please refer to fig2 . fig2 is a diagram illustrating a mos amplifier 200 according to a first embodiment of the present invention . as shown in fig2 , the mos amplifier 200 comprises n gain providers gp 1 ˜ gp n , and n gain tuners gt 1 ˜ gt n . each gain tuner further comprises a gain enabling module gem and a gain decreasing module gdm . for example , the gain tuner gt 1 comprises the gain enabling module gem 1 and the gain decreasing module gdm 1 . each gain enabling module gem is realized with a mos transistor . for example , the gain enabling module gem 1 is realized with the mos transistor q 11 having an aspect ratio m 11 . each gain decreasing module gdm is realized with three mos transistors . for example , the gain decreasing module gdm 1 is realized with the mos transistors q 12 , q 13 , and q 14 having aspect ratio m 12 , m 13 , and m 14 , respectively . each gain tuner corresponds to a gain provider . for example , the gain tuner gt 1 corresponds to the gain provider gp 1 . additionally , the sum of the aspect ratios of the gain decreasing module equals the aspect ratio of the corresponding gain enabling module . for example , in the gain tuner gt 1 , the sum of the aspect ratios ( m 12 + m 13 + m 14 ) for the gain decreasing module gdm 1 equals the aspect ratio ( m 11 ) for the gain enabling module gem 1 . however , the aspect ratios of the gain enabling module and the gain decreasing module can be set depending on design , which means the sum of the aspect ratios of one decreasing module does not have to be the same as the aspect ratio of the corresponding gain enabling module , and furthermore , the number of the mos transistors employed by each gain decreasing module can be set as desired and is not limited to be the same as others . the present embodiment is only illustrated as an exemplary embodiment for easier understanding . the gain provider is realized with a mos transistor . for example , the gain provider gp 1 is realized with the mos transistor q 1 having an aspect ratio m 1 . therefore , as shown in fig2 , the control end ( gate ) of the mos transistor q 1 is coupled to an input end of the mos amplifier 200 for receiving an input signal v in , the biasing end ( source ) of the mos transistor q 1 is coupled to a biasing source v ss , and the output end of the mos transistor q 1 is coupled to the corresponding gain tuner gt 1 . in the gain enabling module gem 1 , the control end ( gate ) of the mos transistor q 11 receives a controlling signal s 11 , the first end ( drain ) of the mos transistor q 11 is coupled to a first end of a load , and the second end ( source ) of the mos transistor q 11 is coupled to the output end of the mos transistor q 1 . in the gain decreasing module gdm 1 , the control end ( gate ) of the mos transistor q 12 receives a controlling signal s 12 , the first end ( drain ) of the mos transistor q 12 is coupled to a second end of the load , and the second end ( source ) of the mos transistor q 12 is coupled to the output end of the mos transistor q 1 ; the control end ( gate ) of the mos transistor q 13 receives a controlling signal s 13 , the first end ( drain ) of the mos transistor q 13 is coupled to the second end of the load , and the second end ( source ) of the mos transistor q 13 is coupled to the output end of the mos transistor q 1 ; the control end ( gate ) of the mos transistor q 14 receives a controlling signal s 14 , the first end ( drain ) of the mos transistor q 14 is coupled to a second end of the load , and the second end ( source ) of the mos transistor q 14 is coupled to the output end of the mos transistor q 1 . the rest gain providers , gain tuners , gain enabling modules , and gain decreasing modules have similar structure and connections as described above and therefore the related description is omitted . the first end of the load is set as the output end of the mos amplifier 200 for outputting an output signal v out , and the second end of the load is coupled to a biasing source v dd . of course , the output signal v out is amplified according to the programmable gain of the mos amplifier 200 from the input signal v in . additionally , the mos transistors of the gain providers are utilized as amplifiers , and the mos transistors of the gain enabling modules and the gain decreasing modules are only utilized as switches merely . each gain provider provides a gain of different value as desired . however , same value of the gains of some gain providers can be designed as well . under the condition that a mos transistor is appropriately biased , when the mos transistor is utilized as a common source amplifier , the gain of the amplifier is proportional to the aspect ratio m of the mos transistor . therefore , the mos transistors q 1 ˜ q n of the gain providers gp 1 ˜ gp n are designed with different aspect ratios m 1 ˜ m n and the corresponding gains g 1 ˜ g n are obtained . assuming the gain g 1 is g ini , and thus the gain g 2 is ( m 2 / m 1 ) g ini , the gain g 3 is ( m 3 / m 1 ) g ini , and so on . when a gain enabling module is turned on ( by the corresponding controlling signal ), the gain of the corresponding gain provider is added to the total gain of the mos amplifier 200 . for example , when the gain enabling module gem 1 is enabled , which means that the mos transistor q 11 is turned on by the controlling signal s 11 , the total gain of the mos amplifier 200 is added with the gain g 1 , which is g ini ; when the gain enabling modules gem 1 and gem 2 is enabled , the total gain of the mos amplifier 200 is added with the gains g 1 and g 2 , which equals to [( m 2 + m 1 )/ m 1 ] g ini ; when all the gain enabling modules gem 1 ˜ gem n are enabled , the total gain of the mos amplifier 200 is [( m n + m ( n − 1 ) + . . . + m 3 + m 2 + m 1 )/ m 1 ] g ini . when a gain decreasing module is turned on while the corresponding gain enabling module is turned on , the gain of the corresponding gain provider is decreased , and the decreasing degree can be set by controlling the gain decreasing module . in other words , when a gain enabling module is turned on , the current sunk by the mos transistor of the corresponding gain provider directly flows from the load to the output end of the mos transistor , and the gain of the mos transistor of the corresponding gain provider is completely added to the total gain of the mos amplifier 200 ; when a gain decreasing module is turned on while the corresponding gain enabling module is turned on , the current sunk by the mos transistor of the corresponding gain provider partially flows from the load and partially flows from the biasing source v dd through the mos transistors turned on in the gain decreasing module , the gain of the mos transistor of the corresponding gain provider is partially added to the total gain of the mos amplifier 200 . for example , when the mos transistor q 11 is turned on , the gain of the mos amplifier 200 is g 1 ; when the mos transistor q 11 and q 12 are turned on , the gain of the mos amplifier 200 is [ m 11 /( m 11 + m 12 )]× g 1 ; when the mos transistor q 11 , q 12 , and q 13 are turned on , the gain of the mos amplifier 200 is [ m 11 /( m 11 + m 12 + m 13 )]× g 1 ; when the mos transistor q 11 , q 12 , q 13 , and q 14 are turned on , the gain of the mos amplifier 200 is [ m 11 /( m 11 + m 12 + m 13 + m 14 )]× g 1 . therefore , the total gain of the mos amplifier 200 can be adjusted by controlling the on / off states of the mos transistors of the gain enabling modules and the gain decreasing modules of the mos amplifier 200 . please refer to fig3 . fig3 is a diagram illustrating a realization of the linear - to - db mos amplifier 300 according to the first embodiment of the present invention . as shown in fig3 , the basic structure of the mos amplifier 300 is similar to the mos amplifier 200 and the related description is omitted . the aspect ratios of the mos transistors q 1 ˜ q n are designed to be m a , 2m a , 4m a , 8m a , . . . , 2 ( n − 1 ) m a , respectively , and the gain of the mos transistor q 1 is assumed to be g ini . in each gain tuner , the aspect ratio of the mos transistor of the gain enabling module is designed to be 8 m b , the aspect ratios of the three mos transistors of the gain decreasing module are designed to be m b , 2m b , and 5m b , respectively . when the gain enabling module gem 1 is turned on , the gain provided is g ini ; when the gain enabling module gem 2 is turned on , the gain provided is 2g ini ; when the gain enabling module gem 3 is turned on , the gain provided is 4g ini ; . . . ; when the gain enabling module gem n is turned on , the gain provided is 2 ( n − 1 ) g ini . in this way , the gain generated from one gain enabling module is double than the gain generated from the previous gain enabling module , and thus the gain difference between each adjacent gain enabling module equals to be 6 db ( 20 log 2 = 20 ×( 0 . 3010 )≈ 6 ). consequently , the gain of the mos amplifier 300 increases with the interval of 6 db by only controlling the on / off states of the mos transistors q 11 ˜ q n1 of the gain enabling modules gem 1 ˜ gem n . the design of the aspect ratios of the mos transistor of the gain enabling module and the mos transistors of the gain decreasing module in one gain tuner in the mos amplifier 300 allows the gain of the mos amplifier 300 to adjusted with 1 db per gain step in the 6 db gap . for example , when the transistors q 11 ˜ q 14 are turned on , the gain of the mos amplifier 300 is ( 0 . 5 ) g ini (− 6 db × g ini ); when the transistors q 11 , q 12 , and q 14 are turned on , the gain of the mos amplifier 300 is ( 0 . 57 ) g ini (− 5 db × g ini ); when the transistors q 11 and q 14 are turned on , the gain of the mos amplifier 300 is ( 0 . 61 ) g ini (− 4 db × g ini ); when the transistors q 11 , q 12 and q 13 are turned on , the gain of the mos amplifier 300 is ( 0 . 72 ) g ini (− 3 db × g ini ); when the transistors q 11 and q 13 are turned on , the gain of the mos amplifier 300 is ( 0 . 8 ) g ini (− 2 db × g ini ); when the transistors q 11 and q 12 are turned on , the gain of the mos amplifier 300 is ( 0 . 88 ) g ini (− 1 db × g ini ); when only the transistor q 11 is turned on , the gain of the mos amplifier 300 is ( 1 ) g ini ( 0 db × g ini ). therefore , the gain of the linear - to - db mos amplifier 300 can be adjusted with 1 db per gain step and adjusted to any value as desired by turning on the mos transistors of the gain enabling modules and the gain decreasing modules of the gain tuners respectively . please refer to fig4 . fig4 is a diagram illustrating a mos amplifier 400 according to a second embodiment of the present invention . as shown in fig4 , the mos amplifier 400 comprises two gain providers gp 1 and gp 2 , two gain tuners gt 1 and gt 2 , n gain providers gp t1 ˜ gp tn , and n gain tuners gt ti ˜ gt tn . the functional description for the gain providers , the gain tuners in fig4 is similar to those components in fig2 and is omitted . however , structurally , mos amplifier 400 is slightly changed according to the mos amplifier 200 . as shown in fig4 , only two gain providers gp 1 and gp 2 and their corresponding gain tuners gt 1 and gt 2 are utilized for low gain adjustment range , but n gain provider gp t1 ˜ gp tn and their corresponding gain tuners gt t1 ˜ gt tn are utilized for high gain adjustment range . more particularly , the aspect ratios of the mos transistors q 1 and q 2 respectively of the gain providers gp 1 and gp 2 can be designed to be m 1 and 2m 1 and the aspect ratio of the mos transistor q t1 of the gain provider gp t1 can be designed to be 4m 1 , the aspect ratio of the mos transistor q t2 of the gain provider gp t2 can be designed to be 8m 1 , and in such manner , the aspect ratio of the mos transistor q tn can be designed to be 2 ( n + 1 ) m 1 . therefore , the gain providers gp 1 and gp 2 provide g ini , the gain provider gp t1 provides 4g ini , the gain provider gp t2 provides 8g ini , . . . , and the gain provider gp tn provide 2 ( n + 1 ) g ini . additionally , the number of the gain providers gp t1 ˜ gp tn depends on the total gain adjustment range as desired . that is , if the gain adjustment range goes higher , the number n goes higher as well , and vice versa . it is noticeable that as for each of the gain tuners gt t1 ˜ gt tn ( for the high gain adjustment ), the corresponding gain enabling module and the corresponding gain decreasing module employ only one mos transistor respectively and both of the employed mos transistors have the same aspect ratio as that of the mos transistor of the corresponding gain provider . therefore , when the mos transistor of the gain enabling module for the high gain adjustment is turned on and the mos transistor of the gain decreasing module for the high gain adjustment is turned off , the gain of the corresponding gain provider for the high gain adjustment is added to the mos amplifier 400 , which means the branch for that one particular high gain adjustment is turned on . on the other hand , when the mos transistor of the gain enabling module for the high gain adjustment is turned off and the mos transistor of the gain decreasing module for the high gain adjustment is turned on , the gain of the corresponding gain provider for the high gain adjustment is not added to the mos amplifier 400 , which means the branch for that one particular high gain adjustment is turned off . the above - mentioned mechanism is designed for keeping the input impedance ( from where v in is inputted ) the same without the relations of the on / off states of mos transistors of the gain enabling modules and the gain decreasing modules for the high gain adjustment . however , if the input impedance variation is not important in the design , the gain decreasing modules for the high gain adjustment can be removed from the design . the design of the mos amplifier 400 saves mos transistors for the gain tuners because for the high gain adjustment range , only two mos transistors are used for each gain tuners , and still provides the same advantage as the mos amplifier 200 . please refer to fig5 . fig5 is a diagram illustrating a mos amplifier 500 according to a third embodiment of the present invention . the mos amplifier 500 is disclosed as a design example for 36 db gain adjustment range . as shown in fig5 , the mos amplifier 500 comprises the gain providers gp 5 , gp 6 , gp 7 , gp 8 , gp 9 , and gp 10 , and the gain tuners gt 5 , gt 6 , gt 7 , gt 8 , gt 9 , and gt 10 . it is noticeable that each of the gain tuners gt 5 , gt 8 , gt 9 , and gt 10 only comprises the gain enabling module , and the corresponding gain decreasing module is saved . it is noticeable that the embodiment as shown in fig5 is designed as the above - mentioned mechanism that when the input impedance variation is not important in the design , the gain decreasing modules for the high gain adjustment can be removed . the aspect ratios of the mos transistors q 5 , q 6 , q 7 , q 8 , q 9 , and q 10 of the gain providers gt 5 , gt 6 , gt 7 , gt 8 , gt 9 , and gt 10 are designed to be m 5 , m 5 , 2m 5 , 4m 5 , 8m 5 , and 16m 5 , respectively . in the gain tuner gt 6 , the aspect ratio of the mos transistor q 61 of the gain enabling module gem 6 is designed to be 8m 6 , the aspect ratios of the mos transistors q 62 , q 63 , q 64 of the gain decreasing module gdm 6 are respectively designed to be m 6 , 2m 6 , 5m 6 . in the gain tuner gt 7 , the aspect ratio of the mos transistor q 71 of the gain enabling module gem 7 is designed to be 16m 7 , the aspect ratios of the mos transistors q 72 , q 73 , q 74 of the gain decreasing module gdm 7 are respectively designed to be 2m 7 , 4m 7 , 7m 7 . the mos transistors q 51 , q 61 , q 62 , q 63 , q 64 , q 71 , q 72 , q 73 , q 74 , q 81 , q 91 , and q 101 are turned on / off respectively according to the controlling signals s 0 , s 1 , s 2 , s 3 , s 4 , s 5 , s 6 , s 7 , s 8 , s 9 , s 10 , and s 11 . please refer to fig6 and fig7 . fig6 is a diagram illustrating the magnitude , and the magnitude in db , corresponding to the gain level of the mos amplifier 500 . fig7 is a diagram illustrating the controlling signals of each gain level of the mos amplifier 500 . since the gain range of the mos amplifier 500 is 36 db and the 36 db gain range is divided into 36 gain levels , increasing one gain level is adding 1 db of the gain of the mos amplifier 500 . as shown in fig6 , each gain level corresponds to a gain magnitude and the gain magnitude in db , and the gain level can be the combination of other gain levels . for example , the gain level “ 15 ” corresponds to 2 . 488889 and 7 . 92011 db (≈ 8 db ), and the gain level “ 15 ” is generated by combining the gain level “ 11 ” and the gain level “ 6 ”. the gain level “ 37 ” corresponds to 31 . 6 and 29 . 99374 db (≈ 30 db ), and the gain level “ 37 ” is generated by combining the gain levels “ 0 ”, “ 3 ”, “ 13 ”, “ 19 ”, “ 25 ”, and “ 31 ”. as shown in fig7 , to achieve each gain level , the controlling signals s 0 ˜ s 11 are accordingly set . for example , to achieve the gain level “ 15 ”, the controlling signals s 0 ˜ s 11 , are set as [ 011001010000 ]. to achieve the gain level “ 37 ”, the controlling signals s 0 ˜ s 11 are set as [ 110011000111 ]. please refer to fig8 . fig8 is a diagram illustrating a decoder 800 as an auxiliary component for the mos amplifier 500 . for controlling convenience , the decoder 800 is disposed between the gain level data d 0 ˜ d 5 and the controlling signals s 0 ˜ s 11 . for example , to achieve the gain level “ 15 ”, the gain level data d 0 ˜ d 5 are [ 001111 ], and the controlling signals s 0 ˜ s 11 are [ 011001010000 ] decoded by the decoder 800 according to the gain level data [ 0011111 ]. to achieve the gain level “ 37 ”, the gain level data d 0 ˜ d 5 are [ 100101 ], and the controlling signals s 0 ˜ s 11 are [ 110011000111 ] decoded by the decoder 800 according to the gain level data [ 100101 ]. to sum up , as those disclosed above , the programmable gain mos amplifier of the present invention provides adjusting gain in db domain linearly by linearly adjusting the controlling data , and for the high gain adjustment , the programmable gain mos amplifier of the present invention provides an easier mechanism to save required components , which increases great convenience . those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention .