Abstract:
A transconductor including circuitry for automatically selecting a non-linear class A operation or a linear class AB operation based on an input signal to be processed to generate an output signal, and for automatically adjusting current from a power supply to a level needed for operation of the transconductor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of prior application Ser. No. 10/483,463, filed Apr. 6, 2005, entitled A-AB TRANSCONDUCTOR, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to the field of transconductors. More specifically, the present invention relates to transconductors likely to be used in multimode devices, that is, capable of processing signals having different frequency characteristics.  
         [0004]     So-called class A non-linear transconductors and so-called class AB transconductors are more specifically considered in the present description.  
         [0005]     2. Discussion of the Related Art  
         [0006]     Class A or class AB transconductors are used to perform many functions such as, in particular, amplifications or signal mixings. For example, transconductors are used in chains of transmission or reception of signals to implement, in particular, so-called low-noise receive amplifiers (LNA), so-called power transmission amplifiers (PA) or mixers.  
         [0007]     The type—class A or class AB—of the transconductor used to implement such function depends on the application. Consider for example the case of transmission chains in the field of radiotelephony or mobile telephony. In this field, there exist different standards especially characterized by different frequency ranges, among which standards UMTS or WCDMA, of a frequency on the order of 2.16 Hz, standard GSM of a frequency of 900 MHz, or standard DCS of a frequency of 1.8 GHz.  
         [0008]      FIG. 1A  schematically illustrates the structure of a transconductor  1 . Such a transconductor  1  includes an NPN-type bipolar transistor  2 . Base  3  of transistor  2  forms a first input/output terminal of the transconductor. Base  2  receives an input signal IN, for example, in mobile telephony, a radiofrequency signal. Emitter  4  of transistor  2  is connected to a low-voltage reference line or ground GND, via a series connection of an impedance  5  and of a constant D.C. current source  6 . Collector  7  of transistor  2  forms a second input/output terminal of the transconductor and provides output signal OUT of transconductor  1 . Collector  7  is connected to any device, illustrated by an input/output terminal  8 , providing a signal to be mixed with input signal IN. For example, device  8  provides a signal enabling switching input signal IN, or a carrier signal to be modulated by input signal IN. Device  8  is not necessarily unique. It may be an intermediary stage of a mixer of several signals, for acquiring one of the signals to be mixed.  
         [0009]      FIGS. 1B and 1C  respectively illustrate characteristics of transconductance gain G and of current I as a function of the level of voltage input V in the transconductor of  FIG. 1A . Transconductance gain G, which is proportional to the value of the bias current of transistor  2 , is a constant go for low levels of base-emitter voltage V across transistor  2 . Beyond a given threshold V 0 , gain G decreases. Short of threshold V 0 , the dynamic output current I OUT  then is, as illustrated in  FIG. 1C , proportional to base-emitter voltage V and limited by value I DC  of D.C. current source  6 . Beyond threshold V 0 , the behavior of the class A transconductor is poorly defined. The input signals are thus limited to those for which the input voltage is smaller than V 0 .  
         [0010]      FIG. 2A  schematically illustrates the structure of a class AB transconductor  20 .  FIGS. 2B and 2C , which are homologous to  FIGS. 1B and 1C , illustrate the gain and current characteristics according to the input voltage of transconductor  20 .  
         [0011]     Transconductor  20  includes an NPN-type bipolar transistor  21 , base  22  of which forms a first input/output terminal, receiving a signal IN, for example, a radiofrequency signal, collector  23  of which forms an output terminal of a current OUT and emitter  24  of which is degenerated, that is, connected to a reference supply GND by an impedance  25 . Further, base  22  is connected to a current bias source  26  by a resistor  27 . As previously for class A transconductor  1  of  FIG. 1A , collector  23  may be connected to an input device  28  of a signal to be mixed or of an amplification order or of a carrier signal or other.  
         [0012]     Transconductor  20  exhibits an exponential characteristic of gain G according to input base-emitter voltage V, illustrated in  FIG. 2B . This enables, as illustrated in  FIG. 2C , obtaining a static component (or mean current)  I OUT     of the output signal current which varies as a function of the input signal. The dynamic component of output signal current I out  is then no longer limited by the bias signal, but follows, or even exceeds, the mean current.  
         [0013]     In the example of application to telephony, to enable a user to keep a given device when a standard changes, one mixer per frequency range must be provided for each function. Such a solution goes against the miniaturization of portable devices.  
         [0014]     It could then be devised to use class A mixers formed of transconductors similar to transconductor  1  of  FIG. 1A , which would be forced to have a linear component by imposing a constant current (source  6 ) sufficiently high for transistor  2  to operate in linear state. Such a solution would have many disadvantages. In particular, such mixers would have a relatively poor linearity as compared to class AB mixers formed of class AB transconductors. Further, this would be obtained at the expense of high power consumption. Such a power consumption would then also exist for small signals although it is not necessary. This would be particularly disadvantageous due to the large number of transconductors used in a multimode transceiver device. The required power supplies would then become bulky and impose at best frequent recharges, which is incompatible with the mobile character of the device. Further, the high power consumption, useless in the case of small signals, would impose additional dissipation constraints in the form of thermal power.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention aims at providing a multimode transconductor.  
         [0016]     The present invention aims at providing such a transconductor which behaves, according to the input signal level, as a class A or as a class AB transconductor.  
         [0017]     The present invention aims at providing such a transconductor which automatically adjusts its consumption level to the level necessary to its operation.  
         [0018]     To achieve these and other objects, the present invention provides a transconductor including means for automatically selecting a non-linear class A operation or a linear class AB operation based on an input signal to be processed, and for automatically adjusting the current sampled from a power supply to a level necessary to its operation.  
         [0019]     According to an embodiment of the present invention, the transconductor includes at least two bipolar transistors, the common bases of which define a first input/output terminal of the transconductor;  
         [0020]     the interconnected collectors of which define a second input/output terminal of the transconductor;  
         [0021]     the emitters of which are individually connected to a low voltage reference line by a respective impedance; and  
         [0022]     the bases of which are connected to a same D.C. current source.  
         [0023]     According to an embodiment of the present invention, the impedances of each individual connection of the emitters of the two bipolar transistors to the low voltage reference line (GND) are just resistive.  
         [0024]     According to an embodiment of the present invention, the bases are connected to the D.C. current source by an isolating resistor.  
         [0025]     According to an embodiment of the present invention, the ratios of the isolating resistance to the resistance of the individual connection of the emitters to the reference line are different.  
         [0026]     According to an embodiment of the present invention, the two bipolar transistors are of different sizes, and the impedance of the individual resistive connection of each of their emitters to the voltage reference line is implemented by a respective resistor.  
         [0027]     According to an embodiment of the present invention, the D.C. current source is formed by the connection, between a high supply line and the low voltage reference line, of a D.C. current source, of a first bipolar transistor of a given type, the junction of the current source and of the first transistor being connected to the base of a second bipolar transistor of the same type as the first transistor, a collector/emitter terminal of which is connected to the high supply line and an emitter/collector terminal of which is connected to the base of the first transistor and forms the current source output.  
         [0028]     According to an embodiment of the present invention, a resistor is interposed between the base of the first bipolar transistor and the emitter/collector terminal of the second bipolar transistor forming the output of the D.C. current source.  
         [0029]     The present invention also provides an input/output stage of a mixer or of a power amplifier or of a low-noise amplifier, formed of a transconductor according to any of the preceding embodiments.  
         [0030]     The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1A , previously described, shows the circuit diagram of a conventional class A transconductor;  
         [0032]      FIGS. 1B and 1C , previously described, illustrate characteristics of the transconductor of  FIG. 1A ;  
         [0033]      FIG. 2A , previously described, shows the circuit diagram of a conventional class AB transconductor;  
         [0034]      FIGS. 2B and 2C , previously described, illustrate characteristics of the transconductor of  FIG. 2A ;  
         [0035]      FIG. 3A  schematically shows the structure of a transconductor according to the present invention;  
         [0036]      FIG. 3B  illustrates a characteristic of the gain versus an input voltage of the transconductor of  FIG. 3A ; and  
         [0037]      FIG. 3C  illustrates a characteristic of the output current versus the input voltage of the transconductor of  FIG. 3A . 
     
    
     DETAILED DESCRIPTION  
       [0038]     Only those elements necessary to the understanding of the present invention have been shown and will be described hereafter. In particular, the upstream and downstream circuits of a transconductor according to the present invention have not been detailed. The invention applies whatever the input and output signals. Further, the same elements have been designated with the same references in the different drawings. Moreover, the characteristics of  FIGS. 1B, 1C ,  2 B,  2 C,  3 B and  3 C are not drawn to scale.  
         [0039]      FIG. 3A  schematically shows the structure of a class AB transconductor  30  according to the present invention.  
         [0040]     Transconductor  30  includes at least two bipolar transistors  31 ,  32  of the same type, for example, NPN. Bases  33  and  34  of transistors  31  and  32  are interconnected at a common point. Common base  33 - 34  forms an input/output terminal to receive, for example, an input signal IN. An input signal IN of radiofrequency type is decoupled by a capacitor  35 . Common base  33 - 34  is further connected to a D.C. current source  50  by an isolation resistor  36 . The collectors of transistors  31  and  32  are interconnected and form an output/input terminal  37 . Emitters  38  and  39  of transistors  31  and  32  are individually connected to a voltage reference line GND by a respective resistive connection. The resistance of the connection is shown in  FIG. 3A  by interposing a resistor  40 ,  41  between reference line GND and emitter  38 ,  39  of transistor  31 ,  32 , respectively.  
         [0041]      FIG. 3A  illustrates a possible embodiment of D.C. current source  50 . Source  50  includes, interposed in series between a high voltage supply line Vcc and reference line GND, a current source  51  (for example, a MOS transistor based circuit) and an NPN-type bipolar transistor  52 . Transistor  52  is connected to source  51  by its collector  53  and to reference line GND by its emitter  54 . A resistor  55  is interposed between emitter  54  and reference GND. The base of transistor  52  is connected to a first terminal  56  of an isolating resistor  57 . A second terminal  58  of isolating resistor  57  is connected to bases  33 ,  34  by the respective isolating resistor  36 . Source  50  further includes an NPN-type transistor  59 , the base of which is connected to collector  53 , and thus also to source  51 , the emitter of which is connected to second terminal  58  of isolating resistor  57  and the collector of which is connected to high power supply Vcc.  
         [0042]     The values of the different isolating resistors  36  of resistive connections  40  and  41  are chosen according to the following constraints.  
         [0043]     Isolating resistor  36  should have a value sufficiently high to guarantee the isolation of D.C. current source  50  against the variations of input signal IN. Indeed, if isolating resistance  36  is too small, the possible noise from source  50  will reach common base  33 - 34 . Isolating resistance  36  should however be sufficiently small to enable passing of the D.C. current necessary to the biasing of transistors  31  and  32 .  
         [0044]     The choice of resistances  40  and  41  of the resistive connections is performed according to the choice of the value of isolating resistance  36  as follows. On the one hand, the two transistors  31  and  32  should exhibit different transconductances, that is, different products of the bias current by the degenerescence resistance. The determination of the transconductance values to be used is performed, for example, by so-called Volterra developments.  
         [0045]     According to an embodiment of the present invention, all transistors are identical.  
         [0046]     The behavior of transconductor  30  will be described hereafter in relation with  FIGS. 3B and 3C .  FIG. 3B , which is to be compared with  FIGS. 1B and 2B , illustrates gain G of the stage according to input/output voltage V.  FIG. 3C , which is to be compared with  FIGS. 1C and 2C , illustrates the variations of the transconductor output current I OUT  according to the same input voltage V. The considered input voltage V is the base-emitter voltage applied across the transistors in parallel  31  and  32 . The gain of the transconductor remains constant at a value g 5 . However, each transistor  31 ,  32  has a specific gain g 3 , g 4 , characteristic of a class AB stage. Especially, beyond a given threshold V 0 , each gain g 3 , g 4  varies. A multiple-transconductance implementation according to the present invention enables obtaining inverse variations and same amplitudes. For example, gain g 3  tends to increase from threshold V 0  while gain g 4  tends to decrease.  
         [0047]     For small input signals, the transconductor then exhibits a class A behavior. The static output current signal  I OUT     remains constant without taking the input signal variations into account. Beyond a given input power, the transconductor adopts a class AB behavior, whatever the input power, that is, voltage V. Output signal I OUT  varies exponentially according to the input signal. In parallel, the level of the current (not shown) sampled by source  50  from supply GND-Vcc remains constant at a minimum value I DC , whether the transconductor operates in class A or in class AB. Static current  I OUT     of the transconductor, the minimum value of which is set by bias source  50 , varies exponentially according to the input signal from as soon as the transconductor switches to the AB operating mode.  
         [0048]     The present invention thus advantageously provides a transconductor likely to automatically switch from a class A operation to a class AB operation and conversely according to the input signal.  
         [0049]     Such a stage is thus advantageously usable as an input stage of a mixer or of a low-noise or power amplifier, whatever the standard of the input signal. A single input stage can then be used in multimode applications. It is no longer necessary to manufacture stages specifically dedicated to an operating mode, that is, to provide as many specific manufacturing lines as there are modes.  
         [0050]     Further, the power consumption is advantageously automatically adjusted and limited to the needs of the transconductor according to the operating mode.  
         [0051]     Further, in a class AB operation, the multimode transconductor according to the present invention advantageously exhibits a linearity greater than known class AB transconductors. Indeed, the connection of multiple transconductances enables obtaining smaller intermodulation products than known class AB transconductors.  
         [0052]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention has been described in the case of an equivalent transconductor including two distinct transconductances. However, it is possible to increase the number of transistors.  
         [0053]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.