherein the differential amplifier further includes a pair of BJTs, each BJT including a base and wherein the control signal is coupled to the bases of the BJTs. 7. The apparatus of claim 6, further including a pair of FET current mirrors operatively coupled to the differential amplifier. 8. The apparatus of claim 7, wherein I boost is proportional to a difference between a minimum value I ωeed and L ^ and the feedback circuit includes a current source capable of generating a current equal to l bleed . 9. The apparatus of claim 8, wherein the feedback circuit further includes at least one FET operatively coupled to the pair of FET current mirrors and the current source. 10. The apparatus of claim 9, wherein the at least one FET of the feedback circuit senses L ^ . 11. An apparatus for generating a current pair, L p , I m wherein the ratio of the pair is exponentially related to a control signal, and wherein one of L p and j , is one of greater than a minimum value and less than a maximum value, the apparatus comprising: a differential amplifier means for generating the current pair L ^ , L p as a function of the control signal and a bias current, I bias , where I bias is a function of a boost current I boost and the sum of L p and I m equals I bias ; and a correction feedback circuit means, coupled to the differential amplifier means, the feedback circuit means for sensing one of I m and L p and generating the boost current I boost when one of I m and I p is one of less than the minimum value and greater than the maximum value. 12. The apparatus of claim 11, wherein I boost is proportional to a difference between one of the minimum value and the maximum value and one of ^ and L p . 13. The apparatus of claim 11, wherein I boost is proportional to a difference between the minimum value and I m . 14. The apparatus of claim 12, wherein I bιas is equal to the sum of I boost and a fixed current, I fιxed . 15. The apparatus of claim 14, wherein the differential amplifier includes a current source that generates the current L flxed . 16. The apparatus of claim 15, wherein the differential amplifier further includes a pair of BJTs, each BJT including a base and wherein the control signal is coupled to the bases of the BJTs. 17. The apparatus of claim 16, further including a pair of FET current mirrors operatively coupled to the differential amplifier. 18. The apparatus of claim 17, wherein I boost is proportional to a difference between a minimum value ϊeed and ^ and the feedback circuit includes a current source capable of generating a current equal to I bleed . 19. The apparatus of claim 18, wherein the feedback circuit further includes at least one FET operatively coupled to the pair of FET current mirrors and the current source. 20. The apparatus of claim 19, wherein the at least one FET of the feedback circuit senses I m . 21. The apparatus of claim 20, wherein the control signal is a voltage control signal. 22. The apparatus of claim 20, wherein I boost is equal to a constant, k times the difference of I bleed and I m . 23. The apparatus of claim 20, wherein the constant, k is approximately 100. 24. A method of generating a current pair, L, I m wherein the ratio of the pair is exponentially related to a control signal, and wherein one of L p and I m is one of greater than a minimum value and less than a maximum value, comprising the steps of: a. inputting the control signal; b. generating the current pair I m , L as a function of the control signal and a bias current, I bias , where I bias is a function of a boost current I boost and the sum of L and I m equals I bias ; c. sensing one of I m and 1^; and d. generating the boost current 1, ^ ^ when one of I m and L p is one of less than the minimum value and greater than the maximum value. 25. The method of claim 24, wherein I boost is proportional to a difference between one of the minimum value and the maximum value and one of I m and L p . 26. The method of claim 24, wherein step d. 27. The method of claim 26, wherein I bias is equal to the sum of I boost and a fixed current, I fixed . 28. The method of claim 27, wherein the control signal is a voltage control signal. 29. The method of claim 28, wherein step iii includes setting I boost equal to a constant, k times the difference of I bleed and I m . 30. The method of claim 29, wherein the constant, k is approximately lOO.comprises the steps of: i. comparing I m to a minimum value, I bleed ; ii. setting I boost to zero when I m is greater than or equal to I bleed ; and iii. setting I boost to a proportional difference of I bleed and I m when I m I bleed .

Inventors: GAZZERO PETER C, WALKER BRETT CHRISTOPHER, GAZZERO, PETER, C., WALKER, BRETT, CHRISTOPHER
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 1999
Main classification: H03F3
IPC codes: G06G7, H03F3, H03G1, H03G3