Abstract:
The variable gain amplifier of the present invention includes at least an operation amplifier. By choosing one of output stages, a feedback resistor is selected and the gain of the variable gain amplifier is decided according to the resistance of the selected feedback resistor, as desired. By adjusting the gain of the variable gain amplifier, the received signals can be amplified or attenuated in accordance with design requirement. The variable gain amplifier can include a two-stage architecture, in which a first stage is used for coarse gain adjustment and a second stage is used for fine gain adjustment. The gain of the two-stage variable gain amplifier can be easily adjusted to a desired value.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an operational amplifier, and more particularly, to a variable gain amplifier. 
   BACKGROUND OF THE INVENTION 
   Please refer to  FIG. 1 , which is a schematic diagram depicting the framework of a conventional variable gain amplifier  100 . The variable gain amplifier  100  has an operational amplifier  110 , a resistor set  120  having a plurality of serial-connected resistors Rf 1 , Rf 2 , . . . , Rfn, and a switch set  130  having a plurality of switches SW 1 , SW 2 , . . . , SWn corresponding to the resistors respectively. The switches are generally implemented with MOS transistors. Under such configuration of  FIG. 1 , a gain G of the variable gain amplifier  100  is given by: G=1+Rf/Rg, wherein Rf is a resistance seen between the output Vo and the inverting input (−) of the operational amplifier  110 , while Rg is a resistance seen between the inverting input (−) of the operational amplifier  110  and a ground node Vag. Both the resistance Rf and Rg depend on the on/off state of the corresponding switches in the switch set  130 . The conventional variable gain amplifier  100  can ensure a monotonic variation in gain, and can avoid nonlinear distortion and gain error caused by the MOS switches since there is no current flowing through the MOS switches. 
   However, since the signal at the non-inverting input (+) of the operational amplifier  110  varies with the input signal Vi and the input dynamic range of the operational amplifier  110  is comparatively small, the conventional variable gain amplifier  100  tends to suffer from more significant distortion. In addition, the conventional variable gain amplifier  100  cannot perform signal attenuation and possesses inferior gain accuracy. 
   Please refer to  FIG. 2 , which is a schematic diagram depicting the framework of another conventional variable gain amplifier  200 . The variable gain amplifier  200  has an operational amplifier (OPA)  210 , an input resistor Ri, a feedback resistor set  220  having a plurality of serial-connected resistors Rk 1 , Rk 2 , . . . , Rkn, and a switch set  230  having a plurality of switches Sk 1 , Sk 2 , . . . , Skn corresponding respectively to the resistors. The switches are generally implemented with MOS transistors. Under such configuration of  FIG. 2 , a gain G of the variable gain amplifier  100  is given by: G=Rk/Ri, wherein Rk is an equivalent resistance shown by the resistor set  220 . As seen in  FIG. 2 , the non-inverting input (+) of the OPA  210  is fixed to a ground node Vag. Therefore, the OPA  210  tends to have smaller distortion. 
   However, since current will flow through the MOS switches in this framework, the nonlinearity of the MOS switch may incur signal distortion. Also, gain error may be induced by the impedance of the MOS switch. In addition, in  FIG. 2  if the tolerable noise level is low, the input resistor Ri with a small impedance and the feedback resistors with small impedances are needed. However, when the impedances of the input resistor and the feedback resistors are of small values, the MOS switches with large equivalent resistances is required so as to reduce distortion and gain error. Unfortunately, when large-resistance MOS switches are adopted, the parasitic capacitors thereof tend to cause loop instability and substrate coupling issues. 
   SUMMARY OF THE INVENTION 
   It is one of the many objects of the present invention to provide a variable gain amplifier capable of amplifying and attenuating the received signals with the characteristics of low distortion and low noise. 
   According to embodiments of the present invention, a variable gain amplifier is disclosed. The variable gain amplifier comprises an input resistor coupled to an input signal; an operational amplifier comprising a pre-drive stage coupled to the input resistor and a plurality of output stages, each of which coupled to the pre-drive stage; and a plurality of feedback resistors, each feedback resistor being coupled to the pre-drive stage by one end thereof and coupled to one of the plurality of output stages by another end thereof. A feedback loop is formed by one of the output stages chosen by a first control signal and the feedback resistor corresponding to the chosen output stage. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a framework of a conventional variable gain amplifier. 
       FIG. 2  is a framework of another conventional variable gain amplifier. 
       FIG. 3  is a schematic diagram of a variable gain amplifier according to an embodiment of the present invention. 
       FIG. 4  is a schematic diagram of a variable gain amplifier according to another embodiment of the present invention. 
       FIG. 4A  is a schematic diagram of a variable gain amplifier according to yet another embodiment of the present invention. 
       FIG. 5  is an embodiment of the switch of  FIG. 4 . 
       FIG. 6  is an embodiment of the operational amplifier of the operational amplification unit with reference to  FIG. 4 . 
       FIG. 7  is an embodiment of the output stage of the operational amplifier with reference to  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   According to embodiments of the present invention, variable gain amplifiers comprising operational amplifiers and resistors are disclosed. The variable gain amplifier may comprise an input resistor, a pre-drive stage of an operational amplifier, a plurality of output stages of the operational amplifier and a plurality of feedback resistors each coupled respectively to a corresponding output stage. In one embodiment of the present invention, the pre-drive stage, the output stages and the feedback resistors may be lumped on a single integrated circuit (IC), and the input resistor may either be integrated on the same IC or be disposed externally. One end of each input resistor is connected to the input signal, and the other end thereof is connected to an input pad of the IC. In another embodiment, the input resistor may be connected to the IC by way of an electrostatic discharge (ESD) protection device so as to prevent the damage to the variable gain amplifier from happening while the magnitude of the received signal is too large. 
   In one embodiment of the present invention as seen in  FIG. 3 , a variable gain amplifier  300  comprises an operational amplifier  310 , an input resistor Ri, and a plurality of feedback resistors Rf 1 , Rf 2 , . . . , Rfn. The operational amplifier  310  further comprises a pre-drive stage  312 , and a plurality of output stages  3141 ,  3142 , . . . ,  314   n . The non-inverting output of the pre-drive stage  312  is connected to the non-inverting inputs of the plurality of output stages  3141 ,  3142 , . . . ,  314   n , and the inverting output of the pre-drive stage  312  is connected to the inverting inputs of the plurality of output stages  3141 ,  3142 , . . . ,  314   n . Each output stage  314   i  is coupled to a corresponding feedback resistor Rfi and also is coupled to a corresponding control signal Cti, where i=1, . . . , n. The output of each output stage  314   i  is coupled to the inverting input of the pre-drive stage  312  via a corresponding feedback resistor Rfi. One end of the input resistor Ri is connected to the input signal Vi, and the other end of the input resistor Ri is connected to an input pad and is further connected to the inverting input of the pre-drive stage  312  via an ESD  330 . 
   Controlled by the control signals Ct 1 , Ct 2 , . . . , Ctn, a gain G of the operational amplifier  300  is determined by the output stage  314   i  chosen and driven by the foregoing control signals accompanying the feedback resistor Rfi coupled thereto, and is given as: G=Rfi/Ri. It is noted that the resistances of the feedback resistors are properly predetermined according to design requirement so as to provide different gain values with different output stages being chosen. Thus, the gain G of the variable gain amplifier  300  can be adjusted as desired and the received signal can be amplified/attenuated by employing the control of the output stages. In this regard, the phenomena of loop instability and substrate coupling, as well as non-linear distortion can be alleviated. Hence, the variable gain amplifier  300  can be utilized in applications demanding very low noise level, such as the asymmetric digital subscriber line (ADSL) communication system. 
   Please refer to  FIG. 4 , which is a schematic diagram of a variable gain amplifier  400  according to another embodiment of the present invention. The variable gain amplifier  400  comprises a first stage operational amplification unit  410  and a second stage operational amplification unit  420 , in which the first stage operational amplification unit  410  is used for coarse gain adjustment and the second stage operational amplification unit  420  is used for fine gain adjustment. By adopting the two-stage gain adjustment approach, since the input referred noise of the second stage operational amplification unit  420  can be far higher than that of the first stage operational amplification unit  410 , both input resistors with larger resistances and feedback resistors with larger resistances can be adopted, and MOS switches with smaller resistances can also be used, without causing obvious non-linear distortion and gain error. 
   The first stage operational amplification unit  410  as seen in  FIG. 4  comprises an operational amplifier  412 , a input resistor Ri and a plurality of feedback resistors  4131 ,  4132 , . . . ,  413   n . The operational amplifier  410  comprises a pre-drive stage  411 , and a plurality of output stages.  4131 ,  4132 , . . . , 413   n . Each output stage  413   i  is controlled by a corresponding control signal Cti, where i=1, . . . ,n. As those skilled in the art can appreciate, the first stage operational amplification unit  410  in  FIG. 4  is similar to the variable gain amplifier  300  in  FIG. 3 , and detailed description is thus omitted herein for simplicity. 
   The second stage operational amplification unit  420  as seen in  FIG. 4  comprises an operational amplifier  422 , a plurality of input resistors Rs 1 , Rs 2 , . . . , Rsn, a plurality of switches Sw 1 , Sw 2 , . . . , Swn corresponding respectively to each input resistor, a plurality of feedback resistors Rk 1 , Rk 2 , . . . , Rkn, and a plurality of switches Sk 1 , Sk 2 , . . . , Skn corresponding respectively to each feedback resistor. Each input resistor Rsi is coupled to the inverting input of the operational amplifier  422  by way of the corresponding switch Swi, wherein every switch Swi is controlled by a control signal Cti of the first stage operation amplification unit  410 , wherein i=1, 2, . . . , n. The non-inverting input of the operation amplifier  422  is connected to a virtual ground Vag. The output of the operational amplifier  422  is coupled to the inverting input thereof by way of the feedback resistors Rk 1 , Rk 2 , . . . , Rkn and the corresponding switches Sk 1 , Sk 2 , . . . , Skn. 
   In this embodiment, the first stage operational amplification unit  410  is used for coarse gain adjustment and the second stage operational amplification unit  420  is used for fine gain adjustment, where the overall operation process is illustrated using the following example. Initially, one output stage out of the output stages  4131 ,  4132 , . . . ,  413   n , that is chosen by the plurality of control signals Ct 1 , Ct 2 , . . . , Ctn, is couple to the pre-drive stage  414 , for enabling a gain Gc of the first stage amplification unit  410 , which is also the coarse gain of the overall variable gain amplifier, to be Gc=Rfi/Ri, wherein Rfi is the resistance of the feedback resistor corresponding to the chosen output stage. It is noted that the framework of the second stage amplification unit  420  is similar to that of the aforementioned variable gain amplifier with reference to  FIG. 2  and the corresponding description. Thus, the fine gain Gf of the second operational amplification unit  420  is determined by the MOS switch Swi corresponding to the chosen output stage and a selected switch Ski. Finally, the total gain of the variable gain amplifier  400  is G=Gc×Gf. 
   The adjustment ranges of both the coarse gain and the fine gain can be varied as desired. For instance, the adjustment range of the coarse gain Gc depends, among other factors, on the number of the output stages  413   i , i=1, . . . ,n and the corresponding feedback resistors Rfi, i=1, . . . ,n. Similarly, the adjustment range of the fine gain Gf depends, among other factors, on the number the feedback resistors Rki, i=1, . . . ,n and the corresponding Ski, i=1, . . . ,n. Thereby the present invention has good design flexibility for fulfilling all kinds of requirement. 
   For example, when designing a low-noise variable gain amplifier with −18 dB˜23 dB gain range having 1 dB step, the variable gain range of the first stage amplification unit  410  may be set to be −18 dB˜18 dB with 6 dB step, and the variable gain range of the second stage amplification unit  420  may be set to be 0 dB ˜5 dB having 1 dB step. Assuming that the first stage input impedance Ri is 1 kΩ, then seven feedback resistors Rf 1 , Rf 2 , . . . , Rf 7  are required in the first stage operational amplification unit  410  since the variable gain range is −18 dB˜18 dB with 6 dB step, wherein resistances of the seven feedback resistors are 125Ω, 250Ω, 500Ω, 1 kΩ, 2 kΩ, 4 kΩ, 8 kΩ, respectively. All the second stage input impedance Rs 1 , Rs 2 , . . . , Rs 7  are then set to be 20 kΩ. Six feedback resistors Rk 1 , Rk 2 , . . . , Rk 6  are required in the second stage operational amplification unit  420  since the variable gain range is 0 dB˜5 dB with 1 dB step, wherein resistances of the six feedback resistors are 20 kΩ, 2 kΩ, 3 kΩ,3 kΩ, 4 kΩ, 4 kΩ. 
   Please refer to  FIG. 4A , which is a schematic diagram of a variable gain amplifier according to yet another embodiment of the present invention. The variable gain amplifier  400  has a first stage operational amplification unit  410  and a second stage operational amplification unit  420 , which both are basically similar to those of  FIG. 4 . The difference between the embodiment of  FIG. 4  and the embodiment of  FIG. 4A  is that the plurality of input resistors Rsi, i=1, . . . ,n, are replaced by a single input resistor Rs. As a result, the adjustment range of the fine gain of the second stage amplification unit  420  depends, among other factors, on the number the feedback resistors Rki, i=1, . . . ,n, and the corresponding switches Ski, i=1, . . . ,n. 
     FIG. 5  is an embodiment of the switches of  FIG. 4 . As seen in  FIG. 5 , the switch  510  comprises a transmission gate configuration composed of two MOS transistors. A PMOS transistor and an NMOS transistor are controlled to turn on/off by a signal SwiB and a complementary signal Swi thereof, respectively. The switch  510  composed of the two MOS transistors is illustrated as a circuit structure  520 . When referencing to the aforementioned embodiments, the signal Swi of the switch  510  is the respective control signal Cti of the operational amplification unit  410 . 
   Please refer to  FIG. 6 , which is an embodiment of an operational amplifier  610  used in the operational amplification unit  410 . The operational amplifier  610  comprises a pre-drive stage  612  and a chosen output stage  614 . 
     FIG.7  is an embodiment of the output stage  614  of the operational amplifier  610  illustrated by an output stage  710  and a circuit symbol thereof  720 . The output stage  710  comprises two output MOS transistors Mon, Mop, and four MOS switches Msw 1 , Msw 2 , Msw 3  and Msw 4 , wherein the transistor Mon is a N-type transistor and the transistor Mop is a P-type transistor. The MOS switches Msw 1  and Msw 4  are controlled by the control signal Cti, while the MOS switches Msw 2  and Msw 3  are controlled by the complementary signal CtiB of the control signal Cti. 
   When the output stage  710  is chosen, that is, in this embodiment, the control signal Cti being set to high, the MOS switches Msw 1  and Msw 2  are on and the MOS switches Msw 3  and Msw 4  are off, and the output stage  710  is therefore activated. 
   While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.