Patent Publication Number: US-2023163738-A1

Title: Gain-adjustable Amplifier Circuit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/294,857, filed on Dec. 30, 2021. Further, this application is a continuation-in-part of U.S. application Ser. No. 17/499,831, filed on Oct. 12, 2021. The contents of these applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention is related to an amplifier circuit, and more particularly to a gain-adjustable amplifier circuit. 
     BACKGROUND 
     With increasing popularity of the Internet and mobile devices, users demand more for communication quality. However, the gain range of the traditional single-stage amplifier is limited. The traditional structure cannot meet the current practical application and design requirements. Therefore, it is an important issue to provide a gain-adjustable amplifier circuit. 
     SUMMARY 
     An embodiment provides an amplifier circuit including an amplifier for generating an amplified input signal according to an input signal, and an attenuator circuit coupled to the amplifier. The attenuator circuit includes an input terminal for receiving the input signal or the amplified input signal, an output terminal, a reference voltage terminal, a zeroth resistor-switch circuit, a first resistor-switch circuit, and a second resistor-switch circuit. The zeroth resistor-switch circuit includes a first terminal coupled to one of the input terminal and the output terminal, a second terminal coupled to another one of the input terminal and the output terminal, a zeroth switch coupled to the first terminal of the zeroth resistor-switch circuit and the second terminal of the zeroth resistor-switch circuit, a zeroth resistor coupled between the first terminal of the zeroth resistor-switch circuit and the second terminal of the zeroth resistor-switch circuit, a first resistor coupled between the zeroth resistor and the second terminal of the zeroth resistor-switch circuit, and a first switch. The first switch includes a first terminal coupled to the zeroth resistor and the first resistor, a second terminal coupled to the second terminal of the zeroth resistor-switch circuit, and a control terminal. The first resistor-switch circuit includes a first terminal coupled to one of the input terminal and the reference voltage terminal, a second terminal coupled to another one of the input terminal and the reference voltage terminal, a zeroth resistor coupled between the first terminal of the first resistor-switch circuit and the second terminal of the first resistor-switch circuit, a first resistor coupled between the zeroth resistor and the second terminal of the first resistor-switch circuit, and a first switch. The first switch includes a first terminal coupled to the zeroth resistor and the first resistor, a second terminal coupled to the second terminal of the first resistor-switch circuit, and a control terminal. The second resistor-switch circuit includes a first terminal coupled to one of the output terminal and the reference voltage terminal, a second terminal coupled to another one of the output terminal and the reference voltage terminal, a zeroth resistor coupled between the first terminal of the second resistor-switch circuit and the second terminal of the second resistor-switch circuit, a first resistor coupled between the zeroth resistor and the second terminal of the second resistor-switch circuit, and a first switch. The first switch includes a first terminal coupled to the zeroth resistor and the first resistor, a second terminal coupled to the second terminal of the second resistor-switch circuit, and a control terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of a gain-adjustable amplifier circuit of a first embodiment of the present invention. 
         FIG.  2    is a block diagram of a gain-adjustable amplifier circuit of a second embodiment of the present invention. 
         FIG.  3    is a block diagram of a gain-adjustable amplifier circuit of a third embodiment of the present invention. 
         FIG.  4    is a circuit diagram of the gain-adjustable amplifier circuit  1  of  FIG.  1   . 
         FIG.  5    is a circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit of  FIG.  1 ,  2  or  3   . 
         FIG.  6    is another circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit of  FIG.  1 ,  2  or  3   . 
         FIG.  7    is another circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit of  FIG.  1 ,  2  or  3   . 
         FIG.  8    is another circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit of  FIG.  1 ,  2  or  3   . 
         FIG.  9    is another circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit of  FIG.  1 ,  2  or  3   . 
     
    
    
     DETAILED DESCRIPTION 
     Below, exemplary embodiments will be described in detail with reference to accompanying drawings so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout. 
     Please refer to  FIG.  1   .  FIG.  1    is a block diagram of a gain-adjustable amplifier circuit of an embodiment of the present invention. The gain-adjustable amplifier circuit  1  includes an input terminal  11 , an output terminal  12 , amplifier  20  and an attenuator circuit  40 . 
     The input terminal  11  is for receiving an input signal Sin, and the output terminal  12  is for sending an output signal Sout. The output signal Sout can be sent to the backend circuit for further process. The input signal Sin and the output signal Sout are, for example, but not limited to, radio frequency signals. 
     An input terminal of the amplifier  20  can receive the input signal Sin through the input terminal  11  and can amplify the input signal Sin according to its gain, so as to output the amplified input signal Sin′ at an output terminal of the amplifier  20 . The amplifier  20  includes a low-noise amplifier or a power amplifier. 
     An input terminal of the attenuator circuit  40  may be coupled to the output terminal of the amplifier  20 . The output terminal may be coupled to the output terminal  12 . The attenuator circuit  40  can provide a plurality of attenuations to the amplified input signal Sin′, and to generate the attenuated signal Sat 1  accordingly. For example, the attenuator circuit  40  can select one of a plurality of attenuations according to the gain required by the amplifier circuit  1  to attenuate the amplitude of the amplified input signal Sin′ to a specific value, thereby generating the attenuated signal Sat 1 . The output signal Sout can be generated by the attenuated signal Sat 1 . In this way, the gain of the amplifier circuit  1  can be related to the attenuation of the amplified input signal Sin′. That is, the gain required by the amplifier circuit  1  can be realized based on the attenuation of the amplified input signal Sin′. The gain of the amplifier circuit  1  may be 0 dB or greater than 0 dB. It should be noted that since the attenuator circuit  40  can provide a plurality of attenuations to the amplified input signal Sin′, the gain of the amplifier circuit  1  is not limited by the gain range of the amplifier  20 . Through the attenuator circuit  40  the gain range of the amplifier circuit  1  can be expended and the gain of the amplifier circuit  1  can be adjusted. Not only that, since the attenuator circuit  40  is coupled to the output of the amplifier  20 , the attenuator circuit  40  would not easily generate loading effect at the input of the amplifier  20 . Thus it is hard to affect the performance of the amplifier circuit  1 , so the linearity and noise figure of the amplifier circuit  1  are maintained. 
     Please refer to  FIG.  2   .  FIG.  2    is a block diagram of a gain-adjustable amplifier circuit of a second embodiment of the present invention. The major difference between the embodiment of  FIG.  2    and the embodiment of  FIG.  1    is that the gain-adjustable amplifier circuit  2  of  FIG.  2    includes an amplifier  20  and an attenuator circuit  50 . 
     The input terminal of the attenuator circuit  50  may be coupled to the input terminal  11 , and the output terminal may be coupled to the output terminal  12 . The attenuator circuit  50  can be configured to provide a plurality of attenuations to the input signal Sin, and generate the attenuated signal Sat 2  accordingly. For example, the attenuator circuit  50  can select one of the attenuations according to the gain required by the amplifier circuit  2  to attenuate the amplitude of the input signal Sin to a specific value, thereby generating the attenuated signal Sat 2 . 
     Furthermore, the amplifier circuit  2  can operate in either amplifying mode or bypass mode. When the intensity of the input signal Sin is not strong enough and needed to be amplified by the amplifier  20 , the amplifier  20  can be enabled. The amplifier circuit  2  can operate in an amplifying mode with the first gain. Further, the input signal Sin can be inputted from the input terminal  11  to the amplifier  20 . The amplifier  20  can amplify the input signal Sin according to its gain and output the amplified input signal Sin′ at its output terminal. The output signal Sout can be generated by the amplified input signal Sin′. Therefore, the first gain is related to the gain of the amplifier  20 . When the intensity of the input signal Sin is high enough that it does not need to be amplified by the amplifier  20 , the amplifier  20  can be disabled. The amplifier circuit  2  can operate in the bypass mode with the second gain (equal or less than 0 dB). It should be noted that the disabled amplifier  20  can reduce unnecessary current consumption. Further, the input signal Sin is input to the attenuator circuit  50  from the input terminal  11  and the attenuator circuit  50  can attenuate the input signal Sin according to the selected attenuation to generate the attenuated signal Sat 2 . The output signal Sout can be generated by the attenuated signal Sat 2 . In this way, the second gain can be related to the attenuation of the input signal Sin. That is, the second gain required by the amplifier circuit  2  can be achieved based on the attenuation of the input signal Sin. The second gain may be less than the first gain. The first gain may be greater than 0 dB, and the second gain may be equal or less than 0 dB. It should be noted that since the attenuator circuit  50  can provide a plurality of attenuations to the input signal Sin, the gain range of the amplifier circuit  2  can be expanded and the gain of the amplifier circuit  2  can be adjusted through the attenuator circuit  50 . 
     Please refer to  FIG.  3   .  FIG.  3    is a block diagram of a gain-adjustable amplifier circuit of a third embodiment of the present invention. The major difference between the embodiment of  FIG.  3    and the embodiment of  FIG.  2    is that the gain-adjustable amplifier circuit  3  of  FIG.  3    may further include an attenuator circuit  40 . The input terminal of the attenuator circuit  40  may be coupled to the output terminal of the amplifier  20 , and the output terminal of the amplifier  20  may be coupled to the output terminal  12 . The attenuator circuits  40  and  50  can respectively provide a plurality of attenuations to the amplified input signal Sin′ and respectively generate attenuated signals Sat 1  and Sat 2  accordingly. 
     Comparing to the amplifier circuit  2  of the second embodiment, when the amplifier circuit  3  operates in amplifying mode, the attenuator circuit  40  can attenuate the amplified input signal Sin′ according to the selected attenuation to generate the attenuator signal Sat 1 . The output signal Sout can be generated by the attenuator signal Sat 1 . In this way, the first gain can be related to the attenuation of the amplified input signal Sin′. That is, in this embodiment, the first gain required by the amplifier circuit  3  can be realized based on the attenuation of the amplified input signal Sin′, and the second gain required by the amplifier circuit  3  can be realized based on the attenuation of the input signal Sin. Since the amplified input signal Sin′ is generated after the input signal Sin passing through the amplifier  20 , the first gain may be greater than the second gain. The first gain may be equal or greater than 0 dB, and the second gain may be equal or less than 0 dB. 
     Please refer to  FIG.  4   .  FIG.  4    is a circuit diagram of the gain-adjustable amplifier circuit  1  of  FIG.  1   . The amplifier  201  and the attenuator circuit  401  of  FIG.  4    may correspond respectively to the amplifier  20  and the attenuator circuit  40  of  FIG.  1   . 
     As shown in  FIG.  4   , amplifier  201  can include transistors M 1  and M 2 . The first terminal of the transistor M 1  is coupled to the output terminal of the amplifier  201 , and the control terminal is for receiving the operation signal VGG. The operation signal VGG is for enabling or disabling the amplifier  201 . The first terminal of the transistor M 2  is coupled to the second terminal of the transistor M 1 . The second terminal is coupled to the reference potential terminal  13 , and the control terminal is coupled to the input terminal of the amplifier  201  and can further receive the bias signal Vbias. The bias signal Vbias can bias the transistor M 2 . The reference voltage terminal REF may be, but is not limited to, a ground terminal in the system. 
     The attenuator circuit  401  can include a plurality of attenuator units ATU 0 -ATUn, each attenuator unit ATU 0 -ATUn is coupled between the input terminal and the output terminal of the attenuator circuit  401 , and each attenuator unit ATU 0 -ATUn can respectively provide one of the plurality of attenuations. For example, the attenuator unit ATU 0  can provide attenuation of 0 dB; the attenuator unit ATU 1  can provide attenuation of 5 dB; the attenuator unit ATUi can provide attenuation of 10 dB and the attenuator unit ATUn can provide attenuation of 20 dB. In other words, attenuator units ATU 0 -ATUn can provide different attenuations. It should be noted that when one of the attenuator units (e.g., the attenuator unit ATU 0 ) is enabled, the rest of the attenuator units (e.g., the attenuator units ATU 1 -ATUn) are disabled. 
     One of the attenuator units (e.g., the attenuator unit ATU 0 ) can include a switch SW 40 , and a control terminal of the switch SW 40  can receive the control signal CS 40 . The control signal CS 40  can turn on or off the switch SW 40 , thereby enabling or disabling the attenuator unit ATU 0  (e.g., turning on the switch SW 40  can enable the attenuator unit ATU 0 , thereby providing attenuation of 0 dB). 
     The remaining attenuator units (e.g., the attenuator units ATU 1 -ATUn) of the plurality of attenuator units ATU 0 -ATUn may each include a π-type attenuator. Further, the π-type attenuator in the attenuator unit ATU 1  can include switches SW 411 , SW 421  and SW 431 . The control terminals of the switches SW 411 , SW 421  and SW 431  can receive the control signal CS 41 . The control signal CS 41  can turn on or off the switches SW 411 , SW 421  and SW 431 , thereby enabling or disabling the attenuator unit ATU 1 . For example, turning on the switches SW 411 , SW 421  and SW 431  can enable the attenuator unit ATU 1  to provide attenuation of 5 dB. Then-type attenuator in the attenuator unit ATUi can include switches SW 41   i , SW 42   i  and SW 43   i . The control terminals of the switches SW 41   i , SW 42   i  and SW 43   i  can receive the control signal CS 4   i . The control signal CS 4   i  can turn on or off the switches SW 41   i , SW 42   i  and SW 43   i , thereby enabling or disabling the attenuator unit ATUi. The π-type attenuator in the attenuator unit ATUn can include switches SW 41   n , SW 42   n  and SW 43   n . The control terminals of switches SW 41   n , SW 42   n  and SW 43   n  can receive the control signal CS 4   n . The control signal CS 4   n  can turn on or off the switches SW 41   n , SW 42   n  and SW 43   n , thereby enabling or disabling the attenuator unit ATUn, and so on. It should be noted that “i” and “n” are positive integers. The control signals CS 40 -CS 4   n  can be provided by an internal circuit of the attenuator circuit  401  or an external circuit outside of the attenuator circuit  401 . It should be noted that although the number of attenuator units shown in  FIG.  4    is more than two, the actual number of attenuator units may be determined according to the actual application or design requirements. Hence, the invention is not limited to the number of attenuator units shown in  FIG.  4   . 
     In addition, the first terminal of the switch SW 40  is coupled to the input terminal of the attenuator circuit  401 , and the second terminal is coupled to the output terminal of the attenuator circuit  401 . The first terminal of each switch SW 411 -SW 41   n  is coupled to the input terminal of the attenuator circuit  401 , and the second terminal is coupled to reference voltage terminal REF. The first terminal of each switch SW 421 -SW 42   n  is coupled to the output terminal of the attenuator circuit  401 , and the second terminal of each switch SW 421 -SW 42   n  is coupled to the reference voltage terminal REF. The first terminal of each switch SW 431 -SW 43   n  is coupled to the input terminal of the attenuator circuit  401 , and the second terminal of each switch SW 431 -SW 43   n  is coupled to the output terminal of the attenuator circuit  401 . In other embodiments, the attenuator circuit  40  shown in  FIG.  3    may also include the attenuator circuit  401  shown in  FIG.  4   . 
     The attenuation corresponding to each attenuator unit ATU 1 -ATUn may be related to its resistance. For example, the on-resistance of the switches SW 411 , SW 421  and SW 431  can be chosen based on the attenuation corresponding to the attenuator unit ATU 1 . The on-resistance of the switches SW 41   i , SW 42   i  and SW 43   i  can be chosen based on the attenuation corresponding to the attenuator unit ATUi. The on-resistance of the switches SW 41   n , SW 42   n  and SW 43   n  can be chosen based on the attenuation corresponding to the attenuator unit ATUn, and so on. That is, the switches SW 411 -SW 43   n  not only can switch the corresponding attenuator units ATU 1 -ATUn, but also can provide the corresponding attenuations. In other embodiments, the on-resistance of the switches SW 411  and SW 421  can be substantially the same; the on-resistance of the switches SW 41   i  and SW 42   i  can be substantially the same; the on-resistance of the switches SW 41   n  and SW 42   n  can be substantially the same, and so on. In another example, the attenuator units ATU 1 , ATUi and ATUn can provide 5 dB, 10 dB and 20 dB of attenuations respectively, and the on-resistance of the switch SW 431  can be less than the on-resistance of the switch SW 43   i ; the on-resistance of the switch SW 43   i  may be less than the on-resistance of the switch SW 43   n . In this embodiment, by properly choosing the channel width-to-length ratio of the switches SW 411 -SW 43   n , the switches SW 411 -SW 43   n  can have appropriate on-resistance. In this way, when one of the attenuator units ATU 1 -ATUn is enabled, the appropriate corresponding attenuation can be provided. 
     The amplifier circuit  1  may further include a matching circuit  60 , and capacitors AC 1 , AC 2  and AC 3 . The matching circuit  60  is coupled between the output terminal of the amplifier  201  and the input terminal of the attenuator circuit  401 , and is for providing an output impedance matching to the amplifier  201 . It can include resistors, inductors, capacitors or any combination of the foregoing three elements. Since the attenuator circuit  401  is substantially symmetrical, the difference between the impedance at the input terminal and the impedance at the output terminal of the attenuator circuit  401  can be within a predetermined range. Thus, the attenuator circuit  401  would not significantly impact the output impedance matching of the amplifier  201 . The capacitor AC 1  is coupled between the matching circuit  60  and the attenuator circuit  401 . The capacitor AC 2  is coupled between the attenuator circuit  401  and the output terminal  12 . The capacitor AC 3  is coupled between the attenuator circuit  401  and the reference voltage terminal REF. The capacitor AC 1  can block the DC bias voltage from the amplifier  201 , and the capacitors AC 2  and AC 3  can block the DC bias voltage from the attenuator circuit  401 . In other embodiments, the amplifier circuit  1  may selectively include capacitor AC 1 , AC 2  or AC 3  according to practical applications or design requirements. 
     Please refer to  FIG.  5   .  FIG.  5    is a circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit  1 ,  2  or  5  of the embodiment of the present invention. The attenuator circuit  403  of  FIG.  5    may correspond to the attenuator circuit  40  of  FIG.  1    or  FIG.  3   , or the attenuator circuit  50  of  FIG.  2    or  FIG.  3   . 
     The attenuator circuit  403  includes an input terminal IN for receiving the input signal or the amplified input signal, an output terminal OUT, a reference voltage terminal REF, resistor-switch circuits RS 0 , RS 1 , and RS 2 . The resistor-switch circuit RS 0  includes a first terminal coupled to one of the input terminal IN and the output terminal OUT, a second terminal coupled to the other one of the input terminal IN and the output terminal OUT, switches SW 60  and SW 631 , and resistors R 631  and RA 631 . The resistor-switch circuit RS 1  includes a first terminal coupled to one of the input terminal IN and the reference voltage terminal REF, a second terminal coupled to the other one of the input terminal IN and the reference voltage terminal REF, resistors R 611  and RA 611 , and a switch SW 611 . The resistor-switch circuit RS 2  includes a first terminal coupled to one of the output terminal OUT and the reference voltage terminal REF, a second terminal coupled to the other one of the output terminal OUT and the reference voltage terminal REF, resistors R 621  and RA 621  and a switch SW 621 . 
     The first terminal of the switch SW 60  is coupled to the input terminal IN of the attenuator circuit  403 . The second terminal of the switch SW 60  is coupled to the output terminal OUT of the attenuator circuit  403 . The control terminal of the switch SW 60  is for receiving the control signal CS 60 . The control signal CS 60  can turn on or turnoff the switch SW 60 . The first terminal of the resistor R 611  is coupled to the input terminal IN of the attenuator circuit  403 . The second terminal of the resistor R 611  is coupled to the first terminal of the switch SW 611 . The second terminal of the switch SW 611  is coupled to the reference voltage terminal REF. The control terminal of the switch SW 611  is for receiving the control signal CS 61 . The first terminal of the resistor RA 611  is coupled to the second terminal of the resistor R 611  and the first terminal of the switch SW 611 . The second terminal of the resistor RA 611  is coupled to the second terminal of the switch SW 611 . The first terminal of the resistor R 621  is coupled to the output terminal OUT of the attenuator circuit  403 . The second terminal of the resistor R 621  is coupled to the first terminal of the switch SW 621 . The second terminal of the switch SW 621  is coupled to the reference voltage terminal REF, and the control terminal of the switch SW 621  is for receiving the control signal CS 61 . The first terminal of the resistor RA 621  is coupled to the second terminal of the resistor R 621  and the first terminal of the switch SW 621 . The second terminal of the switch SW 621  is coupled to the second terminal of the switch SW 621 . The first terminal of the resistor R 631  is coupled to the input terminal IN of the attenuator circuit  403 , and the second terminal of the resistor R 631  is coupled to the first terminal of the switch SW 631 . The second terminal of the switch SW 631  is coupled to the output terminal OUT of the attenuator circuit  403 . The control terminal of the switch SW 631  is for receiving the control signal CS 61 . The first terminal of the resistor RA 631  is coupled to the second terminal of the resistor R 631  and the first terminal of the switch SW 631 . The second terminal of the resistor RA 631  is coupled to the second terminal of the switch SW 631 . The control signal CS 61  can turn on and turn off the switches SW 611 -SW 631 . The control signals CS 60  and CS 61  can be provided by an internal circuit in the attenuator circuit  403  or an external circuit outside of the attenuator circuit  403 . 
     For example, by turning on the switch SW 60  and turning off the switches SW 611 -SW 631 , the attenuator circuit  403  can provide the first attenuation. Alternatively, turning off the switch SW 60  and turning on the switches SW 611 -SW 631  can form the first π-type attenuator. The first π-type attenuator equivalently includes resistors R 611 -R 631  and switches SW 611 -SW 631 , and the attenuator circuit  403  can provide the second attenuation. Alternatively, the switches SW 60 , SW 611 -SW 631  can be turned off to form the second π-type attenuator. The second π-type attenuator equivalently includes resistors R 611 -R 631  and RA 611 -RA 631 , and the attenuator circuit  403  can provide the third attenuation. The third attenuation may be greater than the second attenuation, and the second attenuation may be greater than the first attenuation. That is, resistors R 611 , R 621  and R 631  are shared in the first and second π-type attenuators, so occupied areas of the attenuator circuit are reduced. 
     Further, the on-resistance of the switch SW 60  can be chosen based on the first attenuation. The on-resistance of the switches SW 611 -SW 631  and/or the resistance of the resistors R 611 -R 631  can be chosen based on the second attenuation. The resistance of the resistors RA 611 -RA 631  are chosen based on the third attenuation and the resistance of the resistors R 611  to R 631 . That is, the switches SW 611 -SW 631  can operate to switch the attenuator circuit  403  between the first π-type attenuator and the second π-type attenuator to provide corresponding attenuations. In other embodiments, the overall equivalent resistance of the switch SW 611  and the resistor R 611  and the overall equivalent resistance of the switch SW 621  and the resistor R 621  can be substantially the same. In another example, the first attenuation, the second attenuation and the third attenuation can be 0 dB, 5 dB and 6 dB respectively, and the resistance of the switch SW 60  can be less than the overall equivalent resistance of the switch SW 631  and the resistor R 631 ; the overall equivalent resistance of switch SW 631  and the resistor R 631  can be less than the overall equivalent resistance of the resistors R 631  and RA 631 . In this embodiment, the switches SW 60 , SW 611 -SW 631  can have appropriate on-resistance by carefully choosing the channel width-to-length ratio of the switches SW 60  and SW 611 -SW 631 . 
     Please refer back to  FIG.  4   . When the control signals CS 40 -CS 4   n  turn off the switches SW 40 -SW 43   n  respectively, the attenuator circuit  401  is disabled. At this time, the output terminal of the amplifier  201  and the output terminal  12  of the amplifier circuit  1  are electrically disconnected. In other words, the output terminal  12  of the amplifier circuit  1  would not output the output signal Sout because the amplifier circuit  1  is now a zero gain amplifier. However, in the case where the attenuator circuit  401  being replaced by the attenuator circuit  403 , when the control signals CS 60 -CS 61  turn off the switches SW 60 -SW 631 , the attenuator circuit  403  can forma π-type attenuator. That is, the attenuator circuit  403  is still enabled, and the output terminal of the amplifier  201  and the output terminal  12  of the amplifier circuit  1  can be electrically connected through the attenuator circuit  403 . In other words, the output signal Sout can be generated from the attenuator signal Sat 1 , and the output terminal  12  of the amplifier circuit  1  can still output the output signal Sout. In this way, the gain of the amplifier circuit  1  is associated with the attenuation of the amplified input signal Sin′. 
     It should be noted that since the attenuator circuit  403  has a plurality of different π-type attenuators (e.g., the first π-type attenuator and the second π-type attenuator), it can be regarded as having a substantially symmetrical circuit structures. In each of the circuit structure, the difference between the impedance at the input terminal IN and the impedance at the output terminal OUT of the attenuator circuit  403  can be within a predetermined range. The predetermined range may be ±10%. In other embodiments, the impedance at the input terminal IN of the attenuator circuit  403  and the impedance at the output terminal OUT of the attenuator circuit  403  may be substantially equal. Substantially equal means conjugate matching between the impedance at the input terminal IN and the impedance at the output terminal OUT of the attenuator circuit  403 . That is, the impedance at the input terminal IN and the impedance at the output terminal OUT of the attenuator circuit  403  are complex conjugate of each other. 
     Further, the impedance at the input terminal IN of the attenuator circuit  403  can be the equivalent impedance looking from the input terminal IN into the internal of the attenuator circuit  403 . The impedance at the output terminal OUT of the attenuator circuit  403  can be the equivalent impedance looking from the internal towards the output terminal OUT of the attenuator circuit  403 . In addition, the difference between the equivalent impedance looking from the matching circuit  60  into the attenuator circuit  403  and the equivalent impedance looking from the attenuator circuit  403  towards the output terminal  12  of the amplifier circuit  1  can also be within a predetermined range. 
     The relationship between the various attenuations and the switches SW 60  and SW 611 -SW 631  described in the above paragraphs is summarized in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SW60 
                 SW611-SW631 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 First attenuation 
                 ON 
                 OFF 
               
               
                   
                 Second attenuation 
                 OFF 
                 ON 
               
               
                   
                 Third attenuation 
                 OFF 
                 OFF 
               
               
                   
                   
               
            
           
         
       
     
     Third attenuation&gt;Second attenuation&gt;First attenuation 
     Please refer to  FIG.  6   .  FIG.  6    is another circuit diagram of the attenuator circuit in the gain-adjustable amplifier circuit  1 ,  2  or  3  of the embodiment of the present invention. The attenuator circuit  404  of  FIG.  6    may correspond to the attenuator circuit  40  of  FIG.  1    or  FIG.  3   , or the attenuator circuit  50  of  FIG.  2    or  FIG.  3   . 
     The major difference between the attenuator circuit  404  of  FIG.  6    and the attenuator circuit  403  is that the resistor-switch circuits RS 1 , RS 2 , and RS 0  of the attenuator circuit  404  further include at least resistors R 61   n , R 62   n  and R 63   n  respectively, and switches SW 61   n , SW 62   n  and SW 63   n  respectively. In the above symbols, “n” may be a positive integer greater than 1. As shown in  FIG.  6   , the first terminal of the resistor R 61   n  is coupled to the second terminal of the resistor R 61  (n−1) and the first terminal of the switch SW 61  (n−1). The second terminal of the resistor R 61   n  is coupled to the first terminal of the switch SW 61   n  and the first terminal of the resistor RA 611 . The second terminal of the switch SW 61   n  is coupled to the reference voltage terminal REF. The control terminal of the switch SW 61   n  is for receiving the control signal CS 6   n . The first terminal of the resistor R 62   n  is coupled to the second terminal of the resistor R 62  (n−1) and the first terminal of the switch SW 62 ( n −1). The second terminal of the resistor R 62   n  is coupled to the first terminal of the switch SW 62   n  and the first terminal of the resistor RA 621 . The second terminal of the switch SW 62   n  is coupled to the reference voltage terminal REF. The control terminal of the switch SW 62   n  is for receiving the control signal CS 6   n . The first terminal of the resistor R 63   n  is coupled to the second terminal of the resistor R 63  (n−1) and the first terminal of the switch SW 63 ( n −1). The second terminal of the resistor R 63   n  is coupled to the first terminal of the switch SW 63   n  and the first terminal of the resistor RA 631 . The second terminal of the switch SW 63   n  is coupled to the output terminal OUT of the attenuator circuit  404 . The control terminal of the switch SW 63   n  is for receiving the control signal CS 6   n . The control signal CS 6   n  can turn on or turn off the switches SW 61   n -SW 63   n . The control signals CS 60 , CS 61  to CS 6   n  may be provided by an internal circuit of the attenuator circuit  404  or by an external circuit outside of the attenuator circuit  404 . 
     Taking n=2 as an example, when turning on the switch SW 60  and turning off the switches SW 611 -SW 63   n , the attenuator circuit  404  can provide the first attenuation. Alternatively, when turning off the switches SW 60 , SW 61   n -SW 63   n , and turning on the switches SW 611 -SW 631 , the attenuator circuit  404  can form a first π-type attenuator. The first π-type attenuator includes the resistors R 611 -R 631 , the switches SW 611 -SW 631 . In this configuration, the attenuator circuit  404  can provide second attenuation. Alternatively, when turning off the switches SW 60 , SW 611 -SW 631 , and turning on the switches SW 61   n -SW 63   n , the attenuator circuit  404  can form a second π-type attenuator. The second π-type attenuator includes the resistors R 61   n -R 63   n , the switches SW 61   n -SW 63   n . In this configuration, the attenuator circuit  404  can provide third attenuation. Alternatively, turning off the switches SW 60 , SW 611 -SW 63   n  can form a third π-type attenuator. The third π-type attenuator includes resistors RA 611 -RA 631 . In this configuration, the attenuator circuit  404  can provide the fourth attenuation. The fourth attenuation may be greater than the third attenuation; the third attenuation may be greater than the second attenuation, and the second attenuation may be greater than the first attenuation. 
     Further, the on-resistance of the switch SW 60  can be chosen based on the first attenuation. The on-resistance of the switches SW 611 -SW 631  and/or the resistance of the resistors R 611 -R 631  can be chosen based on the second attenuation. The on-resistance of the switches SW 61   n -SW 63   n  and/or the resistance of the resistors R 61   n -R 63   n  can be chosen based on the third attenuation and the resistance of the resistors R 611 -R 631 . The resistance of the resistors RA 611 -RA 631  can be chosen based on the fourth attenuation and the resistance of the resistors R 611 -R 63   n , and so on. That is, the switches SW 611 -SW 63   n  not only can switch between the first π-type attenuator, the second π-type attenuator and the third π-type attenuator, but also can provide the corresponding attenuation. In other embodiments, the overall equivalent resistance of the switch SW 611  and the resistor R 611  can be the same as the overall equivalent resistance of the switch SW 621  and the resistor R 621 . For example, the first attenuation, the second attenuation, the third attenuation and the fourth attenuation can be 0 dB, 5 dB, 10 dB and 11 dB respectively. The overall equivalent resistance of the switch SW 60  can be less than the overall equivalent resistance of the switch SW 631  and the resistor R 631 , the overall equivalent resistance of the switch SW 631  and the resistor R 631  can be less than the overall equivalent resistance of the switch SW 63   n  and the resistors R 63   n  and R 631 , and the overall equivalent resistance of the switch SW 63   n  and the resistors R 63   n  and R 631  can be less than the overall equivalent resistance of the resistors R 631 , R 63   n  and RA 631 , and so on. In this embodiment, the channel width-to-length ratios of the switches SW 60 , SW 611 -SW 63   n  can be chosen so that the switches SW 60 , SW 611 -SW 63   n  have appropriate on-resistance. 
     It should be noted that since the attenuator circuit  404  can form a plurality of different π-type attenuators, which can be regarded as having substantially symmetrical circuit structures. In each of the circuit structures, the difference between the impedance at the input terminal IN of the attenuator circuit  404  and the impedance at the output terminal OUT of the attenuator circuit  404  is within a predetermined range. The predetermined range may be ±10%. In other embodiments, the impedance at the input terminal IN of the attenuator circuit  404  and the impedance at the output terminal OUT of the attenuator circuit  404  may be substantially equal. Substantially equal means conjugate matching between the impedance at the input terminal IN of the attenuator circuit  404  and the impedance at the output terminal OUT of the attenuator circuit  404 . That is, the impedance at the output terminal OUT of the attenuator circuit  404  and the impedance at the input terminal IN of the attenuator circuit  404  are complex conjugate of each other. 
     Further, the impedance at the input terminal IN of the attenuator circuit  404  can be the equivalent impedance looking from the input terminal IN into the internal of the attenuator circuit  404 . The impedance at the output terminal OUT of the attenuator circuit  404  can be the equivalent impedance looking from the internal towards the output terminal OUT of the attenuator circuit  404 . In addition, the difference between the equivalent impedance looking from the matching circuit  60  into the attenuator circuit  404  and the equivalent impedance looking from the attenuator circuit  404  towards the output terminal  12  of the amplifier circuit  1  can also be within a predetermined range. 
     When n=2, the relationship between the various attenuations and the switches SW 60 , SW 611 -SW 631  and SW 61   n -SW 63   n  described in the above paragraphs is summarized in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 SW60 
                 SW611-SW631 
                 SW61n-SW63n 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 First attenuation 
                 ON 
                 OFF 
                 OFF 
               
               
                 Second attenuation 
                 OFF 
                 ON 
                 OFF 
               
               
                 Third attenuation 
                 OFF 
                 OFF 
                 ON 
               
               
                 Fourth attenuation 
                 OFF 
                 OFF 
                 OFF 
               
               
                   
               
            
           
         
       
     
     Fourth attenuation&gt;Third attenuation&gt;Second attenuation&gt;First attenuation 
     Please note that the integer “n” can be greater than 2. As such, there can be as many chain coupled switches SW 61   n -SW 63   n  and resistors R 61   n -R 63   n  in the attenuator circuit  404  as possible. This configuration allows the attenuator circuit  404  to provide more choice of attenuations for flexible implementation. 
     Please refer to  FIG.  7   .  FIG.  7    is another circuit diagram of the attenuator circuit  40  of the gain-adjustable amplifier circuit  1 ,  2  or  3  of the embodiment of the present invention. The attenuator circuit  405  of  FIG.  7    may correspond to the attenuator circuit  40  of  FIG.  1    or  FIG.  3    or attenuator circuit  50  of  FIG.  2    or  FIG.  3   . 
     The major difference between the attenuator circuit  404  and the attenuator circuit  405  is that the attenuator circuit  405  may further include a switch SWP. The switch SWP may be coupled to between the second terminal of the resistor-switch circuits RS 1  and/or RS 2 , and the reference voltage terminal REF. The control terminal can receive a control signal CSp. The control signal CSp can turn on or turn off the switch SWP. 
     Taking n=2 as an example, when turning on the switch SW 60  and turning off the switches SW 611 -SW 63   n  and the switch SWP, the attenuator circuit  405  can provide the first attenuation. Alternatively, when turning off the switches SW 60 , SW 61   n -SW 63   n , and turning on the switches SW 611 -SW 631  and the switch SWP, the attenuator circuit  405  can form a first π-type attenuator. The first π-type attenuator includes the resistors R 611 -R 631 , the switches SW 611 -SW 631  and the switch SWP. In this configuration, the attenuator circuit  405  can provide second attenuation. Alternatively, when turning off the switches SW 60 , SW 611 -SW 631 , and turning on the switches SW 61   n -SW 63   n  and the switch SWP, the attenuator circuit  405  can form a second π-type attenuator. The second π-type attenuator includes the resistors R 611 -R 63   n , the switches SW 61   n -SW 63   n  and the switch SWP. In this configuration, the attenuator circuit  405  can provide third attenuation. Alternatively, when turning off the switches SW 60  and SW 611 -SW 63   n  and turning on the switch SWP, the attenuator circuit  405  can form a third π-type attenuator. The third π-type attenuator includes the resistors R 611 -R 631  and RA 611 -RA 631  and the switch SWP. In this configuration, the attenuator circuit  405  can provide the fourth attenuation. The fourth attenuation may be greater than the third attenuation; the third attenuation may be greater than the second attenuation, and the second attenuation may be greater than the first attenuation. 
     When the attenuator circuit  405  provides the first attenuation, the switches SW 611 -SW 61   n , SW 621 -SW 62   n , and SWP are equivalent to capacitors with their respective off-capacitance. The equivalent circuit can be regarded as the capacitors with the off-capacitance of the switches SW 611 -SW 61   n  and the switches SW 621 -SW 61   n  coupled connected in series with the capacitor with the off-capacitance of the switch SWP. Therefore, the switch SWP can reduce the overall capacitance of the attenuator circuit  405 , thereby avoiding load effect. When the attenuator circuit  405  provides the second attenuation, the third attenuation or the fourth attenuation, the turned-on switch SWP is equivalent to a resistor. The switch SWP can provide flexibility to the resistance of the first π-type attenuator, the second π-type attenuator or the third π-type attenuator. Further, the on-resistance of the switches SW 611 -SW 631  and SWP and/or the resistance of the resistors R 611 -R 631  can be chosen based on the second attenuation. The resistance of the resistors R 611 -R 631  can be chosen based on the third attenuation. The on-resistance of the switch SWP can be chosen based on the on-resistance of the switches SW 61   n -SW 63   n  and/or the resistance of the resistors R 61   n -R 63   n . The resistance of the resistors R 611 -R 63   n , the on-resistance of the switch SWP and the resistance of the resistors RA 611 -RA 631  can be chosen based on the fourth attenuation and so on. 
     When n=2, the relationship between the various attenuations and the switches SW 60 , SW 611 -SW 631 , SW 61   n -SW 63   n , and SWP described in the above paragraphs is summarized in Table 3. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 SW60 
                 SW611-SW631 
                 SW61n-SW63n 
                 SWP 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 First attenuation 
                 ON 
                 OFF 
                 OFF 
                 OFF 
               
               
                 Second attenuation 
                 OFF 
                 ON 
                 OFF 
                 ON 
               
               
                 Third attenuation 
                 OFF 
                 OFF 
                 ON 
                 ON 
               
               
                 Fourth attenuation 
                 OFF 
                 OFF 
                 OFF 
                 ON 
               
               
                   
               
            
           
         
       
     
     Fourth attenuation&gt;Third attenuation&gt;Second attenuation&gt;First attenuation 
     Please note that the integer “n” can be greater than 2. As such, there can be as many chain coupled switches SW 61   n -SW 63   n  and resistors R 61   n -R 63   n  in the attenuator circuit  405  as possible. This configuration allows the attenuator circuit  405  to provide more choice of attenuations for flexible implementation. 
     Please refer to  FIG.  8   .  FIG.  8    is another circuit diagram of the attenuator circuit  40  of the gain-adjustable amplifier circuit  1 ,  2  or  3  of the embodiment of the present invention. The attenuator circuit  406  of  FIG.  8    may correspond to the attenuator circuit  40  of  FIG.  1    or  FIG.  3    or the attenuator circuit  50  of  FIG.  2    or  FIG.  3   . 
     The major difference between the attenuator circuit  406  and the attenuator circuit  405  is that the attenuator circuit  406  may further include a resistor RP 1 . The resistor RP 1  may be coupled between the switch SWP and the reference voltage terminal REF. The resistor RP 1  can provide more flexibility to choosing the resistance of each π-type attenuator (e.g., the first to third π-type attenuators). 
     Please refer to  FIG.  9   .  FIG.  9    is another circuit diagram of the attenuator circuit  40  of the gain-adjustable amplifier circuit  1 ,  2  or  3  of the embodiment of the present invention. The attenuator circuit  407  of  FIG.  9    may correspond to the attenuator circuit  40  of  FIG.  1    or  FIG.  3   , or the attenuator circuit  50  of  FIG.  2    or  FIG.  3   . 
     The major difference between the attenuator circuit  407  and the attenuator circuit  405  is that the attenuator circuit  407  may further include a resistor RP 2  coupled in parallel to the switch SWP. The resistor RP 2  can provide more flexibility to choosing the resistance of each π-type attenuator (e.g., the first to third π-type attenuators). 
     In addition, the attenuator circuit  50  shown in  FIG.  2  or  3    may be implemented by any one of the attenuator circuits in  FIGS.  4  to  9   . The implementation detail is not repeated here for brevity. 
     In the above embodiments, the switches and the transistors mentioned may be field effect transistors (FET) or other types of transistors. When the switches and the transistors are FETs, the first terminal may be either the drain or the source; the second terminal may be the other one of the drain and the source; the control terminal may be the gate. 
     The gain-adjustable amplifier circuit provided by the various embodiments of the present invention can expand the gain range of the amplifier circuit and adjust the gain of the amplifier circuit through the attenuator circuits. Moreover, since the attenuator circuit can have substantially symmetrical circuit structure, the attenuator circuit would not easily affect the output impedance matching of the amplifier in the amplifier circuit. In addition, by sharing at least one resistor in different π-type attenuators, occupied areas of the attenuator circuit are reduced. 
     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. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.