Abstract:
A circuit structure for performing current amplification. The circuit structure may be standardized as a current amplifier cell such that many types of applications requiring current amplification may be created. The basic amplifier cell, which may accept voltage or current sources as an input signal, produces two identical output signals which may be used for feedback or serve as input to additional amplifier stages. This simple structure may be extended to perform current amplification with variable gain or AC or DC voltage-to-current conversion through the use of appropriately selected resistive elements.

Description:
PRIORITY  
         [0001]    Foreign Priority under Title 35, U.S. Code, Section 119, is claimed to Chinese Application 01104518.3, filed Feb. 10, 2001, within 12 months prior to the filing date of the instant application  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field  
           [0003]    The present invention relates generally to analog integrated circuits and more specifically to current amplification including voltage-to-current conversion and current amplification.  
           [0004]    2. Discussion of the Prior Art  
           [0005]    While there are many standard circuits available for voltage amplification, current amplification circuits are uncommon. In a typical current amplifier, amplification is achieved through the use of a current mirror with different emitter areas for each transistor and different emitter resistors. At large values of gain, precise matching of emitter areas and resistance becomes critical.  
           [0006]    Referring to FIG. 1, the classical manner of amplifying the current is shown. The current amplifier of FIG. 1 includes current source I 1 , transistors Q 1 , Q 2 , and resistive elements R 1 , R 2 . The transistors labeled Q 1  and Q 2  have differing emitter areas of 1 unit and 2 units, respectively. In all other aspects transistors Q 1  and Q 2  are identical. Because of the difference in emitter areas, the saturation current out of Q 2  will be twice that of Q 1 ; IS 2 =2*IS 1 .  
           [0007]    The voltage between the base and ground is given by:  
             Vb=I 1 *R 1 +VT*In ( I 1 /Is 1)  (1)  
           [0008]    Vb may also be found as:  
             Vb=I 2 *R 2 +VT*In ( I 2 /Is 2)  (2)  
           [0009]    Equating (1) and (2), we have:  
             I 1 *R 1 +VT*In ( I 1 /Is 1)= I 2 *R 2 +VT*In ( I 2 /Is 2)  (3)  
           [0010]    with R 1 =2*R 2 , and Is 2 =2*Is 1   
           [0011]    Equation (3) has the solution I 2 =2*I 1 , and so the output current is twice the input current. This output gain is determined by the choice of resistive elements R 1  and R 2 , in conjunction with the difference in emitter areas.  
           [0012]    This circuit has several drawbacks. For large values of current gain, the resistive elements R 1  and R 2  and the emitter elements of transistors Q 1  and Q 2  must be matched very accurately, as can be seen by inspection of equation (3). A further drawback of this circuit is that the values of gain that may be produced are limited to the relative values of the resistors and the emitter areas. This gain value is restricted to integer values.  
           [0013]    A circuit with a simple structure, accepting either a current source or voltage source as an input, and producing identical current outputs could be used to address the shortcomings of the current amplifier prior art. Also, it would be advantageous if such a circuit could be used as a standard current amplifier cell in a variety of applications in which current amplification or voltage-to-current conversion is needed.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the invention to provide a current amplifier circuit structure having a simple structure, thereby making it suitable for use as a standardized current amplifier circuit cell.  
           [0015]    It is further an object of the invention that such a current amplifier circuit structure be able to accept either an input current source or an input voltage source.  
           [0016]    It is yet another object of the invention that the current amplifier circuit structure be easily adaptable to a variety of applications, such as those applications requiring current amplification with precisely controllable gain characteristics and those applications calling for voltage-to-current conversion.  
           [0017]    Therefore, according to the structure of the present invention, a simple integrated circuit is used to amplify the input signal. The integrated circuit consists of an amplifier cell, which may be used as a building block in many circuit configurations, including a variable gain current amplifier, and a AC or DC voltage-to-current converter. The basic amplifier cell consists of several functional elements, including an input stage, a current mirror stage, and an output stage. The input stage and current mirror stage allow the input signal to be duplicated while isolating the input signal. This duplicated signal is used as input to the output stage. The output stage uses a current mirror to produce two identical output currents, one suitable for feedback or other purposes, at corresponding output terminals. These output currents may then be used to produce current amplification or voltage-to-current conversion through the judicious use of one or more resistive elements coupled to these output terminals.  
           [0018]    Current amplification is enabled through the use of resistive elements that determine the output gain of the amplifier. The gain may be real-valued and may be accurately determined by appropriate selection of the resistive elements. In an exemplary embodiment, a first resistive element is placed in a feedback connection between the first output current and the negative input terminal. A second resistive element is coupled between the first output terminal and ground. The value of these resistive elements determines the amount of current amplification at the output of the final stage.  
           [0019]    AC or DC voltage-to-current conversion may also be performed using the simple current amplification cell by using a voltage differential pair as input to the cell. The output current is determined using a resistive element. In an exemplary embodiment of the present invention, voltage-to-current conversion is enabled by presenting a voltage signal between the two terminals of the input stage. This input voltage signal produces an identical voltage at the input to the output stage through the action of the current mirror. Through the action of the current mirror in the output stage, the identical output currents are present at the two output terminals. By placing a resistive element at the node between the first output terminal and ground, the output current from the second terminal is found to be proportional to the input voltage. The resistive element determines the value of the output current.  
           [0020]    This circuit is able to source or sink current, and thus is capable of providing AC or DC voltage-to-current conversion.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The novel features believed characteristic of the invention are set forth in the claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 is an example of current amplification, according to the prior art.  
         [0023]    [0023]FIG. 1A is an example of voltage-to-current conversion, according to the prior art.  
         [0024]    [0024]FIG. 2 shows the circuit structure of a current amplifier cell, according to the present invention.  
         [0025]    [0025]FIG. 3 shows a current amplification cell with two input terminals and two output terminals, according to the present invention.  
         [0026]    [0026]FIG. 4 shows a block diagram of an application containing the current amplification cell that performs variable gain current amplification, according to the present invention.  
         [0027]    [0027]FIG. 5 shows the circuit structure of an application that performs current amplification, according to the present invention.  
         [0028]    [0028]FIG. 6 shows a block diagram of an application containing the current amplification cell that performs voltage-to-current conversion, according to the present invention.  
         [0029]    [0029]FIG. 7 shows the circuit structure of an application that performs voltage-to-current conversion, according to the present invention.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0030]    The current amplification cell of the present invention produces identical output currents when a one-terminal or two-terminal input signal is applied thereto. Referring now to FIG. 2, the current amplifier cell  10  for creating identical output currents in accordance with the present invention is shown. Current amplifier cell  10  contains the following elements: n-p-n/p-n-p transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6 , capacitive element C 1 , and current sources I 1 , I 2 , and I 3 . A constant current I 1  is coupled to both emitters of transistors Q 1  and Q 2 . The other terminal of current I 1  is coupled to a constant supply voltage Vss. Transistors Q 1  and Q 2  are coupled as a differential pair input stage as shown. The base of transistor Q 1  is coupled to the positive input terminal, while the base of transistor Q 2  is coupled to the negative input terminal. The collector of Q 1  is coupled to the collector of Q 3  in the Q 3 -Q 4  current mirror. The collector of Q 2  is coupled to the collected to the collector of Q 4  in the Q 3 -Q 4  current mirror. The base of the Q 3  transistor is coupled to the base of the Q 4  transistor, and the base of Q 4  transistor is also coupled to the Q 4  collector. The Q 3  emitter is coupled to the Q 4  emitter as shown. The emitter of transistor Q 4  is coupled to the emitters of transistors Q 5  and Q 6  in the output stage. The collector of transistor Q 3  is coupled to the base of transistor Q 5  and also to the base of transistor Q 6 . Further, the collector of transistor Q 3  is coupled to capacitive element C 1 . Capacitive element C 1  is terminated in ground. The output of the current source I 2  is coupled to the collector of transistor Q 5 . The output of the current source I 3  is coupled to the collector of transistor Q 6 . Both current source I 2  and current source I 3  are terminated in the constant supply voltage Vss. A first output O 1  is taken from the connection between current source I 2  and the collector of transistor Q 5 . A second output O 2  is taken from the connection between current source I 3  and the collector of transistor Q 6 .  
         [0031]    The current amplifier cell  10  produces identical output currents at output terminals O 1  and O 2 . The theory of operation of current amplifier cell  10  will now be described. A signal is applied between the positive and negative input terminals of the cell. Through the action of the differential pair Q 1 -Q 2  and the Q 3 -Q 4  current mirror, identical output currents will be present at the collectors of Q 3  and Q 4 . The Q 3 -Q 4  current mirror serves to isolate variations in the input signal and produce a stable signal to the output stage. The output stage also contains a current mirror; in this case, the Q 5 -Q 6  current mirror. The emitters of transistors Q 5  and Q 6  are coupled to the constant supply voltage Vcc, and the bases of transistors Q 5  and Q 6  receive the same input current. Therefore, through the use of identical current sources I 2  and I 3 , the outputs O 1  and O 2  both receive the same current. Either O 1  or O 2  may be used for feedback, which can improve the accuracy of the output signal.  
         [0032]    Referring now to FIG. 3, the input and output terminals for a current amplifier cell  10  are shown. As described above, the input signal is applied between the positive terminal and the negative terminal. The identical output currents are available at terminals O 1  and O 2 . In this configuration, the current amplifier cell  10  may be used as a building block in many applications.  
         [0033]    Referring now to FIG. 4, an application using the current amplifier cell  10  is shown. The application performs current amplification. The positive input terminal is coupled to ground. The negative input terminal is coupled to an input source I_in, which is terminated in ground. The second output terminal O 2  is coupled through resistive element R 1  to the negative input terminal in a feedback connection. The second output terminal O 2  is also coupled to ground through resistive element R 2 . Through the action of the current amplifier cell  10  and the feedback connection, the output current is given by  
           IO 1 =I   —   in *(1.0 +R 1 /R 2)  
         [0034]    This simple feedback circuit is able to produce arbitrary values of current gain with good accuracy.  
         [0035]    Referring now to FIG. 5, a circuit that performs current amplification in accordance with an embodiment of the present invention is shown. The circuit comprises n-p-n/p-n-p transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6 , capacitive element C 1 , current sources I 1 , I 2 , and I 3 , input current source I_in, and resistive elements R 1  and R 2 . A constant current I 1  is coupled to both emitters of transistors Q 1  and Q 2  . The other terminal of the current I 1  is coupled to a constant supply voltage Vss. Transistors Q 1  and Q 2  are coupled as a differential pair input as shown. The base of transistor Q 1  is coupled to the positive input terminal, which terminates in ground. The base of transistor Q 2  is coupled to the negative input terminal, which is coupled to an input current source, I_in. The input current source, I_in, is terminated in ground. The collector of Q 1  is coupled to the collector of Q 3  in the Q 3 -Q 4  current mirror. The collector of Q 2  is coupled to the collected to the collector of Q 4  in the Q 3 -Q 4  current mirror. The base of the Q 3  transistor is coupled to the base of the Q 4  transistor, and the base of Q 4  transistor is also coupled to the Q 4  collector. The Q 3  emitter is coupled to the Q 4  emitter as shown. The emitter of transistor Q 4  is coupled to the emitters of transistors Q 5  and Q 6  in the output stage. The collector of transistor Q 3  is coupled to the base of transistor Q 5  and also to the base of transistor Q 6 . Further, the collector of transistor Q 3  is coupled to capacitive element C 1 .  
         [0036]    Capacitive element C 1  is terminated in ground. The output of the current source I 2  is coupled to the collector of transistor Q 5 . The output of the current source I 3  is coupled to the collector of transistor Q 6 . Both current source I 2  and current source I 3  are terminated in the constant supply voltage Vss. Both resistive elements, R 1  and R 2 , are coupled to the collector of transistor Q 5 . Resistive element R 2  is coupled to ground, while resistive element R 1  is attached in a feedback connection to the base of transistor Q 2 .  
         [0037]    This current amplification system operates correctly because the current out of the collector of transistor Q 5  is the same as the current out of the collector of transistor Q 5 . The input current, I_in, is coupled to the base of transistor Q 2 . Since both resistive elements R 1  and R 2  are coupled to ground, the voltage potential must be the same across R 1  and R 2 . Since the base of transistor Q 1  is grounded, the input current I_in flows entirely through the resistive element R 1 . And so, I_R 1 =I_in. Therefore, I_in*R 1 =I_R 2 *R 2 , so that I_R 2 =I_in*R 1 /R 2 . The output current is: I_O 1 =I_R 1 +I_R 2 =I_in*(1+R 1 /R 2 ).  
         [0038]    It is noted that in addition to resistors R 1 , R 2 , resistive elements may include transistors, operational amplifiers, etc., or other elements or combination of elements that provide the desired resistive characteristics.  
         [0039]    This output current result was obtained because O 1  and O 2  are equal. This is due to the action of the output stage. Through the action of the differential pair Q 1 -Q 2 , identical output currents will be present at the collectors of Q 3  and Q 4 . The Q 3 -Q 4  current mirror serves to isolate variations in the input signal and produce a stable signal to the output stage. The output stage also contains a current mirror; in this case, the Q 5 -Q 6  current mirror. The emitters of transistors Q 5  and Q 6  are coupled to the constant supply voltage Vcc, and the bases of transistors Q 5  and Q 6  receive the same input voltage. Therefore, through the use of identical current sources I 2  and I 3 , the outputs O 1  and O 2  both receive the same current.  
         [0040]    Referring now to FIG. 6, a voltage-to-current conversion application using the current amplifier cell  10  is shown. A voltage Vi is coupled to the positive input terminal of current amplifier cell  10  as shown. The output O 2  of current amplifier cell  10  is coupled to a resistive element R and also coupled to the negative input terminal of the current amplifier cell  10 . The other terminal of resistive element R is coupled to ground. Due to the action of the current amplifier cell  10 , the output current through O 1  is Vi/R. Thus, the output current magnitude is controlled by the input potential voltage Vi and resistive element R.  
         [0041]    In a typical voltage-to-current converter, conversion is achieved through the use of a current mirror and a feedback circuit. Referring now to FIG. 1A, the classical technique for performing voltage-to-current conversion is shown. The voltage-to-current converter of FIG. 1A includes an input voltage Vi, operational amplifier OA, transistors Q 1 , Q 2  and Q 3 , and a resistive element R 1 . The voltage, Vi, is coupled to the positive input terminal of the operational amplifier, while the operational amplifier output is coupled to the control terminal of transistor Q 1 . The first terminal of transistor Q 1  is coupled to the negative input terminal of the operational amplifier OA and the resistive element R 1 . The other terminal of resistive element R 1  is coupled to ground. The second terminal of transistor Q 1  is attached to the first terminal of transistor Q 2 . The output current is taken from the first terminal of transistor Q 3 , while the second terminal of transistors Q 2  and Q 3  are coupled to a constant supply voltage Vcc. The control terminals of transistors Q 2  and Q 3  are coupled, and the control terminal of transistor Q 2  is also coupled to the first terminal of transistor Q 2 .  
         [0042]    The current through R 1  is IR 1 =Vi/R 1 .  
         [0043]    This current is equal to I 0  due to the action of the Q 2 -Q 3  current mirror. So, Io=IR 1 =Vi/R 1 . This arrangement only works for a DC input signal. If the input is AC, then this circuit will be unable to sink current.  
         [0044]    Referring to FIG. 7, a circuit that performs AC or DC voltage-to-current conversion, in accordance with the present invention is shown. The circuit comprises n-p-n/p-n-p transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6 , capacitive element C 1 , and current sources I 1 , I 2 , and I 3 . A constant current I 1  is coupled to both emitters of transistors Q 1  and Q 2 . The other terminal of the current I 1  is coupled to a constant supply voltage Vss. Transistors Q 1  and Q 2  are coupled as a differential pair input as shown. The base of transistor Q 1  is coupled to an input voltage Vi, which is terminated in ground. The base of transistor Q 2  is coupled to resistive element R, which is coupled to ground. The base of transistor Q 2  is also coupled to the collector of transistor Q 5  and the output of current source I 2 . The collector of Q 1  is coupled to the collector of Q 3  in the Q 3 -Q 4  current mirror. The collector of Q 2  is coupled to the collector of Q 4  in the Q 3 -Q 4  current mirror. The base of the Q 3  transistor is coupled to the base of the Q 4  transistor, and the base of Q 4  transistor is also coupled to the Q 4  collector. The Q 3  emitter is coupled to the Q 4  emitter as shown. The emitter of transistor Q 4  is coupled to the emitters of transistors Q 5  and Q 6  in the output stage. The collector of transistor Q 3  is coupled to the base of transistor Q 5  and also to the base of transistor Q 6 . Further, the collector of transistor Q 3  is coupled to capacitive element C 1 . Capacitive element C 1  is terminated in ground. The output of the current source I 2  is coupled to the collector of transistor Q 5 . The output of the current source I 3  is coupled to the collector of transistor Q 6 . Both current source I 2  and current source I 3  are terminated in the constant supply voltage Vss.  
         [0045]    This circuit produces an output current that is proportional to the input voltage. The constant of proportionality is the value of the resistive element R. The feedback connection between the collector of transistor Q 5  and the negative input of the differential pair Q 1 -Q 2  causes the voltage potential at the base of transistor Q 2  to be the same as the input voltage potential Vi. Thus, the current through resistive element R is Vi/R. The current out of the collector of transistor Q 5  is the same as the current out of the collector of transistor Q 6 . And so, the output current I_O 1  is given by Vi/R. It is noted that in addition to resistor R, resistive element R may include transistors, operational amplifiers, etc., or other elements or combination of elements that provide the desired resistive characteristic.  
         [0046]    While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, one of ordinary skill in the art will recognize that p-n-p transistors may be substituted for the n-p-n transistor configurations described above, with minor circuit modification, without departing from the spirit and scope of the invention.