Patent Publication Number: US-11664799-B2

Title: Analog switch circuit and control circuit and control method thereof

Description:
CROSS REFERENCE 
     The present invention claims priority to U.S. 63/163,019 filed on Mar. 18, 2021 and claims priority to TW 110142062 filed on Nov. 11, 2021. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     The present invention relates to an analog switch circuit; particularly, it relates to such analog switch circuit capable of improving harmonic distortion. The present invention also relates to a control circuit and a control method of such analog switch circuit. 
     Description of Related Art 
     Please refer to  FIG.  1   , which shows a schematic diagram of a conventional analog switch circuit  10 . The analog switch circuit  10  comprises: a first switch Q 1  and a second switch Q 2 , which are connected in series between an input signal Vin and the output signal Vout. The first switch Q 1  and the second switch Q 2  have a channel resistance R 0 , and when the first switch Q 1  and the second switch Q 2  operate to convert the input signal Vin to the output signal Vout, there is a channel resistance variation ΔR, wherein the channel resistance variation ΔR is correlated with a variation of the input signal Vin, a variation of the output signal Vout and a variation of ambient temperature. Besides, the analog switch circuit  10  has a parasitic resistance Rp. The output signal Vout is applied to a load resistance RL which is electrically connected to a ground level. Under such situation, the output signal Vout can be represented by the following equation: 
               V   OUT     =       V   IN             Δ   ⁢   R       R   L       ⁢     V   IN       +         R   0     +     R   Parasitic         R   L       +   1             
Through Fourier transformation of the above equation, the following equation can be obtained:
 
                 f   [     V   in     ]     =         V   in     B     -       A     B   2       ⁢     V   in   2       +         A   2       B   3       ⁢     V   in   3       -         A   3       B   4       ⁢     V   in   4       +         A   4       B   5       ⁢     V   in   5           ⁢   
         wherein   ⁢         A     =       Δ   ⁢   R       R   L         ,     B   =           R   0     +     R   Parasitic         R   L       +   1               
In a case when the input signal Vin is a sinusoidal wave, which can be represented as: V in =sin(2πft), the following equations can be obtained:
 
               fundamental   =       (       1   B     +       3   ⁢     A   2         4   ⁢     B   3         +       5   ⁢     A   4         8   ⁢     B   5           )     ⁢     sin   ⁡   (     2   ⁢   π   ⁢   ft     )         ⁢   
       2   ⁢   nd   ⁢         harmonic     =       (       A     2   ⁢     B   2         +       A   3       2   ⁢     B   4           )     ⁢     sin   ⁡   (       4   ⁢   π   ⁢   f     +     π   2       )         ⁢   
       3   ⁢   nd   ⁢         harmonic     =       (         A   2       4   ⁢     B   3         +       5   ⁢     A   4         16   ⁢     B   5           )     ⁢     sin   ⁡   (       6   ⁢   π   ⁢   f     +   π     )         ⁢   
       4   ⁢   nd   ⁢         harmonic     =       (       A   3       8   ⁢     B   4         )     ⁢     sin   ⁡   (       8   ⁢   π   ⁢   f     +       3   ⁢   π     2       )         ⁢   
       5   ⁢   nd   ⁢         harmonic     =       (       A   4       16   ⁢     B   5         )     ⁢     sin   ⁡   (     10   ⁢   π   ⁢   f     )               
In light of above, when ΔR/R L  is smaller, a distortion of the resonant waveform will be better alleviated, and if the load resistance RL is small, the harmonic distortion will become more serious. Therefore, in light load condition (i.e., in a case when the load resistance RL is low), it is a challenge to reduce the harmonic distortion.
 
     Please refer to  FIG.  2 A  and  FIG.  2 B .  FIG.  2 A  shows a schematic diagram of a conventional analog switch circuit  20  constituted by PMOS/NMOS devices.  FIG.  2 B  illustrates the harmonic distortion of the output signal Vout of the conventional analog switch circuit  20  of  FIG.  2 A . In the analog switch circuit  20 , because the PMOS device and the NMOS device both receive a constant gate voltage Vg, a gate-source voltage of the PMOS device and a gate-source voltage of the NMOS device will change as the input signal Vin changes. A channel resistance of the analog switch circuit  20  will change accordingly due to the characteristics of the PMOS and NMOS devices. However, the variation of the channel resistance will cause the harmonic distortion of the output signal Vout to become more serious. The distortion of the resonant waveform of the analog switch circuit  20  is shown in  FIG.  2 B .  FIG.  2 B  illustrates that the analog switch circuit  20  constituted by PMOS and NMOS devices shown in  FIG.  2 A  can only provide a performance of approximately −90 dB. 
     Please refer to  FIG.  3 A ,  FIG.  3 B  and  FIG.  3 C .  FIG.  3 A  shows a schematic diagram of a conventional analog switch circuit  30 .  FIG.  3 B  shows a simulation schematic diagram of electrical characteristics of the conventional analog switch circuit  30  of  FIG.  3 A .  FIG.  3 C  illustrates the harmonic distortion of the output signal Vout of the conventional analog switch circuit  30  of  FIG.  3 A . As shown in  FIG.  3 A , by providing a constant voltage Vgs as a gate-source voltage of each MOS device of the analog switch circuit  30 , the gate-source voltage of each MOS device will not change as an input signal Vin changes. However as shown in  FIG.  3 B , although the gate-source voltage is a constant voltage Vgs, as the input signal Vin changes, the channel voltage VRon, the channel resistance Ron and the load current ILoad change greatly, which will accordingly affect the performance of the harmonic distortion. As shown in  FIG.  3 C , as compared to the analog switch circuit  20  and the analog switch circuit  10 , although the analog switch circuit can provide a better performance of harmonic distortion (approximately −110 dB), such configuration is still not satisfactory. 
     Note that, as well known by one skilled in this art, a gate-source voltage refers to a voltage difference between a gate and a source; a drain-source voltage refers to a voltage difference between a drain and a source; a drain-source current refers to a current flowing between a drain and a source. 
     As compared to the above-mentioned prior arts, the present invention provides an analog switch circuit, and a control circuit and a control method of such analog switch circuit, which are capable of adaptively adjusting a gate-source voltage via feedback mechanism according to a channel voltage variation, so as to maintain a channel resistance at a constant while the channel voltage changes. 
     SUMMARY OF THE INVENTION 
     From one perspective, the present invention provides an analog switch circuit, comprising: a switch unit including a first switch, wherein the first switch is coupled to a current path formed between an input terminal and an output terminal, and wherein the first switch is configured to operably convert an input signal of the input terminal to an output signal of the output terminal according to a first gate-source voltage of the first switch; and a control circuit including: a sensor circuit coupled between the input terminal and the output terminal, wherein the sensor circuit is configured to operably generate a sensing signal according to a voltage difference between the input signal and the output signal; and a gate-source voltage adjustment circuit coupled to the sensor circuit, wherein the gate-source voltage adjustment circuit is configured to operably generate and adaptively adjust the first gate-source voltage according to the sensing signal, so as to maintain a conduction resistance of the switch unit at a constant while the voltage difference changes. 
     From another perspective, the present invention provides a control circuit of an analog switch circuit, comprising: a sensor circuit coupled between an input terminal and an output terminal, wherein the sensor circuit is configured to operably generate a sensing signal according to a voltage difference between an input signal of the input terminal and an output signal of the output terminal; and a gate-source voltage adjustment circuit coupled to the sensor circuit, wherein the gate-source voltage adjustment circuit is configured to operably generate and adaptively adjust a first gate-source voltage according to the sensing signal, so as to operate a first switch of a first switch in a switch unit of the analog switch circuit, thereby converting the input signal to the output signal and thereby maintaining a conduction resistance of the switch unit at a constant while the voltage difference changes; wherein the first switch is coupled in a current path formed between the input terminal and the output terminal. 
     From yet another perspective, the present invention provides a control method of an analog switch circuit, comprising the following steps: operating a first switch in the analog switch circuit according to a first gate-source voltage of the first switch, so as to convert an input signal of the input terminal to an output signal of the output terminal; generating a sensing signal according to a voltage difference between the input signal and the output signal; and adaptively adjusting the first gate-source voltage according to the sensing signal, so as to maintain a conduction resistance of the analog switch circuit at a constant while the voltage difference changes; wherein the first switch is coupled in a current path formed between the input terminal and the output terminal. 
     In one embodiment, the control circuit further includes: a voltage-divider circuit coupled between the input terminal and the output terminal, wherein voltage divider circuit is configured to operably provide a bulk-source divided voltage into the voltage difference, so that the gate-source voltage adjustment circuit adaptively adjusts the first gate-source voltage further according to the bulk-source divided voltage. 
     In one embodiment, the switch unit further includes a second switch, wherein the first switch and the second switch are coupled in series between the input terminal and the output terminal. 
     In one embodiment, a relationship between the first gate-source voltage and the voltage difference is represented by a following equation: Vgs 1 =Vc 1 +K×Vdif, wherein the first gate-source voltage Vgs 1  is equal to a sum of a constant voltage Vc 1  plus a product of the voltage difference Vdif multiplied by a coefficient K; wherein the coefficient K is a real number greater than one. 
     In one embodiment, the gate-source voltage adjustment circuit is further configured to operably generate a second gate-source voltage of the second switch according to the sensing signal, so as to control the second switch, to thereby maintain the conduction resistance of the switch unit at the constant while the voltage difference changes. 
     In one embodiment, the second gate-source voltage is a constant. 
     In one embodiment, the control circuit further includes: a voltage divider circuit coupled between the input terminal and the output terminal, wherein voltage divider circuit is configured to operably provide a bulk-source divided voltage into the voltage difference, so that the gate-source voltage adjustment circuit adaptively adjusts the first gate-source voltage further according to the a bulk-source divided voltage. 
     In one embodiment, the sensor circuit includes: an amplifier circuit having a first amplifier input terminal and a second amplifier input terminal; a first resistor coupled between the first amplifier input terminal and the input terminal; and a second resistor coupled between the second amplifier input terminal and the output terminal; wherein the amplifier circuit regulates a voltage at the first amplifier input terminal and a voltage at the amplifier second input terminal to be a same level by a negative feedback control mechanism; wherein the amplifier circuit generates an amplification current, and wherein the sensing signal is proportional to the amplification current. 
     In one embodiment, the sensor circuit further includes: a current mirror circuit, which is configured to operably duplicate and amplify the amplification current, so as to generate the sensing signal. 
     In one embodiment, the amplifier circuit includes: a first super source follower. 
     In one embodiment, the gate-source voltage adjustment circuit includes: a first impedance circuit coupled between a first gate of the first switch and a first source of the first switch, wherein the first impedance circuit is configured to operably adaptively adjust the first gate-source voltage according to the sensing signal. 
     In one embodiment, the gate-source voltage adjustment circuit further includes: a second super source follower coupled between the sensor circuit and the first impedance circuit, wherein the second super source follower is configured to operably generate two summation currents according to a sensing current correlated with the sensing signal and a constant current; and a second impedance circuit coupled to the second super source follower; wherein the two summation currents flow through the first impedance circuit and the second impedance circuit, respectively, so as to adaptively adjust the first gate-source voltage. 
     In one embodiment, the first switch includes a first metal oxide semiconductor (MOS) device, and wherein the voltage difference is correlated with a drain-source voltage of the first MOS device, and wherein the conduction resistance is correlated with a channel resistance of the first MOS device when the first MOS device is in ON operation; wherein the sensing signal is proportional to a drain-source current of the first MOS device when the first MOS device is in ON operation; wherein the gate-source voltage adjustment circuit is configured to operably adaptively adjust the first gate-source voltage according to the voltage difference, so as to maintain the channel resistance of the first MOS device at the constant when the first MOS device is in ON operation. 
     In one embodiment, the first switch includes a first metal oxide semiconductor (MOS) device, whereas, the second switch includes a second MOS device, and wherein the voltage difference is correlated with a sum of a drain-source voltage of the first MOS device plus a drain-source voltage of the second MOS device; wherein the conduction resistance is correlated with a sum of a channel resistance of the first MOS device when the first MOS device is in ON operation plus a channel resistance of the second MOS device when the second MOS device is in ON operation; wherein the sensing signal is proportional to a drain-source current of the first MOS device when the first MOS device is in ON operation; wherein the gate-source voltage adjustment circuit is configured to operably adaptively adjust the first gate-source voltage according to the voltage difference, so as to maintain the sum of the channel resistance of the first MOS device plus the channel resistance of the second MOS device at the constant. 
     The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic diagram of a conventional analog switch circuit  10 . 
         FIG.  2 A  shows a schematic diagram of a conventional analog switch circuit  20  constituted by PMOS and NMOS devices. 
         FIG.  2 B  illustrates the harmonic distortion of the output signal Vout of the conventional analog switch circuit  20  of  FIG.  2 A . 
         FIG.  3 A  shows a schematic diagram of a conventional analog switch circuit  30 . 
         FIG.  3 B  shows a simulation schematic diagram of electrical characteristics of the conventional analog switch circuit  30  of  FIG.  3 A . 
         FIG.  3 C  illustrates the harmonic distortion of the output signal Vout of the conventional analog switch circuit  30  of  FIG.  3 A . 
         FIG.  4 A  shows a schematic diagram of an analog switch circuit  40  according to an embodiment of the present invention. 
         FIG.  4 B  illustrates a simulation schematic diagram depicting the harmonic distortion of the output signal Vout of the analog switch circuit  40  of  FIG.  4 A . 
         FIG.  5    shows a schematic diagram of an analog switch circuit  50  according to an embodiment of the present invention. 
         FIG.  6    shows a schematic diagram of an analog switch circuit  60  according to a specific embodiment of the present invention. 
         FIG.  7    shows a schematic diagram of an analog switch circuit  70  according to an embodiment of the present invention. 
         FIG.  8    shows a schematic diagram of an analog switch circuit  80  according to an embodiment of the present invention. 
         FIG.  9    shows a schematic diagram of an analog switch circuit  90  according to an embodiment of the present invention. 
         FIG.  10    shows a schematic diagram of an analog switch circuit  100  according to an embodiment of the present invention. 
         FIG.  11    shows a specific embodiment of a sensor circuit  103 . 
         FIG.  12 A  to  FIG.  12 E  illustrate a Fast Fourier Transform (FFT) simulation result depicting the harmonic distortion of the output signal Vout of the analog switch circuit of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings as referred to throughout the description of the present invention are for illustration only, to show the interrelations between the circuits and the signal waveforms, but not drawn according to actual scale of circuit sizes and signal amplitudes and frequencies. 
     Please refer to  FIG.  4 A  and  FIG.  4 B .  FIG.  4 A  shows a schematic diagram of an analog switch circuit  40  according to an embodiment of the present invention.  FIG.  4 B  illustrates a simulation schematic diagram depicting the harmonic distortion of the output signal Vout of the analog switch circuit  40  of  FIG.  4 A . The analog switch circuit  40  comprises: a switch unit  41  and a control circuit  42 . The control circuit  42  includes: a sensor circuit  43  and a gate-source voltage adjustment circuit  45 . As shown in  FIG.  4 A , the switch unit  41  is configured to operate a first switch Q 1  therein according to a first gate-source voltage Vgs 1  of the first switch Q 1 , so as to convert an input signal Vin of an input terminal T 1  to the output signal Vout of an output terminal T 2 . The sensor circuit  43  is coupled between the input terminal T 1  and the output terminal T 2 . The sensor circuit  43  is configured to operably generate a sensing signal Ssen according to a voltage difference between the input signal Vin and the output signal Vout. The gate-source voltage adjustment circuit  45  is coupled to the sensor circuit  43  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen, so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference changes. 
     In this embodiment, the first switch Q 1  is for example an N type metal oxide semiconductor (MOS) device (i.e., NMOS device). The above-mentioned voltage difference is correlated with a drain-source voltage of the NMOS device. The above-mentioned conduction resistance is correlated with a channel resistance of the NMOS device when the NMOS device is in ON operation. In one embodiment, the above-mentioned voltage difference is equal to the drain-source voltage of the NMOS device. In one embodiment, the above-mentioned conduction resistance is equal to the channel resistance of the NMOS device when the NMOS device is in ON operation. The sensing signal Ssen is proportional to a drain-source current of the NMOS device when the first MOS device is in ON operation. The gate-source voltage adjustment circuit  45  is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the above-mentioned voltage difference, so as to maintain the channel resistance of the NMOS device at a constant when the NMOS device is in ON operation. For example, as shown in  FIG.  4 A , the first gate-source voltage Vgs 1  is for example equal to a sum of a constant voltage Vc 1  plus a product of a parameter K multiplied by the sensing signal Ssen, wherein the parameter K can be a constant or an adjustable variable which is set or adjusted according to a user&#39;s requirement or according to a condition of the harmonic distortion. 
     In this embodiment, the gate-source voltage adjustment circuit  45  is coupled to the sensor circuit  43 , and an output of the gate-source voltage adjustment circuit  45  is coupled between a gate and a source of a MOS device of the first switch Q 1 . In an implementation wherein the first switch Q 1  is an NMOS device, when a voltage difference of the input signal Vin minus the output signal Vout has a positive sign, the gate-source voltage adjustment circuit  45  is coupled between the gate of the NMOS device and the output terminal T 2 , as shown in  FIG.  4 A . On the other hand, when a voltage difference of the input signal Vin minus the output signal Vout has a negative sign, the gate-source voltage adjustment circuit  45  is coupled between the gate of the NMOS device and the input terminal T 1  (not shown in  FIG.  4 A ). In another embodiment, a selection switch can be employed, which selectively connects the output terminal of the gate-source voltage adjustment circuit  45  to the output terminal T 2  or the input terminal T 1  according to the voltage difference of the input signal Vin minus the output signal Vout. 
     The drawback of the prior art is that when the input signal and the output signal vary, the channel resistance variation ΔR changes to result in harmonic distortion (“channel resistance” is the conduction resistance in ON operation of a MOS device). In view of this, in the analog switch circuit of the present invention which uses a MOS device as the first switch Q 1 , the present invention senses a voltage difference between the input terminal and the output terminal and adjusts the gate-source voltage of the MOS device via feedback mechanism, to maintain the channel resistance at a constant. This embodiment controls the channel resistance by changing a voltage between the gate and the source (i.e., first gate-source voltage Vgs 1  in this embodiment) of the MOS device. As shown in  FIG.  4 A , this embodiment feedback controls the first gate-source voltage Vgs 1  of the MOS device according to a voltage difference between the input terminal T 1  and the output terminal T 2 , to reduce the variation of the channel resistance. The first gate-source voltage Vgs 1  of the MOS device can be determined according to the constant voltage Vc 1  and the sensing signal Ssen, so as to adaptively adjust the first gate-source voltage Vgs 1  of the MOS device to the target level. The sensing signal Ssen is correlated with a voltage difference between the input terminal Vin and the output terminal Vout. In this embodiment, the sensing signal Ssen is correlated with a voltage difference between the drain and the source of the MOS device. A simulation result is as shown in  FIG.  4 B . According to the simulation result of this embodiment, high order harmonic distortion can be greatly reduced by accurate compensation. 
     Please refer to  FIG.  5   , which shows a schematic diagram of an analog switch circuit  50  according to an embodiment of the present invention. The analog switch circuit  50  of  FIG.  5    is different from the analog switch circuit  40  of  FIG.  4    in that: in the analog switch circuit  50  of  FIG.  5   , in addition to including a sensor circuit  43  and a gate-source voltage adjustment circuit  45 , a control circuit  52  of the analog switch circuit  50  further includes a voltage divider circuit  57 . The voltage divider circuit  57  is coupled between an input terminal T 1  and an output terminal T 2 . The voltage divider circuit  57  is configured to operably divide a voltage difference between the input terminal Vin and the output terminal Vout, so as to generate a bulk-source divided voltage of a first switch Q 1 , and the gate-source voltage adjustment circuit  45  adaptively adjusts the first gate-source voltage Vgs 1  further according to the bulk-source divided voltage. As one having ordinary skill in the art readily understands, the “bulk-source divided voltage” is a divided voltage generated by a resistor Rbs which lies between a bulk coupled to a MOS device and a source of the MOS device, as referring to the voltage divider circuit  57  in  FIG.  5   . Besides, as shown in  FIG.  5   , the gate-source voltage adjustment circuit  45  can be coupled between the bulk and the gate of the MOS device, and regardless whether the voltage difference of the input signal Vin minus the output signal Vout has a positive sign or a negative sign, the gate-source voltage adjustment circuit  45  can adaptively adjust the first gate-source voltage Vgs 1  in both situations without changing the connection of the gate-source voltage adjustment circuit  45  to the input terminal T 1  or to the output terminal  12 , so that the conduction resistance of the switch unit  41  is maintained at a constant while the voltage difference changes, to thereby improve harmonic distortion. 
     As shown in  FIG.  5   , in this embodiment, the first gate-source voltage Vgs 1  is equal to a sum of a constant voltage Vc 1  plus a product of a parameter K multiplied by the sensing signal Ssen plus the above-mentioned bulk-source divided voltage, which can be represented by the following equation:
 
 Vgs 1= Vc 1+( K×Ssen )+ V 3− V out
 
(in this embodiment, the bulk-source divided voltage is V 3 −Vout). Given the fact that the sensing signal Ssen is equal to a difference of the input terminal Vin minus the output terminal Vout, it is derived that:
 
 Vgs 1= Vc 1+( K×Ssen )+ V 3− V out
 
 Vgs 1= Vc 1+( K×Ssen )+ D ×( V in− V out)
 
wherein D=Rbs/(Rbs+Rbs′), wherein the resistance Rbs′ denotes a resistance between the bulk and the drain of the MOS device of the first switch Q 1 , that is, D denotes a ratio of the resistance Rbs to the total resistance of the entire voltage division resistor group; the voltage V 3  denotes a bulk voltage of the first switch Q 1 .
 
     
       
         
           
             
               
                 
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     That is, in this embodiment, the gate-source voltage adjustment circuit  45  generates and adaptively adjusts the first gate-source voltage Vgs 1  according to the sensing signal Ssen generated by the voltage difference between the input terminal Vin and the output terminal Vout (in one embodiment, the sensing signal Ssen is equal to the voltage difference of the input terminal Vin minus the output terminal Vout), so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference changes. 
     In one embodiment, the parameter K is a real number greater than one. When the parameter K is greater than one, the gate-source voltage adjustment circuit  45  is not limited by the approach of using a voltage divider circuit constituted by passive devices such as resistors to adjust the first gate-source voltage Vgs 1  by a divided voltage of a voltage difference Vdif between the input terminal Vin and the output terminal Vout. According to the embodiment of the present invention, the present invention can sense the voltage difference Vdif and adjust the first gate-source voltage Vgs 1  via an active feedback control mechanism, to thereby more precisely maintain the conduction resistance of the switch unit  41  at a constant. 
     Please refer to  FIG.  6   , which shows a schematic diagram of an analog switch circuit  60  according to a specific embodiment of the present invention. In this embodiment, the analog switch circuit  60  comprises: a switch unit  41  and a control circuit  62 . The control circuit  62  includes: a sensor circuit  63  and a gate-source voltage adjustment circuit  65 . As shown in  FIG.  6   , the switch unit  41  is configured to operate a first switch Q 1  therein according to a first gate-source voltage Vgs 1  of the first switch Q 1 , so as to convert an input signal Vin of an input terminal T 1  to the output signal Vout of an output terminal T 2 . The sensor circuit  63  is configured to operably generate a sensing signal Ssen according to a voltage difference Vdif between the input signal Vin and the output signal Vout. As shown in  FIG.  6   , in one embodiment, the sensing signal Ssen can be for example a current K 1 *Isen. The gate-source voltage adjustment circuit  65  is coupled to the sensor circuit  63  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen, so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference Vdif changes. 
     As shown in  FIG.  6   , in one embodiment, the parameter K 1  is equal to one whereby the sensing signal Ssen is the sensing current Isen. Under such situation, a regulation signal generation circuit  651  receives the sensing current Isen and generates a current K*Isen flowing through a resistor R 3 , thereby generating a voltage difference K*Isen*R 3 . Under such circumstance, Vgs 1 =Vc 1 +K*Isen*R 3 . 
     This embodiment shows a specific embodiment of the sensor circuit  63 . The sensor circuit  63  includes: an amplifier circuit  631 , a current mirror circuit  633 , a first resistor R 1  and a second resistor R 2 . The amplifier circuit  631  has a first input terminal N 1  coupled to the input terminal T 1  and a second input terminal N 2  coupled to the output terminal T 2 . The amplifier circuit  631  regulates a voltage at the first input terminal N 1  and a voltage at the second input terminal N 2  to be the same level by a negative feedback control mechanism. The first resistor R 1  is coupled between the first input terminal T 1  and the input terminal N 1 . The second resistor R 2  is coupled between the second input terminal N 2  and the output terminal T 2 . The amplifier circuit  631  generates a sensing current Isen, wherein the sensing signal Ssen is proportional to the sensing current Isen. The current mirror circuit  633  is configured to operably duplicate and amplify the sensing current Isen, so as to generate the sensing signal Ssen. In this embodiment, the current mirror circuit  633  amplifies the sensing current Isen by K 1 -fold to generate a current K 1 *Isen to be the sensing signal Ssen. The amplifier circuit  631  includes an amplifier A 1  which can be implemented via numerous ways, and a specific embodiment thereof will be described later. 
     The gate-source voltage adjustment circuit  65  is coupled to the sensor circuit  63  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the current K 1 *Isen (which serves as the sensing signal Ssen), so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference Vdif changes. 
     In this embodiment, the gate-source voltage adjustment circuit  65  includes: a regulation signal generation circuit  651 , a voltage source VS and a resistor R 3 . In this embodiment, the regulation signal generation circuit  651  generates a current K*Isen flowing through the resistor R 3 , so as to generate a voltage difference K*Isen*R 3 . Thus, the first gate-source voltage Vgs 1  is equal to a sum of a constant voltage Vc 1  plus the voltage difference K*Isen*R 3 . The current K*Isen is correlated with the voltage difference Vdif between the input terminal Vin and the output terminal Vout. Consequently, the regulation signal generation circuit  651  can adaptively adjust the first gate-source voltage Vgs 1  according to the voltage difference Vdif, so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference Vdif changes. The resistor R 3  is coupled between the gate and source of the first switch Q 1 , to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal. 
     As shown in  FIG.  6   , each of two current sources provide a constant current Ib, wherein the two constant currents Ib and Ib flow through the first resistor R 1  and the second resistor R 2 , respectively. The voltage difference across two ends of the first resistor R 1  is equal to V 1 , whereas, the voltage difference across two ends of the second resistor R 2  is equal to V 2 . The amplifier circuit  631  regulates a voltage at the first input terminal N 1  and a voltage at the second input terminal N 2  to be the same level by a negative feedback control mechanism. Assuming that the first switch Q 1  has a channel resistance of Ron, the output current flowing through the first switch Q 1  is equal to Iout, and the resistance R 1  is equal to the resistance R 2 , under such situation, the negative feedback control mechanism of the amplifier circuit  631  can be represented by the following equations: 
                   V   ⁢   1     +     Iout   ×   Ron       =     V   ⁢   2       ⁢   
         Ib   ×   R   ⁢   1     +       I   ⁢   out     ×   Ron       =       Isen   ×   R   ⁢   2     +     Ib   ×   R   ⁢   2         ⁢   
         Ib   ×   R   ⁢   1     +       I   ⁢   out     ×   Ron       =       Isen   ×   R   ⁢   1     +     Ib   ×   R   ⁢   1         ⁢   
     Isen   =         I   ⁢   out     ×   Ron       R   ⁢   1               
Therefore, the first gate-source voltage Vgs 1  can be represented by the following equation:
 
                 V   ⁢   gs     ⁢   1     =       Vc   ⁢   1     +     K   ×         I   ⁢   out     ×   Ron       R   ⁢   1       ×   R   ⁢   3             
In a case when the resistance R 3  is equal to the resistance R 1 , the first gate-source voltage Vgs 1  can be represented by the following equations:
 
                   V   ⁢   gs     ⁢   1     =       Vc   ⁢   1     +     K   ×       Iout   ×   Ron       R   ⁢   1       ×   R   ⁢   3         ⁢   
       Vgs   ⁢   1     =       Vc   ⁢   1     +     K   ×   I   ⁢       out   ×   Ron         ⁢   
       Vgs   ⁢   1     =         V   ⁢   c     ⁢       1     +     K   ×   Vdif               
The above-mentioned equations demonstrate that the sensor circuit  63  regulates the voltage at the first input terminal N 1  and the voltage at the second input terminal N 2  to be the same level by a negative feedback control mechanism, so as to generate the sensing current Isen, wherein the sensing current Isen is correlated with the voltage difference Vdif between the input terminal Vin and the output terminal Vout. The gate-source voltage adjustment circuit  65  adjusts the first gate-source voltage Vgs 1  according to the sensing current Isen, so as to maintain the conduction resistance of the switch unit  41  at a constant while the voltage difference Vdif changes.
 
     In one embodiment, as shown in  FIG.  6   , the amplifier circuit  631  includes: a first super source follower. In one embodiment, the parameter K is a real number greater than one. 
     Please refer to  FIG.  7   , which shows a schematic diagram of an analog switch circuit  70  according to an embodiment of the present invention. In this embodiment, the analog switch circuit  70  of  FIG.  7    is different from the analog switch circuit  60  of  FIG.  6    in that: in the analog switch circuit  70  of  FIG.  7   , in addition to including a sensor circuit  63  and a gate-source voltage adjustment circuit  65 , a control circuit  62  of the analog switch circuit  70  further includes a voltage divider circuit  77 . The voltage-divider circuit  77  is coupled between an input terminal T 1  and an output terminal T 2 . The voltage divider circuit  77  is configured to operably divide a voltage difference between the input terminal Vin and the output terminal Vout, so as to generate a bulk-source divided voltage of a first switch Q 1 , and the gate-source voltage adjustment circuit  65  adaptively adjusts the first gate-source voltage Vgs 1  further according to the bulk-source divided voltage. Besides, as shown in  FIG.  7   , the gate-source voltage adjustment circuit  65  is coupled between the bulk and a gate of the first switch Q 1 . Thus, regardless whether the voltage difference of the input signal Vin minus the output signal Vout has a positive sign or a negative sign, the gate-source voltage adjustment circuit  65  can adaptively adjust the first gate-source voltage Vgs 1  in both situations without changing the connection of the gate-source voltage adjustment circuit  65  to the input terminal T 1  or to the output terminal T 2 , so that the conduction resistance of the switch unit  41  is maintained at a constant while the voltage difference changes, to thereby improve harmonic distortion. 
     Please refer to  FIG.  8   , which shows a schematic diagram of an analog switch circuit  80  according to an embodiment of the present invention. The analog switch circuit  80  comprises: a switch unit  81  and a control circuit  82 . The control circuit  82  includes: a sensor circuit  83  and a gate-source voltage adjustment circuit  85 . As shown in  FIG.  8   , the switch unit  81  is configured to operate a first switch Q 1  therein according to a first gate-source voltage Vgs 1  of the first switch Q 1  and is configured to operate a second switch Q 2  therein according to a second gate-source voltage Vgs 2  of the second switch Q 2 , so as to convert an input signal Vin of an input terminal T 1  to the output signal Vout of an output terminal T 2 . The sensor circuit  83  is coupled between the input terminal T 1  and the output terminal T 2 . The sensor circuit  83  is configured to operably generate a sensing signal Ssen according to a voltage difference between the input signal Vin and the output signal Vout. The gate-source voltage adjustment circuit  85  is coupled to the sensor circuit  83  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen, so as to maintain the conduction resistance of the switch unit  81  at a constant while the voltage difference changes. 
     In this embodiment, the analog switch circuit  80  of  FIG.  8    is different from the analog switch circuit  40  of  FIG.  4 A  in that: in addition to the first switch Q 1 , the analog switch circuit  80  further includes a second switch Q 2 , wherein the first switch Q 1  and the second switch Q 2  are coupled in series between the input terminal T 1  and the output terminal T 2 . 
     In one embodiment, as shown in  FIG.  8   , the gate-source voltage adjustment circuit  85  further includes a second regulation circuit  852 . The second regulation circuit  852  generates a second gate-source voltage Vgs 2  according to a sensing signal Ssen (as indicated by a dashed arrow in  FIG.  8   ), wherein the second gate-source voltage Vgs 2  serves to control the second switch Q 2 , to thereby maintain the conduction resistance of the switch unit  81  at a constant while the voltage difference changes. In one embodiment, the second gate-source voltage Vgs 2  is a constant. 
     As shown in  FIG.  8   , in this embodiment, the first switch Q 1  includes for example a first metal oxide semiconductor (MOS) device, such as an N type MOS device (i.e., NMOS device). The second switch Q 2  includes for example a second MOS device, such as an N type MOS device (i.e., NMOS device). The above-mentioned voltage difference is correlated with a sum of a drain-source voltage of the first MOS device plus a drain-source voltage of the second MOS device. The above-mentioned conduction resistance is correlated with a sum of a channel resistance of the first MOS device when the first MOS device is in ON operation plus a channel resistance of the second MOS device when the second MOS device is in ON operation. 
     In one embodiment, the above-mentioned voltage difference is equal to a sum of the drain-source voltage of the first MOS device plus the drain-source voltage of the second MOS device. The above-mentioned conduction resistance is equal to a sum of the channel resistance of the first MOS device when the first MOS device is in ON operation plus the channel resistance of the second MOS device when the second MOS device is in ON operation. The sensing signal Ssen is proportional to a drain-source current of the first MOS device when the first MOS device and the second MOS device are both in ON operation. The gate-source voltage adjustment circuit is configured to operably adaptively adjust the first gate-source voltage Vgs 1  according to the above-mentioned voltage difference (i.e., the sum of the drain-source voltage of the first MOS device plus the drain-source voltage of the second MOS device), so as to maintain the sum of the conduction resistance at the constant. For example, as shown in  FIG.  8   , the first gate-source voltage Vgs 1  is for example equal to a sum of a constant voltage Vc 1  plus a product of a parameter K multiplied by the sensing signal Ssen, wherein the parameter K can be a constant or an adjustable variable which is set or adjusted according to a user&#39;s requirement or according to a condition of the harmonic distortion. 
     Please refer to  FIG.  9   , which shows a schematic diagram of an analog switch circuit  90  according to an embodiment of the present invention. The analog switch circuit  90  comprises: a switch unit  81  and a control circuit  92 . The control circuit  92  includes: a sensor circuit  83 , a gate-source voltage adjustment circuit  85  and a voltage divider circuit  97 . As shown in  FIG.  9   , the switch unit  81  is configured to operate a first switch Q 1  therein according to a first gate-source voltage Vgs 1  of the first switch Q 1  and to operate a second switch Q 2  therein according to a second gate-source voltage Vgs 2  of the second switch Q 2 , so as to convert an input signal Vin of an input terminal T 1  to the output signal Vout of an output terminal T 2 . The sensor circuit  83  is coupled between the input terminal T 1  and the output terminal T 2 . The sensor circuit is configured to operably generate a sensing signal Ssen according to a voltage difference between the input signal Vin and the output signal Vout. The gate-source voltage adjustment circuit  85  is coupled to the sensor circuit  83  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen, so as to maintain the conduction resistance of the switch unit  81  at a constant while the voltage difference changes. The voltage divider circuit  97  is coupled between an input terminal T 1  and an output terminal T 2 . The voltage divider circuit  97  is configured to operably divide a voltage difference between the input terminal Vin and the output terminal Vout, so as to generate a bulk-source divided voltage of a first switch Q 1 , so that the gate-source voltage adjustment circuit  85  adaptively adjusts the first gate-source voltage Vgs 1  further according to the bulk-source divided voltage. In this embodiment, the second gate-source voltage Vgs 2  for example can be a constant. 
     Please refer to  FIG.  10   , which shows a schematic diagram of an analog switch circuit  100  according to an embodiment of the present invention. The analog switch circuit  100  comprises: a switch unit  101  and a control circuit  102 . The control circuit  92  includes: a sensor circuit  103 , a gate-source voltage adjustment circuit  105  and a voltage divider circuit  107 . As shown in  FIG.  10   , the switch unit  101  is configured to operate a first switch Q 1  therein according to a first gate-source voltage Vgs 1  of the first switch Q 1  and is configured to operate a second switch Q 2  therein according to a second gate-source voltage Vgs 2  of the second switch Q 2 , so as to convert an input signal Vin of an input terminal T 1  to the output signal Vout of an output terminal T 2 . The sensor circuit  103  is coupled between the input terminal T 1  and the output terminal T 2 . The sensor circuit  103  is configured to operably generate a sensing signal Ssen according to a voltage difference between the input signal Vin and the output signal Vout. The gate-source voltage adjustment circuit  105  is coupled to the sensor circuit  103  and is configured to adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen, so as to maintain the conduction resistance of the switch unit  101  at a constant while the voltage difference Vdif changes. 
     It is worthwhile mentioning that, the first gate voltage Vg 1  shown in  FIG.  10    is the voltage difference between the gate of the first switch Q 1  (NMOS device in this embodiment) and the ground level. The second gate voltage Vg 2  shown in  FIG.  10    is the voltage difference between the gate of the second switch Q 2  (NMOS device in this embodiment) and the ground level. As shown in  FIG.  10   , the input signal Vin and the output signal Vout are with reference to the ground level. 
     In this embodiment, the switch unit  101  includes: the first switch Q 1  and the second switch Q 2 , wherein the first switch Q 1  and the second switch Q 2  are coupled in series between the input terminal T 1  and the output terminal T 2 . As shown in  FIG.  10   , in one embodiment, the second switch Q 2  is an NMOS device and is controlled by the second gate voltage Vg 2 , whereas, the first switch Q 1  is an NMOS device and is controlled by the first gate voltage Vg 1 . 
     In one embodiment, in a case when Vin&gt;Vout, the source of the first switch Q 1  and the source of the second switch Q 2  are at the right side which is the side nearer to the output signal Vout. The second gate-source voltage Vgs 2  of the second switch Q 2  can be represented by the following equation: 
                 Vgs   ⁢   2     =       2   ⁢   Ib   ×   2   ⁢   Rfod     +       V   ⁢   R     ⁢   5         ⁢   
       Vgs   ⁢   2     =       2   ⁢   Ib   ×   2   ⁢   Rfod     +     Vds   ⁢   2   ×       R   ⁢   5         R   ⁢   4     +     R   ⁢   5                     
The first gate-source voltage Vgs 1  of the first switch Q 1  can be represented by the following equation:
 
                 Vgs   ⁢   1     =         (       2   ⁢   Ib     +   Isen     )     ×   2   ⁢   Rfod     +       V   ⁢   R     ⁢       7         ⁢   
       Vgs   ⁢   1     =         (       2   ⁢   Ib     +   Isen     )     ×   2   ⁢   Rfod     +     Vds   ⁢   1   ×       R   ⁢   7         R   ⁢   6     +     R   ⁢   7                     
In a case when the resistances of the resistor R 4 , the resistor R 5 , the resistor R 6  and the resistor R 7  are equal to one another, the following equations will be obtained:
 
 Vgs 2=2 Ib× 2 Rfod+|Vds 2|×½
 
 Vgs 1=(2 Ib+Isen )×2 Rfod+|Vds 1|×½
 
wherein |Vds 2 |+|Vds 1 |=|Vout−Vin|, in this equation the absolute value signs are given to take both cases Vin&lt;Vout and Vin&gt;Vout into consideration; wherein VR 5  denotes a voltage difference across two ends of the first resistor R 5 , whereas, VR 7  denotes a voltage difference across two ends of the second resistor R 7 ; wherein Vds 1  denotes the drain-source voltage of the first switch Q 1 , whereas, Vds 2  denotes the drain-source voltage of the second switch Q 2 .
 
     The second gate-source voltage Vgs 2  is equal to the sum of the voltage drops of two resistors Rfod and Rfod plus the bulk-source divided voltage of the second switch Q 2 . In the case when Vin&gt;Vout, the source is at the right side which is the side nearer to the output signal Vout; the bulk-source divided voltage is the voltage drop across the resistor R 5 , and the positive terminal is at the bulk. In the case when Vin&lt;Vout, the source is at the left side which is the side nearer to the input signal Vin; the bulk-source divided voltage is the voltage drop across the resistor R 4 , and the positive terminal is at the bulk. 
     The second gate-source voltage Vgs 2  of the second switch Q 2  is equal to a difference of the second gate voltage Vg 2  minus the input signal Vin, in the case when Vin&lt;Vout; or, the second gate-source voltage Vgs 2  of the second switch Q 2  is equal to a difference of the second gate voltage Vg 2  minus the voltage V 4  (wherein the voltage V 4  is the voltage at the node between the resistor R 5  and the resistor R 6 ), in the case when Vin&gt;Vout. 
     The first gate-source voltage Vgs 1  of the first switch Q 1  is equal to a difference of the first gate voltage Vg 1  minus the voltage V 4 , in the case when Vin&lt;Vout; or, the first gate-source voltage Vgs 1  of the first switch Q 1  is equal to a difference of the first gate voltage Vg 1  minus the output signal Vout, in the case when Vin&gt;Vout. 
     In a case when the resistances of the resistor R 4 , the resistor R 5 , the resistor R 6  and the resistor R 7  are equal to one another, the equation having absolute values as shown above will be obtained. 
     As shown in  FIG.  10   , the voltage-divider circuit  107  includes four voltage division resistors R 4 -R 7  connected in series between the input terminal T 1  and the output terminal T 2 , and the voltage divider circuit  107  further includes a switch Q 11 , a switch Q 12 , a switch Q 21 , and a switch Q 22 . The switch Q 11 , the switch Q 12 , the switch Q 21  and the switch Q 22  are configured to concurrently turn OFF a current path between the input signal Vin and the output signal Vout through four voltage division resistors R 4 -R 7  when the first switch Q 1  and the second switch Q 2  are controlled to be OFF. The switch Q 11  is coupled between the source and the bulk of the first switch Q 1 . The switch Q 12  is coupled between the drain and the bulk of the first switch Q 1 . The switch Q 21  is coupled between the source and the bulk of the second switch Q 2 . The switch Q 22  is coupled between a drain and the bulk of the second switch Q 2 . The switch Q 11  and the switch Q 12  are controlled by the first gate voltage Vg 1 . The switch Q 21  and the switch Q 22  are controlled by the second gate voltage Vg 2 . In a case when the first gate voltage Vg 1  and the second gate voltage Vg 2  control the first switch Q 1  and the second switch Q 2  to be OFF, the switch Q 11 , the switch Q 12 , the switch Q 21  and the switch Q 22  are configured to prevent the input signal Vin of the input terminal T 1  from being converted to output signal Vout of the output terminal T 2  via the four voltage division resistors R 4 -R 7 . 
     This embodiment shows a specific embodiment of gate-source voltage adjustment circuit  105 . The gate-source voltage adjustment circuit  105  includes: a super source follower  1051 , a first impedance circuit  1053  and a second impedance circuit  1055 . The super source follower  1051  is coupled between the sensor circuit  103  and the first impedance circuit  1053 . The super source follower  1051  generates two summation currents  2 Ib+Isen and  2 Ib+Isen according to a sensing current Isen related to the sensing signal Ssen and constant currents Ib and  2 Ib. The first impedance circuit  1053  is coupled between the gate of the first switch Q 1  and the source of the first switch Q 1 , wherein the first impedance circuit  1053  is configured to operably adaptively adjust the first gate-source voltage Vgs 1  according to the sensing signal Ssen. The second impedance circuit  1055  is coupled to the super source follower  1051 . The two summation currents  2 Ib+Isen and  2 Ib+Isen flow through the first impedance circuit  1053  and the second impedance circuit  1055 , respectively, so as to adaptively adjust the first gate-source voltage Vgs 1 . 
     In this embodiment, the super source follower  1051  is located at a top side of  FIG.  10   . In the super source follower  1051 , two transistors whose gates are coupled to each other correspond to the amplifier A 1  in the amplifier circuit shown in  FIG.  6    and  FIG.  7   . 
     Please refer to  FIG.  11   , which shows a specific embodiment of a sensor circuit  103 . As shown in  FIG.  11   , in this embodiment, switch devices M 1 , M 2  and M 3  together constitute a super source follower, which serves as an amplifier circuit  1031  having feedback control mechanism (referring to the amplifier circuit  631  in  FIG.  6   ). By a negative feedback path provided by the switch device M 3 , a source of the switch device M 1  (i.e., first input terminal N 1 ) and a source of the switch device M 2  (i.e., second input terminal N 2 ) are regulated at the same voltage. Resistor devices M 4  and M 5  for example are field oxide devices to serve as resistors (referring to the first resistor R 1  and the second resistor R 2  in  FIG.  6   ). In this embodiment, the channel resistance Ron indicates the conduction resistance of the switch unit. The amplifier circuit  1031  generates the sensing current Isen according to the voltage difference Vdif generated by an output current Iout flowing through the channel resistance Ron. 
     Switch devices M 6 , M 7  and M 8  together constitute another super source follower, which serve as an amplifier circuit  1032  having feedback control mechanism. by a negative feedback path provided by the switch device M 8 , a source of the switch device M 6  (i.e., third input terminal N 3 ) and a source of the switch device M 7  (i.e., fourth input terminal N 4 ) are regulated at the same voltage. Resistor devices M 9  and M 10  for example are field oxide devices to serve as resistors. The amplifier circuit  1032  serves as an amplifier circuit having feedback control mechanism in a case when the input signal Vin is smaller than the output signal Vout, so that regardless whether the input signal Vin is smaller than or not smaller than the output signal Vout, the sensor circuit  103  can operate. To be more specific, on one hand, when the input signal Vin is not smaller than the output signal Vout, the amplifier circuit  1031  generates the sensing current Isen according to the voltage difference Vdif generated by the output current Iout flowing through the channel resistance Ron. On the other hand, when the input signal Vin is smaller than the output signal Vout, the amplifier circuit  1032  generates the sensing current Isen according to the voltage difference Vdif generated by the output current Iout flowing through the channel resistance Ron. 
     In addition to including the amplifier circuit  1031  and  1032  and the resistor devices M 4 , M 5 , M 9  and M 10 , the sensor circuit  103  further includes: a current mirror circuit  1033  and a current mirror circuit  1034 . The current mirror circuits  1033  and  1034  for example mirror the sensing current Isen by 1-fold to generate the sensing signal Ssen which has the same level as the sensing current Isen. Certainly, the amplification ratio of the current mirror circuits  1033  and  1034  is not limited to 1:1. In other embodiments, it is also practicable and within the broadest scope of the present invention that the amplification ratio of the current mirror circuits  1033  and  1034  can be K which is not equal to one. 
     Assuming that the amplification ratio of the current mirror circuit  1033  to the current mirror circuit  1034  is 1, and the resistances of the resistor devices M 4 , M 5 , M 9  and M 10  are all equal to Ron, the sensing current Isen can be derived and represented by the following equation: 
                   Rfod   ×   2   ⁢   Ib     +     Ron   ⁢       ❘   &#34;\[LeftBracketingBar]&#34;     Iout     ❘   &#34;\[RightBracketingBar]&#34;           =       Rfod   ×   Isen     +     Rfod   ×   2   ⁢   Ib         ⁢   
     Isen   =       Ron   ×       ❘   &#34;\[LeftBracketingBar]&#34;     Iout     ❘   &#34;\[RightBracketingBar]&#34;         Rfod             
The final equation indicates that the sensing current Isen of the sensing signal Ssen is proportional to the voltage difference Vdif generated by an output current Iout flowing through the channel resistance Ron.
 
     Please refer to  FIG.  12 A  to  FIG.  12 E , which illustrate a Fast Fourier Transform (FFT) simulation result depicting the harmonic distortion of the output signal Vout of the analog switch circuit of the present invention. As shown in  FIG.  12 A  to  FIG.  12 E , according to the embodiments of the present invention, the second order or higher order FFT harmonic distortion is lower than −110 dB, which is better than the prior art. In  FIG.  12 A  to  FIG.  12 E , the horizontal axis indicates a frequency of the input signal Vin and a frequency of the output signal Vout. As shown in  FIG.  12 A  to  FIG.  12 E , according to the embodiments of the present invention, the second order or higher order FFT harmonic distortion at signal frequency of 1 KHz, 3 KHz, 5 KHz, 10 KHz and 0.1 KHz is lower than −110 dB. 
     The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the broadest scope of the present invention. An embodiment or a claim of the present invention does not need to achieve all the objectives or advantages of the present invention. The title and abstract are provided for assisting searches but not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, to perform an action “according to” a certain signal as described in the context of the present invention is not limited to performing an action strictly according to the signal itself, but can be performing an action according to a converted form or a scaled-up or down form of the signal, i.e., the signal can be processed by a voltage-to-current conversion, a current-to-voltage conversion, and/or a ratio conversion, etc. before an action is performed. It is not limited for each of the embodiments described hereinbefore to be used alone; under the spirit of the present invention, two or more of the embodiments described hereinbefore can be used in combination. For example, two or more of the embodiments can be used together, or, a part of one embodiment can be used to replace a corresponding part of another embodiment. For example, the gate-source voltage adjustment circuit  105  shown in the embodiment of  FIG.  10    can be correspondingly applied in the embodiments of  FIGS.  4 A,  5 ,  6 ,  7 ,  8  and  9   . In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.