FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to an amplifier and, more particularly, to a self-biased amplifier. 
       BACKGROUND OF THE INVENTION 
       [0002]    The input impedance and input bias are two important factors of the design of an amplifier. For instance, to amplify a signal from a high output impedance signal source, the input stage of the amplifier must have high input impedance to reduce the signal attenuation resulted from signal coupling loss. However, if an amplifying circuit with high amplification is employed in the input stage of the amplifier, the larger size of the circuit components would result in smaller input impedance, which in turn prevents the signal from the high output impedance signal source from being coupled to the amplifier completely, and consequently lead to signal attenuation. The conventional amplifiers often use bias circuits to provide the desired high input impedance thereof, though this gives rise to other problems. On the other hand, the output bias needed at the output of the signal source often differs from the input bias at the input stage of the amplifier, which hinders the direct connection between the amplifier and the signal source. If an amplifier configuration can allow adequate bias for the output of the signal source, the input bias adaptability of the amplifier can be enhanced and the amplifier can be suitable for various signal sources, thus widening the applications of the amplifier. 
         [0003]    When designing the circuitry of an amplifier, factors such as cost and overall size of the circuit must be taken into account, and this is especially true if other factors like noise reduction may be overlooked, where the cost and the size of the amplifier are the most critical factors to consider. Provided that the performance of the amplifier is not overly compromised, reducing the size of the amplifier, such as integration into a single chip, and lowering the cost are the top priority for the designers of the amplifiers currently. 
         [0004]    U.S. Pat. No. 3,595,998 proposed a preamplifier for microphones which uses a polarity-dependent bias circuit to control the gate voltage of the FET of an amplifier, and provides individual bias for the signal source of the amplifier. However, the polarity-dependent bias circuit is complex and huge and, as described in U.S. Pat. No. 6,812,788, it is required to have a resistance up to tens or even hundreds of GΩ and will induce severe noise problem. In addition, to provide so much resistance, external resistor is required, thereby causing that the circuitry cannot be miniaturized and has higher cost. 
         [0005]    U.S. Pat. No. 5,337,011 proposed a preamplifier for microphones which uses two cascode stages to improve the impedance matching, in order to prevent severe gain loss and inhibit noises. Unfortunately, this circuit is also complex and never solves the adaptability problem of the input bias. 
         [0006]    U.S. Pat. No. 7,110,560 proposed a preamplifier for microphones which uses a pair of cross-coupled diodes to provide high input impedance, and a coupling capacitor to prevent DC leakage. However, the cross-coupled diodes at the input will introduce other problems as described in U.S. Pat. Publication No. 20030194100, and further, this art still cannot solve the adaptability problem of the input bias. 
         [0007]    U.S. Pat. Publication No. 20030194100 proposed an input buffer bias circuit for microphones which uses a current limiter to limit the current of the cross-coupled input bias diodes to increase the voltage level of the input signal. However, this art still do not solve the adaptability problem of the input bias. 
         [0008]    U.S. Pat. No. 6,888,408 proposed a preamplifier for microphones which uses a two-stage amplifier to replace the conventional junction transistor (JFET), and in which the first stage amplifier minimizes the input capacitance, and the second stage amplifier optimizes the gain. However, the first stage amplifier must be designed to match the output capacitance of the signal source, and thus it is designed according to the signal sources of various output capacitances one by one, which not only restricts the applications of the amplifier but also increases the cost. In-addition, this art does not solve the adaptability problem of the input bias. 
         [0009]    U.S. Pat. Publication No. 20050151589 proposed an amplifying circuit of a capacitive transducer, which also uses a pair of cross-coupled diodes to provide high input impedance, and a servo-amplifier to feed back the output to the pair of cross-coupled diodes in order to control the input bias point. However, the cross-coupled diodes at the input will induce other problems, and this art still does not solve the adaptability problem of the input bias. 
         [0010]    U.S. Pat. No. 6,812,788 proposed an amplifying circuit for a capacitive microphone which uses independent bias power supplies to set the bias voltages of the signal source and the amplifier input respectively, and a network of diodes and resistors with high resistance to replace conventional coupling resistor with high resistance and feed back the output to the input coupling network. However, this art needs two bias power supplies and uses the input coupling network for providing high input impedance, resulting in increased complexity and cost of the amplifier circuit design. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is to provide an amplifier with independent input bias. 
         [0012]    Particularly, one object of the present invention is to provide an amplifier with self-biased input. 
         [0013]    Another object of the present invention is to provide an amplifier with high input impedance. 
         [0014]    Particularly, one object of the present invention is to provide an amplifier suitable to high output impedance signal sources. 
         [0015]    Yet another object of the present invention is to provide an amplifier that may be integrated in a single chip. 
         [0016]    Particularly, one object of the present invention is to provide an amplifier with lower complexity and cost. 
         [0017]    An amplifier according to the present invention comprises a signal source input to receive an input signal, a load connected to an amplifying circuit, and a self-bias circuit connected to an input of the amplifying circuit for biasing the input of the amplifying circuit at a DC level, wherein the load and the self-bias circuit are in the current path of the amplifying circuit with the amplifying circuit therebetween. 
         [0018]    Alternatively, the amplifier further comprises a coupling circuit connected between the signal source input and the input of the amplifying circuit for coupling the input signal from the signal source input to the amplifying circuit, such that the signal source input and the input of the amplifying circuit are biased independently, and a bias circuit connected between a supply voltage and the signal source input for biasing the signal source input at a second DC level. 
         [0019]    With the configuration of the coupling circuit and the bias circuits, the signal source input and the input of the amplifying circuit can be biased with a single bias, or at two independent biased DC levels. 
         [0020]    Because the input of the amplifying circuit employs a self-bias circuit, the circuits thereof can be simplified and the cost is reduced. 
         [0021]    The coupling circuit may be implemented with capacitor, diode, or diode-configured transistor. 
         [0022]    The bias circuit may be short circuit, resistor, diode, diode-configured transistor, or combination thereof. 
         [0023]    The amplifying circuit may employ single-stage amplifying transistor, differential pair, cascode amplifying circuit, or cascade amplifying circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings; wherein: 
           [0025]      FIG. 1  schematically shows a first embodiment according to the present invention; 
           [0026]      FIG. 2  shows an example of the amplifier in  FIG. 1 ; 
           [0027]      FIG. 3  schematically shows a second embodiment according to the present invention; 
           [0028]      FIG. 4  shows an example of the amplifier in  FIG. 3 ; 
           [0029]      FIG. 5  schematically shows a third embodiment according to the present invention; 
           [0030]      FIG. 6  shows an example of the amplifier in  FIG. 5 ; 
           [0031]      FIG. 7  schematically shows a fourth embodiment according to the present invention; and 
           [0032]      FIG. 8  shows an example of the amplifier in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  schematically shows a first embodiment according to the present invention, in which an amplifier  100  has a signal source input  102  to be connected with a signal source, such as to an output of a signal source with high output impedance, for receiving an input signal S 1  therefrom, to be further amplified by an amplifying circuit  112  to generate an output signal S 2  at an output  104 , a power input  116  of the amplifying circuit  112  is connected with an external supply voltage VDD in order to activate the amplifying circuit  112 , a load  114  is located in a current path of the amplifying circuit  112  and connected between the power input  116  and the amplifying circuit  112 , a bias circuit  110  is connected in the current path of the amplifying circuit  112  on the opposite side of the amplifying circuit  112 , a bias circuit  108  is connected between an input  106  and the bias circuit  110  such that the bias circuits  108  and  110  constitute a self-bias circuit for generating low voltage as the bias of the input  106  and the signal source input  102 , and thus it is not required to provide an additional supply voltage to generate the bias of the input  106  and the signal source input  102 . Moreover, because the bias of the input  106  and the signal source input  102  is generated via the self-bias circuit that is connected at the opposite side to where the amplifying circuit  112  is connected to the load  114 , the bias may be varied by modifying a design thereof, and the bias will not be varied along with changes in the load  114 , thus the resultant bias is more stable. All of the components of the aforementioned amplifier  100  may be integrated in a single chip. 
         [0034]    Referring to an example shown in  FIG. 2 , the load  114  is a resistor, the bias circuit  108  is a reverse-biased diode, the bias circuit  110  comprises a resistor connected with a forward-biased diode in series, and the amplifying circuit  112  is a single-stage amplifying transistor, such as PMOSFET. In different embodiments, the bias circuits  108  and  110  may be selected from short circuit, resistor, diode, or combination thereof. As is well known, the diode may be diode-configured transistor, and the amplifying circuit  112  may employ differential pair, cascode amplifying circuit, or cascade amplifying circuit. 
         [0035]      FIG. 3  schematically shows a second embodiment according to the present invention, in which an amplifier  150  comprises not only the structure shown in  FIG. 1 , but also a coupling circuit  118  connected between the signal source input  102  and the input  106  of the amplifying circuit  112 , so that the signal source input  102  and the input  106  of the amplifying circuit  112  can be biased independently, and a bias circuit  120  is connected between a supply voltage VB and the signal source input  102  for biasing the signal source input  102  at a DC voltage. Therefore, the signal source input  102  and the input  106  of the amplifying circuit  112  can be biased at different DC levels, so that an output of the signal source can be coupled to the amplifying circuit  112  with minimal loss. Because the signal source input  102  and the input  106  of the amplifying circuit  112  are independently biased, the amplifier  150  is adaptive to different signal sources. All of the components of the aforementioned amplifier  150  may be integrated in a single chip. 
         [0036]    Referring to another embodiment shown in  FIG. 4 , the coupling circuit  118  is a capacitor. As is well known, this capacitor may be implemented with the structure of polysilicon-insulator-diffusion, metal-insulator-diffusion, polysilicon-insulator-polysilicon, metal-insulator-polysilicon, or metal-insulator-metal on a semiconductor chip. The load  114  is a resistor, the bias circuit  120  is a reverse-biased diode, the bias circuit  108  is also a reverse-biased diode, the bias circuit  110  comprises a resistor connected with a forward-biased diode in series, and the amplifying circuit  112  is a single-stage amplifying transistor, such as PMOSFET. In different embodiments, the coupling circuit  118  may employ a reverse-biased diode or any other circuits that allow the signal source input  102  and the input  106  of the amplifying circuit  112  to be independently biased, the bias circuits  108 ,  110  and  120  may be selected from short circuit, resistor, diode, diode-configured transistor, or combination thereof, and the amplifying circuit  112  may employ differential pair, cascode amplifying circuit, or cascade amplifying circuit. 
         [0037]      FIG. 5  schematically shows a third embodiment according to the present invention, in which an amplifier  200  comprises a signal source input  202  for receiving an input signal S 1 , and an amplifying circuit  212  connected with an external supply voltage VDD by a power input  216 , so as to amplify the input signal S 1  for generating the output signal S 2  at an output  204 . A load  214  is located in a current path of the amplifying circuit  212  and connected between the amplifying circuit  212  and ground GND, a bias circuit  210  is connected in the current path of the amplifying circuit  212  at the opposite side of the amplifying circuit  212  to the load  214 , a bias circuit  208  is connected between an input  206  of the amplifying circuit  212  and the bias circuit  210 , the bias circuits  208  and  210  constitute a self-bias circuit for generating high voltage as the bias of the input  206  of the amplifying circuit  212  and the signal source input  202 , and thus it is not required to provide an additional supply voltage to generate the bias voltage for the input  206  and the signal source input  202 . Moreover, because the bias of the input  206  and the signal source input  202  is generated via the self-bias circuit that is connected at the opposite side to where the amplifying circuit  212  is connected to the load  214 , the bias may be varied by modifying the design thereof, and the bias will not be varied along with changes in the load  214 , thus the resultant bias is more stable. Similarly, the bias circuits  208  and  210  may be selected from short circuit, resistor, diode, diode-configured transistor, or combination thereof, the amplifying circuit  212  may employ single-stage amplifying transistor, differential pair, cascode amplifying circuit, or cascade amplifying circuit. All of the components of the aforementioned amplifier  200  may be integrated in a single chip. 
         [0038]      FIG. 6  shows an example of the amplifier  200  in  FIG. 5 , in which the bias circuit  208  is a short circuit, the bias circuit  210  is a diode-configured transistor, the amplifying circuit  212  employs a single-stage amplifying transistor, such as NMOSFET, and the load  214  is a diode-configured transistor. 
         [0039]      FIG. 7  schematically shows a fourth embodiment according to the present invention, in which an amplifier  250  comprises not only the structure shown in  FIG. 5 , but also a coupling circuit  218  connected between the signal source input  202  and the amplifying circuit  212 , and a bias circuit  220  connected between the supply voltage VB and the signal source input  202 . Therefore, the signal source input  202  and the input  206  of the amplifying circuit  212  can be biased at different DC voltages, so that the amplifier  250  is adaptive to different signal sources. Similarly, the coupling circuit  218  may use capacitor, diode, or any other circuits that allow the signal source input  202  and the input  206  of the amplifying circuit  212  to be independently biased, and the capacitor may be implemented with the structure of polysilicon-insulator-diffusion, metal-insulator-diffusion, polysilicon-insulator-polysilicon, metal-insulator-polysilicon, or metal-insulator-metal on a semiconductor chip. The bias circuits  208 ,  210  and  220  may be selected from short circuit, resistor, diode, diode-configured transistor, or combination thereof, the amplifying circuit  212  may employ single-stage amplifying transistor, differential pair, cascode amplifying circuit, or cascade amplifying circuit. All of the components of the aforementioned amplifier  250  may be integrated in a single chip. 
         [0040]      FIG. 8  shows an example of the amplifier in  FIG. 7 , in which the coupling circuit  218  is a diode-configured transistor, the bias circuit  220  is a resistor, the bias circuit  208  is a short circuit, the bias circuit  210  is a diode-configured transistor, the amplifying circuit  212  employs a single-stage amplifying transistor, such as NMOSFET, and the load  214  employs a diode-configured transistor. 
         [0041]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.
An amplifier has a self-bias circuit to generate the bias voltage for the input of the amplifying circuit in the amplifier, thereby simplifying the circuit complexity to reduce the size and cost of the amplifier.