Abstract:
A circuit includes a transconductance stage which converts an input signal power into a signal current and supplies it to an output of the circuit. A current diverting circuit branch is coupled to selectively divert the current from the transconductance stage away from the output. A feedback network feeds back a portion of the current diverted away from the output to the input of the transconductance stage. When implemented as a variable gain amplifier, the current diverting branch functions to change a gain of the circuit from a high level to one or more lower levels. The feeding back of a portion of the diverted current to the input improves the linearity of the circuit in the lower gain mode(s).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to gain control amplifier circuits and, more particularly, to a feedback technique which improves the linearity when switching from a high gain mode to one or more lower gain modes. 
     2. Description of the Prior Art 
     Variable gain amplifiers (“VGA&#39;s”) are used in numerous electronic products such as global positioning (GPS) receivers, wireless local area networks and mobile communication devices, such as cordless and cellular phones. In particular, VGA&#39;s are used in the various parts of such devices, for example, in the radio frequency (RF) input stage, intermediate frequency (IF) and low frequency or baseband circuits of these devices. 
     The linearity of a circuit may be defined by various parameters. For example, the input third-order intercept point represents third-order non-linearity in the transfer function of a circuit. Due to the third-order non-linearity, two undesired signals in adjacent channels generate output third-order intermodulation products (IM 3 ) which can corrupt the desired signal at the output. The power of the desired signal at the output of a linear circuit increases linearly with the input power of the circuit. However, the power of the output IM 3  increases with the cube of the input power. The input third-order intercept point is the input power level, at which the power of the desired signal at the output of a circuit is equal to the power of the output IM 3 . 
     FIG. 1 shows a typical low-noise amplifier (LNA)  10  for amplifying input signals, such as radio-frequency signals propagating through receiving circuitry of a wireless system, such as a cellular telephone. A signal input RF in  is provided for receiving an input signal from a source, such as a tuner or the previous amplifier stage. The input signal is supplied to the base of an NPN junction transistor Q 1 . A degeneration impedance Ze is coupled between the emitter of the transistor Q 1  and a ground terminal. The transistor Q 1  and the impedance Ze function as a common-emitter transconductance stage. 
     An NPN junction transistor Q 2  is connected to the transistor Q 1  in a cascode configuration. The emitter of the transistor Q 2  is connected to the collector of the transistor Q 1 . The base of the transistor Q 2  is supplied with bias voltage from a bias circuit which can be either on chip or off chip. A resistor R 1  and an inductor L 1  are coupled between the collector of the transistor Q 2  and a source of collector voltage Vcc. A capacitor C 1  is arranged between the collector of the transistor Q 2  and an output RF out  of the low noise amplifier  10 . 
     The resistor R 1  is an output-matching resistor that functions to match the output impedance of the low noise amplifier  10  with the impedance of a load coupled to the output RF out . The inductor L 1  and capacitor C 1  form an impedance transformation network that transforms the output impedance defined by the resistor R 1  to match the impedance of the load. The inductor L 1  also serves as a pull-up inductor that increases the allowable voltage at the collector of the transistor Q 2 . 
     The gain of the amplifier of FIG. 1 is controlled in discrete steps by using a well-known current dividing or splitting technique. A switch in the form of an NPN transistor Q 3  having its emitter coupled to the collector of the transconductance device Q 1  and its collector coupled to the power supply Vcc is provided with a control signal at its base B to render the switch conductive or non-conductive. When the switch Q 3  is in the non-conductive state, the amplifier is in the high gain mode and all of the current from the transconductance stage is delivered to the output RF out . When the switch Q 3  is conductive, the amplifier is in a reduced gain mode, as current is diverted from the output and dumped to the power supply by the switch Q 3 , thereby reducing the gain at the output RF out  relative to the input RF in . In other words, current from the transistor Q 2  is split into two paths, with some of the current diverted through the transistor Q 3  and the remaining current provided to the output. In this current splitting technique, the gain step between the two gain modes depends on the device size ratios between the transistors Q 2  and Q 3  (with all emitters connected to the same node). This gain control scheme can be expanded by connecting additional transistors in parallel with transistors Q 2  and Q 3  (with all emitters connected to the same node). 
     A disadvantage of the gain control scheme of FIG. 1 is that the VGA has the same linearity in both high and lower gain modes. In many applications, the VGA is required to have higher linearity when the VGA is switched from the high to the lower gain modes. When the VGA is switched to lower gain modes, it is typically because of higher input signal power, so increased linearity is desired. 
     It would be desirable to create a new technique that improves the linearity of a VGA when it is switched from a high gain mode to a lower gain mode. 
     SUMMARY OF THE INVENTION 
     Various advantages of the invention are achieved at least in part by providing an electronic circuit, such as an amplifier, which includes: (i) a transconductance stage which converts an input signal power into a signal current and supplies the current to an output of the circuit, (ii) a current diverting circuit branch coupled to selectively divert the current from the transconductance stage away from the output and (iii) a feedback network which feeds back a portion of the current diverted away from the output to the input of the transconductance stage. When implemented as a variable gain amplifier, the current diverting branch functions to change a gain of the circuit from a high level to one or more lower levels. 
     The invention is based in part on the recognition that in known VGA&#39;s as discussed with reference to FIG. 1, the linearity of the VGA is dominated by the linearity of the transconductance stage. The feeding back of a portion of the diverted current to the input improves the linearity of the circuit in the lower gain mode(s). Furthermore, since the feedback network does not couple the output to the input, the isolation of the output relative to the input is not compromised. 
     In accordance with one aspect of the present invention, the transconductance stage includes a first transistor having a control terminal, such as the base of a bipolar transistor or the gate of a MOS transistor, coupled to receive the input signal. A degeneration impedance may be provided between a main current electrode, such as an emitter or source, of the first transistor and a ground terminal. 
     According to another aspect of the invention, the current diverting branch includes a switching transistor which receives a bias voltage to control the conductance of the switch and thereby the diversion of current through said current diverting branch. The circuit may include a plurality of current diverting branches to implement an amplifier having several gain modes. The circuit may have more than one feedback network in dependence on the number of gain modes for which improved linearity is desired. 
     According to yet another aspect of the invention, the feedback network includes an impedance network coupled between an output terminal of the switching transistor of the current diverting branch and the input of the transconductance stage. The invention also concerns a method of feeding back a portion of current diverted from the output of a variable gain amplifier to improve the linearity of one or more gain modes. 
     These and other object, features and advantages of the invention will become apparent with reference to the following detailed description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a prior art current splitting VGA; 
     FIG. 2 is a circuit diagram of a VGA according to the invention with partial current feedback to improve linearity in the lower gain modes; and 
     FIG. 3 is a circuit diagram similar to that of FIG. 2, but illustrating additional current diverting branches for implementing additional gain modes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the invention has general applicability in the field of electronic circuits, the best mode for practicing the invention is based in part on the realization of variable gain amplifiers using partial current feedback. Referring to FIG. 2, a low noise amplifier  100  of the present invention comprises a signal input RFin for supplying signals, for example, radio-frequency signals propagating through receiving circuitry of a wireless system, from a source, such as the previous stage of the receiving circuitry. For example, the amplifier  100  may operate at 0.9 GHz. 
     The input signal is supplied to the base of an NPN junction transistor Q 1 . A degeneration impedance Ze may be coupled between the emitter of the transistor Q 1  and a ground terminal. For example, an inductor having an inductance selected in the range between 1 nH and 3 nH may be used as the degeneration impedance Ze. The transistor Q 1  and the impedance Ze function as a common-emitter transconductance stage  110  to transform the power of the input signal into current produced on the collector of the transistor Q 1 . The impedance Ze is preferred; however, the emitter of the transistor Q 1  may be connected directly to ground. 
     An NPN junction transistor Q 2  is connected to the transistor Q 1  in a cascode configuration to isolate an output RFout of the amplifier  100  from the transconductance stage. The base of the transistor Q 2  is supplied with bias voltage, for example, about 2 V from a bias input Bias. This bias voltage may be supplied from either an on or off chip circuit. A resistor R 1  and an inductor L 1  are coupled between the collector of the transistor Q 2  and a source of collector voltage Vcc. For example, the collector voltage may be equal to about 3 V. 
     A capacitor C 1  is arranged between the collector of the transistor Q 2  and the output RFout of the low noise amplifier  100 . For example, for an operating frequency of 0.9 GHz, the capacitance of the capacitor C 1  may be selected to be 1 pF, the resistance of the resistor R 1  may be 200 Ohm, and the inductance of the inductor L 1  may be selected to be 5 nH. 
     The resistor R 1  functions as an output-matching resistor that matches the output impedance of the low noise amplifier  100  with the impedance of a load coupled to the output RFout. The inductor L 1  and capacitor C 1  form an output impedance transformation network to match the output impedance of the amplifier  100  with the impedance of the load. The inductor L 1  also serves as a pull-up inductor that increases the allowable voltage at the collector of the transistor Q 2 . Although the present invention is disclosed with the example of bipolar transistors, one skilled in the art will realize that field effect transistors may be used as the transistors Q 1  and Q 2 . 
     A current diverting circuit branch  120  includes an NPN transistor Q 3  with its emitter coupled to the collector of the transconductance device Q 1  and its collector coupled to the power supply Vcc. The transistor Q 3  is provided with a control signal at its base B 1  to switch it between a conductive and a non-conductive state. When the switch Q 3  is in the non-conductive state, the amplifier is in the high gain mode and all of the current from the transconductance stage is delivered to the output RF out . When the switch Q 3  is in the conductive state, the amplifier is in a reduced gain mode, as current from transistor Q 1  is diverted to the power supply by the switch Q 3 , thereby reducing the current supplied to the output (RF out ,) and reducing the gain at the output RF out  relative to the input RF in . 
     To improve the linearity of the low noise amplifier  100  when switched to the low gain mode, a feedback network  130  is arranged to at least partially feed the current diverted from the output RF out  back to the input of the transconductance stage, in this case RF in . The feedback network includes a resistor R 2  coupled between the collector of the switch Q 3  and supply Vcc and a feedback capacitor C 2  coupled between the collector of the transistor Q 3  and the base of the transistor Q 1 . The feedback capacitor C 2  may be the internal capacitance of transistor Q 1 . 
     When the transistor Q 3  is biased to divert current from the output, part of the diverted current that flows through the transistor Q 3  is fed back to the input of the transconductance stage at the base of the transistor Q 1  through a shunt feedback network  130 . In this example, the network includes a resistor R 2 , coupled between an output terminal (in this case collector) of the switching transistor Q 3  and the supply Vcc, and a capacitor C 2  coupled between the output terminal of the transistor Q 3  and the control terminal, or base, of the transistor Q 1 . Other impedance elements may be used. The feedback current improves the linearity of the LNA in the low gain mode. 
     The following table illustrates the improvement in input third-order intercept point (IIP3) due to the shunt feedback network of the invention. The measurements were taken at an input signal of 0.9 GHz. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Input IP3 (dBm) 
               
             
          
           
               
                   
                 High-Gain Mode 
                 Low-Gain Mode 
               
               
                   
               
             
          
           
               
                 Without Shunt Feedback (FIG. 1) 
                 −1 
                 −1 
               
               
                 With Shunt Feedback (FIG. 2) 
                 −1 
                 2 
               
               
                   
               
             
          
         
       
     
     The value of the resistor R 2  and the capacitor C 2  are chosen to improve the linearity in the low-gain mode without compromising the input return loss (the amount of input power reflected back to the source) of the LNA. Linearity improves with the increased values of R 2  and C 2 , but the input return loss degrades accordingly. 
     The reverse isolation of a circuit is the isolation of the input of the circuit from a signal at the output of the circuit. Traditional feedback loops in amplifiers establish a connection between the input of an amplifier and its output, and hence reduce reverse isolation of the amplifier. By contrast, since the shunt feedback network  130  does not connect the input RF in  to the output RF out , it does not degrade the reverse isolation of the VGA provided by the transistor Q 2 . 
     The resistors R 3  and R 4 , coupled respectively between the collector of the transistor Q 1  and the emitters of transistors Q 2  and Q 3 , control how the current is split between the output RF out  and the current diverting branch  120 . The gain step between the two gain modes is depends on the resistance ratios between the resistors R 3  and R 4 . For example, if the resistors R 3  and R 4  have the same resistances, the gain step is 6 dB. Although control of the current splitting can be accomplished in the absence of transistors Q 2 , Q 3 , the use of the resistors R 3 , R 4  provides better control. In an integrated circuit, resistors can generally be fabricated with much tighter tolerances than transistors, the use of resistors thus providing tighter control of the current splitting. It should be noted that the resistors typically have enough resistance that the current splitting becomes independent of the sizing of the transistors. The resistors R 3  and R 4  also reduce the noise contribution from the transistors Q 2  and Q 3  in the low gain mode. 
     Thus, the amplifier  100  has a series-feedback circuit provided by the degeneration impedance Ze, and an additional shunt-feedback circuit provided by the feedback network  130  which is effective to feed back a portion of any current diverted by the branch  120  to improve the linearity in the low gain mode. 
     FIG. 3 illustrates the circuit according to the invention extended to include further gain modes. Components corresponding to those in FIG. 2 bear the same references. Two additional current diverting branches  140  and  160  are coupled to the transconductance stage via the collector of the transconductance device Q 1  and include, respectively, a resistor R 5  and an NPN transistor Q 4 , and a resistor R 6  and a further NPN transistor Q 5 . Shunt feedback networks  150  and  170  include a resistor R 7  and capacitor C 3 , and a resistor R 8  and a capacitor C 4 , respectively. The bases B 2  and B 3  of the transistors Q 4  and Q 5  receive appropriate bias voltages to divert or to not divert current from the output RF out  to achieve second and third further gain modes in the same manner as applied to the base B 1  of the transistor Q 3 . The resistors R 5  and R 6 , as well as the feedback networks  150  and  170  function in a corresponding manner to the resistor R 4  and the feedback network  130 , respectively. Those of ordinary skill in the art will appreciate that the sizes of the devices Q 4 , Q 5 , the level of the bias voltages applied at bases B 2  and B 3 , and the resistance values of the resistors R 5  and R 6  may be adjusted to control gain step between the various gain modes. 
     In this disclosure, there are shown and described only the preferred embodiments of the invention, but it is to be understood that the invention is capable of changes and modifications within the scope of the inventive concept as expressed herein. For example, MOS devices may be used in place of the bipolar devices shown.