Abstract:
There is provided a bias arrangement for an amplifier adapted to amplify a varying input signal, the arrangement comprising a control circuit arranged to adaptively vary a bias current to the amplifier in dependence on an envelope of the varying input signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to amplifiers having a biased current input and arranged to amplify a varying input signal. The invention is particularly but not exclusively concerned with transconductance amplifiers in envelope tracking architectures. 
     2. Description of the Related Art 
     Frequency domain duplex (FDD) systems include transceivers that have a transmitter and a receiver which operate at different carrier frequencies. A simple exemplary architecture of such an FDD system is illustrated in  FIG. 1 . A transceiver includes a transmitter block  6  which receives an input signal on line  2  to be transmitted by an antenna  18 . A receiver block  8  receives signals that are detected at the antenna  18  and delivers them on signal line  4 . The output of the transmitter  6  is delivered to a duplex filter  14  on line  10 . The received signal from the antenna  18  is delivered from the duplex filter  14  on a line  12  to the receiver  8 . The antenna  18  is connected to the duplex filter  14  via a line  16 . 
     In such an FDD system it is essential that energy from the transmitter does not block the receiver. This may occur because the duplex filter on the output of the transmitter has only limited attenuation. Any noise present at the receiver frequency on the transmitter output due to noise from the transmitter has the potential to cause receiver blocking. 
     The transmitter circuitry  6  typically includes a transconductance amplifier. A transconductance amplifier generates a current which is proportional to its input voltage. Any noise present at the receiver frequency on the transmitter output due to noise from a transconductance amplifier in the transmitter has the potential to cause the above-mentioned receiver blocking. Any such noise must preferably not be allowed to exceed a low level while the power consumption of the transconductance amplifier is minimised. Thus efficient, low noise, high linearity transconductance amplifiers are required in FDD systems. 
     A typical transconductance amplifier uses a class A or continuous bias scheme that is independent of the input signal level. In a transmitter incorporating an envelope tracking modulated power supply, a typical transconductance amplifier uses a class A or continuous bias scheme that is independent of the modulation envelope. 
     With such a bias scheme, the transconductance amplifier consumes the same power and generates the same noise at the signal troughs as it does at the signal peaks. 
     It is an aim of the invention to control an amplifier, such as a transconductance amplifier, to reduce generated noise and/or reduce consumed power. 
     SUMMARY OF THE INVENTION 
     The invention introduces an adaptive biasing scheme, suitable for transconductance amplifiers, and exploits a feature that during signal troughs the bias current can be “backed off”. This saves power and reduces noise during the quiet periods of the modulation. The invention may be particularly advantageously applied in a transmitter incorporating an envelope tracking modulated power supply. Extra circuitry is required to implement the adaptive biasing scheme, which increases both the die area and design and verification time. However the invention provides significant benefits, not only in reducing noise during the quiet periods of the modulation, but also by saving power. 
     Embodiments of the invention provide a bias arrangement for an amplifier adapted to amplify a varying input signal, the arrangement comprising a control circuit arranged to adaptively vary a bias current to the amplifier in dependence on an envelope of the varying input signal. 
     The bias arrangement may further include a current source for generating the bias current. 
     The control circuit includes a peak detection circuit may be arranged to track the envelope of the varying input signal and to provide an output which is proportional to the instantaneous magnitude of the varying input signal to the bias modulation circuit for controlling the variation of the bias current. The control circuit may include a bias modulation circuit responsive to the output of the peak detection circuit and arranged to control the current source in dependence thereon. 
     The bias arrangement may further comprise a common mode detection circuit for detecting a common mode voltage associated with the varying input signal, and a current generator for generating a DC offset current in the bias current in dependence on the detected common mode voltage. 
     The amplifier is preferably a transconductance amplifier. 
     An envelope-tracking supply for an amplification stage may include such a transconductance amplifier. A mobile communication handset may include an envelope tracking power supply for a radio frequency amplifier including an amplifier as described. A mobile communication infrastructure device including an envelope tracking power supply for a radio frequency amplifier including an amplifier as described. 
     In another embodiment a bias arrangement for a transconductance amplifier is arranged to amplify a varying input signal, and comprises: a peak detection circuit arranged to track the envelope of an input signal for the transconductance amplifier and to provide an output which is proportional to the instantaneous magnitude of the input signal; and a bias modulation circuit responsive to the peak detection circuit and arranged to modulate a bias current to the transconductance amplifier in accordance with the envelope of the input signal. 
     The bias arrangement may further comprise a current source for providing the bias current under the control of the bias modulation circuit. 
     In embodiments there is provided a method of providing a bias current to an amplifier arranged to amplify a varying input signal, the method comprising varying the bias current in dependence on the envelope of the varying input signal. 
     The method may further comprise providing an output which is proportional to the instantaneous magnitude of the varying input signal for controlling the variation of the bias current. 
     The method may further comprise detecting a common mode voltage associated with the varying input signal, and generating a DC offset current in the bias current in dependence on the detected common mode voltage. 
     The method may be applied to a transconductance amplifier. The method may be applied to the transconductance amplifier of an envelope tracking power supply for a radio frequency amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention is now described by way of example with reference to the accompanying figures, in which: 
         FIG. 1  illustrates an FDD transceiver, in which embodiments of the invention may be implemented; 
         FIG. 2  illustrates a prior art transconductance amplifier with constant bias; 
         FIG. 3  illustrates a transconductance amplifier with modified bias in accordance with a preferred embodiment of the invention; 
         FIG. 4  illustrates an exemplary implementation of the modified bias in accordance with the invention; and 
         FIG. 5  illustrates an exemplary application of a transconductance amplifier in accordance with an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is now described by way of example with reference to particular embodiments and exemplary implementations. The invention is not limited to the details of any described embodiments or exemplary implementations. 
     With reference to  FIG. 2  there is illustrated an exemplary arrangement of a transconductance amplifier as known in the art, with a constant bias. 
     As illustrated in  FIG. 2  a transconductance amplifier generally designated by reference numeral  30  receives a voltage input signal on line  36 , and generates a current control signal on line  38 . A current source  32  provides a constant bias current on line  28  to the transconductance amplifier  30 . The current control signal on line  38  controls a current source  34 . The current source  34  generates a current on a line  40  which is an output current of the transconductance amplifier. The output current on line  40  is proportional to the input voltage on line  36 . Each of the current sources  32  and  34  are connected to a supply voltage. 
     In an arrangement such as illustrated in  FIG. 2 , the transconductance amplifier&#39;s constant bias must be set at a sufficiently high level to ensure that in dependence on overall supply voltage, input signal, and load conditions, the transconductance amplifier can deliver sufficient current to the load without clipping or distortion under all operating conditions. 
       FIG. 3  illustrates an improvement to the prior art arrangement of  FIG. 2  in accordance with a preferred embodiment of the invention. Where elements of  FIG. 3  correspond to elements of  FIG. 2  like reference numerals are used. 
     A control circuit  42  is introduced comprising a peak detection circuit  48 , a bias modulation circuit  46 , and a current source  44  which generates a modulated bias current. The peak detection circuit  48  receives the voltage input signal on line  36  as an input, and the current source  44  generates a variable bias current on line  28  to the transconductance amplifier  30 . The current source  44  is additionally connected to the supply voltage. 
     The purpose of the peak detection circuit  48  is to accurately track the peaks of the envelope of the input signal on line  36 , and to provide an output that is proportional to the instantaneous magnitude of the peaks of this input signal. 
     The purpose of the bias modulation circuit  46  is to compensate for excess noise or distortion resulting from the peak detection circuit  48 . 
     The bias modulation circuit  46  modulates the transconductance amplifiers bias current, by controlling the current source  44 , in accordance with the input signal envelope. For small input signals the bias current will be minimised, and for larger input signals sufficient bias will be provided for the transconductance amplifier to accurately track the signal peaks. 
     The peak detection circuit  48  and the bias modulation circuit  46  are preferably designed to track the process, temperature and voltage effects of the transconductance amplifier. In one example, the transconductance of the transconductance amplifier is inversely proportional to resistance, therefore, if the bias varies proportional to resistance then a constant transconductance is obtained. 
     The adaptive biasing technique provided by the control circuit  42  of  FIG. 3  allows the power consumption of a transconductance amplifier for use in a transmitter, or other application, to be minimised such that only the current required to convey the modulation is consumed. By minimising current consumption, the noise of the transconductance amplifier is also minimised. This allows the area of the transconductance amplifier to be reduced for a given noise budget, such as to meet a receiver blocking requirement as described. 
     With reference to  FIG. 4  there is illustrated an exemplary implementation of the control circuit  42  of  FIG. 3 . Where elements of  FIG. 4  correspond to elements of  FIG. 3  like reference numerals are used. 
     The input voltage on line  36  is provided to an envelope detector  68  which generates an envelope signal on line  69 . The envelope signal is input to a peak detector circuit  50  which corresponds to the peak detection circuit  48  of  FIG. 3 . The output of the peak detector  50  on line  71  is a waveform which follows the peaks of the envelope signal on line  69 . 
     The output of the peak detector  50  on line  71  forms an input to the non-inverting input of an operational transconductance amplifier  54 . The output of the operational transconductance amplifier  54  is connected to the gate of a transistor  62 . The source of the transistor  62  is fed back to the inverting input of the operational transconductance amplifier  54 . The source of the transistor  62  is additionally connected to one terminal of a resistor  64 , having a value R set , which has a second terminal connected to electrical ground. 
     The operational transconductance amplifier  54 , the transistor  62 , and the resistor  64  operate to force the voltage at the non-inverting input of the amplifier  54  (which is the voltage of the output of the peak detector  50 ) to be present at the source of the transistor  62  and the first terminal of the resistor  64 . This converts the voltage signal of the output of the peak detector  50  on line  71  into the current domain. 
     This current flows in the drain of the transistor  62 , which is connected to the source of a further transistor  56 , and the transistor  56  has its drain connected to the supply voltage V DD . The transistor  56  is diode connected, and has its gate connected to a further transistor  58 , preferably via a noise filter  60  as shown in  FIG. 4 . 
     The transistor  58  is an implementation of the modulated bias current source  44  of  FIG. 3 . Thus the drain of the transistor  58  is connected to the supply voltage V DD , and the source of the transistor  58  is connected via line  28  to the supply terminal of the transconductance amplifier  30 . 
     The current flowing in the drain of transistor  62 , which is the current corresponding to the voltage output by the peak detector circuit  50 , is mirrored by the current mirror configuration arrangement of transistors  56  and  58  to flow in the transistor  58 . 
     In a preferred embodiment, a common mode detection circuit  52  is additionally provided in the implementation. As illustrated in  FIG. 4 , the common mode detection circuit  52  receives the envelope signal on line  69  as an input. The common mode detection circuit  52  is illustrated as generating a control signal which controls a current source  66 , which is connected between the supply voltage V DD  and the connection point between the first terminal of the resistor  64  and the source of the transistor  62 . It should be noted that the illustration of the current source  66  is exemplary only, and in alternative arrangements a current sink and/or a current sink in combination with a current source may be provided. 
     The purpose of the common mode detection circuit  52  and current source  66  is to inject a current into the node between the source of the transistor  62  and the first terminal of the resistor  64  to meet a desired objective for the DC bias current in the supply to the transconductance amplifier. In the illustration of  FIG. 4 , the common mode detection circuit determines the common mode voltage in the envelope signal on line  69 , and adjusts the current injected from the current source  66  in dependence thereon. In the example illustrated in  FIG. 4  the current source may be used to inject a current in order to eliminate a DC offset current. In alternative arrangements a current sink may be used in order to generate a DC offset current. 
     With reference to  FIG. 5 , there is illustrated an example implementation of a transconductance amplifier such as transconductance amplifier  30  of  FIGS. 3 and 4 , in which the modifications according to the present invention may be implemented. 
       FIG. 5  illustrates an RF power amplifier  70  with an envelope tracking modulated power supply  80 . An RF input signal on line  82  is amplified by the RF amplifier  70  to provide an amplified RF output signal on line  84 , which is delivered to a load such as an RF antenna. 
     An envelope detector  86  additionally receives the RF input signal on line  82  and generates an envelope signal on line  76  representing the envelope of the input signal to be amplified. This forms the input to the envelope tracking power supply  80 . 
     One of a plurality of available voltage levels is selected in a switched supply circuit  71  in dependence upon the envelope signal on line  76 , and the selected switched supply voltage is connected to a first terminal  101  of an inductor  74 . 
     A correction amplifier arrangement generally designated by reference numeral  103  additionally receives the envelope signal on line  76 . The correction amplifier arrangement comprises, in the illustrative arrangement, a transconductance amplifier  105 , an amplifier  113 , a resistor  111 , a capacitor  109 , a battery  115  and a feedback stage  107 . The correction amplifier arrangement  103  is merely illustrative of an exemplary implementation, and one skilled in the art will appreciate that alternative implementation are possible to provide a correction to the voltage generated by the switched supply  71 . 
     With reference to the correction amplifier arrangement  103 , the transconductance amplifier  105  receives the envelope signal on line  76  at its inverting input, and receives an output supply voltage on line  78  from the second terminal  102  of the inductor  74  at its non-inverting terminal, via the feedback stage  107 . The transconductance amplifier may be implemented in accordance with the above-described advantageous techniques in accordance with the invention. 
     The output of the amplifier  105 , which is a voltage-to-current transconductance amplifier, is connected to the inverting input of amplifier  113 . The non-inverting input of amplifier  113  is connected to the battery  115 , the other terminal of which is connected to electrical ground. The battery  115  represents a fixed voltage. 
     The output of the amplifier  113  is connected to the second terminal  102  of the inductor  74 . The capacitor  109  and the resistor  111  are each connected, in parallel, between the input and output of the amplifier  113 . 
     The correction amplifier arrangement  103  operates to compare the output supply voltage at the output of the inductor  74  with the envelope signal on line  76 , which provides a reference, and generate an error signal which indicates an error in the output signal to thereby correct the signal. Thus the voltage signal on line  78  is the selected switched supply voltage corrected by the correction amplifier arrangement  103 , and is delivered as the supply voltage for the RF amplifier  70 . 
     The amplifier  105  may be implemented as a transconductance amplifier having control circuitry in accordance with the invention and embodiments as described herein. 
     An RF amplifier arrangement such as illustrated in  FIG. 5 , with an envelope tracking power supply, may be utilised in various applications, such as in transmitters of mobile communication handsets and mobile communication infrastructure devices. 
     Although the invention has been described herein with reference to its application to a transconductance amplifier, the invention may be more broadly applicable to any amplifier for which a varying input signal is provided, and where there is a need to adapt the bias current of the amplifier to achieve optimum gain efficiency. 
     Low noise, power efficient transconductance amplifiers are required in transceiver circuits and precision digital to analog converters and analog to digital converters, as well as other mixed signal applications. The invention may be advantageously implemented in any such applications. 
     The invention has been described herein with reference to particular advantageous embodiments and exemplary implementations. The invention is not limited to any details of any aspects of such embodiments and implementations. The scope of the invention is defined by the appended claims.