Patent Publication Number: US-2006017526-A1

Title: Balanced gyrator and devices including the balanced gyrator

Description:
TECHNICAL FIELD  
      The present invention relates to a balanced gyrator and to devices, such as gyrator filters and integrated transceivers including at least one of the balanced gyrators.  
     BACKGROUND ART  
      Gyrator filters are frequently used in low power channel filters for wireless transceivers. Currently there is an interest in being able to fabricate complete integrated transceivers/receivers in MOS technology. Channel filters may comprise MOS gyrators which suffer from capacitive feedforward which is the result of non-reciprocal gate-drain capacitance in its MOSTs and this is results in filters with a distorted high frequency response. Gyrators comprise transconductor feedback pairs and ideally transconductors linearly convert an input voltage into an output current with both input and output ports presenting an infinite impedance. A typical transconductor feedback pair is shown in  FIG. 1  in which one transconductor  10  is inverting and the other transconductor  12  is non-inverting.  
       FIG. 2  shows an embodiment of a balanced class AB transconductor which comprises two pairs of MOS transistors, each pair comprising a p-type transistor  14  and a n-type transistor  16  having their drain electrodes coupled together, their source electrodes connected to respective supply voltage lines V dda  and V ss , their gate electrodes connected together with a common junction of each pair of gate electrodes being connected to a respective input terminal  18 ,  20 , and their respective interconnected drain electrodes coupled to output terminals  22 ,  24 . A common mode feedback (cmfb) circuit  26  is coupled between the input terminals  18 ,  20  to provide dc stability.  
      A problem with a balanced gyrator such as that shown in  FIG. 3  using two balanced class AB transconductors  10 ,  12  with the output connections of the feedback transconductor  12  crossed-over is that the capacitances occurring naturally between the drains and gates of the transistors forming the transconductors create a high frequency parasitic feedthrough path and this produces high frequency peaking in the filter&#39;s frequency response. This may be mitigated by using very small transistors in the transconductors but in practice this results in very poor matching.  
      Referring to  FIGS. 4 and 5  this problem may be understood by initially considering the capacitances between the gate g and the drain d of a MOSFET as shown in  FIG. 4 . Y. P. Tsividis, “Operation and Modeling of the MOS transistor”, McGraw-Hill, ISBN 0-07-065381, pp. 370 to 372 points out that transistors which operate in saturation SAT, see  FIG. 5 , such as those of the transconductors used in the balanced gyrators to which the present invention relates, have intrinsic capacitances Cgs, Cdg and Cgd given by:  
               C   gs     =       -       δ   ⁢           ⁢     Q   g         δ   ⁢           ⁢     V   s           =       2   3     ·     C   ox                 (   1   )                 C   dg     =       -       δ   ⁢           ⁢     Q   d         δ   ⁢           ⁢     V   g           =       4   15     ·     C   ox                 (   2   )                 C   gd     =       -       δ   ⁢           ⁢     Q   g         δ   ⁢           ⁢     V   d           =   0             (   3   )             
 
      The MOSFET also has an extrinsic capacitance, C gdol , due to gate-drain overlap and stray fields between the gate and the drain contacts.  
      The transconductor has a feedforward capacitance, C ff , and a feedback capacitance, C fb , where:  
               C   ff     =         C   dg     +     C   gdol       =         2   5     ·     C   gs       +     C   gdol                 (   4   )                 C   fb     =         C   gd     +     C   gdol       =     C   gdol               (   5   )             
 
      Clearly the capacitance is non-reciprocal, i.e. C ff ≠C fb , and simple neutralisation techniques using simple (reciprocal) capacitances are useless.  
     DISCLOSURE OF INVENTION  
      A first object of the present invention is to mitigate the effects of the high parasitic feedthrough path on the performance of a balanced gyrator.  
      A second object of the present invention is to avoid or reduce distortion in the frequency response of a filter implemented using balanced gyrators.  
      According to one aspect of the present invention there is provided a balanced gyrator comprising a plurality of interconnected feedforward and feedback MOS single-ended transconductors, balanced inputs and outputs, common mode feedback means coupled respectively between the balanced inputs and outputs, and means for providing each of the transconductors with a non-reciprocal feedback capacitance for rendering reciprocal the feedthrough capacitance of the transconductor thereby neutralising the feedthrough capacitance of the gyrator.  
      According to a second aspect of the present invention there is provided a filter comprising at least one stage including first and second shunt capacitors and a series inductance stage, characterised in that the series inductance stage comprises first and second balanced gyrators and a shunt capacitance and in that each of the first and second gyrators comprises a plurality of interconnected feedforward and feedback MOS single-ended transconductors, balanced inputs and outputs, common mode feedback means coupled respectively between the balanced inputs and outputs, and means for providing each of the transconductors with a non-reciprocal feedback capacitance for rendering reciprocal the feedthrough capacitance of the transconductor thereby neutralising the feedthrough capacitance of the gyrator.  
      According to a third aspect of the present invention there is provided a transceiver having at least one channel filter, the or each channel filter comprising a plurality of balanced gyrators, each balanced gyrator including a plurality of interconnected feedforward and feedback MOS single-ended transconductors, balanced inputs and outputs, common mode feedback means coupled respectively between the balanced inputs and outputs, and means for providing each of the transconductors with a non-reciprocal feedback capacitance for rendering reciprocal the feedthrough capacitance of the transconductor thereby neutralising the feedthrough capacitance of the gyrator.  
      According to a further aspect of the invention there is provided a device comprising a balanced gyrator in accordance with the first aspect of the invention or a filter in accordance with the second aspect of the invention or a transceiver in accordance with the third aspect of the invention. Such a device may be, for example, an integrated circuit. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
       FIG. 1  is a block schematic diagram showing a gyrator comprising a feedback pair of transconductors,  
       FIG. 2  is a diagram of a balanced class AB transconductor comprising MOS transistor pairs and a common-mode feedback circuit,  
       FIG. 3  is a block schematic diagram of a balanced gyrator block comprising two balanced transconductors as shown in  FIG. 2 ,  
       FIG. 4  is a diagram of a MOSFET showing the various intrinsic and extrinsic capacitances between pairs of electrodes,  
       FIG. 5  is a graph illustrating the intrinsic capacitances of the transistors of the transconductor in various operating regions,  
       FIG. 6  is a schematic circuit diagram of a single ended transconductor with an added feedback circuit,  
       FIG. 7  is a block schematic circuit diagram of a balanced gyrator block comprising four of the single ended transconductors shown in  FIG. 6  and common mode feedback stages,  
       FIG. 8  is a block schematic diagram of a fifth order gyrator filter,  
       FIG. 9  illustrates, in broken lines, the frequency response of a fifth order gyrator filter in which the gyrator feedforward capacitances are not neutralised and, in full lines, the frequency response of the fifth order gyrator filter in which the gyrator feedforward capacitances have been neutralised, and  
       FIG. 10  is a block schematic diagram of a transceiver having a polyphase filter employing balanced gyrators made in accordance with the present invention.  
    
    
      In the drawings the same reference numerals have been used to indicate corresponding features.  
     MODES FOR CARRYING OUT THE INVENTION  
      As FIGS.  1  to  5  have already been described in the preamble of the specification they will not be described again.  
      Referring to  FIG. 6  the illustrated single ended transconductor comprises pMOS and nMOS transistors  14 ,  16 , respectively, whose drain electrodes are connected together and whose source electrodes are connected to respective current supply rails V dda  and V ss . The gates of these transistors are connected to a common input terminal  18 .  
      The gate-source capacitance  30  of the pMOS transistor  14 , C gsp , is shown in broken lines between the gate of the transistor  14  and the supply line V dda . Similarly the gate-source capacitance  32  of the nMOS transistor  16 , C gsn , is shown in broken lines between the gate of the transistor  16  and the supply line V ss . The capacitance C dgt  between the interconnected drains and interconnected gates of the transistors  14 ,  16  is shown in broken lines.  
      The illustrated single ended transconductor further comprises an added feedback circuit C f . This feedback circuit C f  comprises a source follower S, pMOS transistor  36 , which is biased by a current source  1 , pMOS transistor  34 , and driven at its gate by the voltage at the transconductor output  22 . The source follower output is connected to the transconductor input  18  by a capacitor C p  formed from the oxide capacitance of a MOS transistor  38 . In the illustrated embodiment the transistor  38  is a PMOS transistor and if the transistor cuts-off due to signal polarity reversal the capacitance is fairly constant because the channel is replaced by the back-gate.  
      In a non-illustrated embodiment a reverse connected nMOS transistor (with its gate connected to the transconductor output  22  and common source-drain connected to the input  18 ) could be used to make the capacitor C p . In that case, it should be biased permanently in its triode region using the source follower V gs , transistor  36 .  
      Reverting to the embodiment as illustrated, when a signal voltage is applied to the transconductor input  18 , current flows by way of the capacitance C dgt  to the transconductor output  22  and by way of the capacitor C p  to the source follower S which routes it harmlessly to the V ss  rail. So:  
               C   ff     =       C   dgt     =         2   5     ⁢     (       C   gsp     +     C   gsn       )       +     C   gdolp     +     C   gdoln                 (   6   )             
 
      When a signal voltage is applied to the transconductor output  22 , current flows by way of the capacitance C gdt  and the capacitor C p  to the transconductor input  18 . So: 
 
 C   fb   =C   gdt   +C   p   =C   gdolp   +C   gdoln   +C   p   (7) 
 
      If C p  is designed so that:  
                 C   p     =       2   5     ⁢     (       C   gsp     +     C   gsn       )         ⁢     
     ⁢     then   ⁢     :               (   8   )                 C   ff     =       C   fb     =     C   f               (   9   )             
 
 i.e. the feedthrough capacitance is now reciprocal. 
 
       FIG. 7  is a block schematic diagram of a balanced gyrator comprising four single-ended transconductors TC 1  to TC 4  of the type shown in  FIG. 6  in which the reciprocal capacitance is modelled by the capacitor C f  and common mode feedback (cmfb) circuits  26  connected across the input and output, respectively. The outputs of the transconductors TC 1  and TC 4  are coupled to inputs to the transconductors TC 3  and TC 2 , respectively. As the balanced inputs  18 ,  19  and outputs  22 ,  23  always experience equal and opposite signal voltages, the currents fed through the capacitors C f  in the forward transconductor pair are always cancelled by the equal and opposite currents fed through the capacitors C f  in the feedback transconductor pair. In other words, the balanced gyrator feedthrough capacitors are self-neutralised. The cmfb circuits  26  serve to provide dc stabilisation.  
      The illustrated balanced gyrator circuit has been found to give a significant improvement to the frequency response of a Gm-C channel filter.  
       FIG. 8  shows a fifth order bandpass filter. The filter is an inductance/capacitance filter consisting of an input resistance R IN , and output resistance R OUT , shunt capacitors C 1 , C 3  and C 5  and series inductances L 1 , L 2 . The inductance L 1  is implemented by balanced gyrators BG 1 , BG 2  and a capacitor C 2  and the inductance L 2  by balanced gyrators BG 3 , BG 4  and a capacitor C 4 , the balanced gyrators BG 3 , BG 4  being constructed in the same manner as the balanced gyrators BG 1 , BG 2 . As the balanced gyrators BG 1  to BG 4  have been described with reference to  FIG. 7 , then in the interests of brevity they will not be described again.  
      This improvement in the frequency response is illustrated in  FIG. 9  in which the broken line frequency response  40  shows the effect of the feedthrough capacitors not being reciprocal, as demonstrated by equation (9) above, and the full line frequency response  42  illustrates the improvement when the capacitors are reciprocal.  
      The value of the capacitance C p  ( FIG. 6 ) may be determined empirically by simulating the filter containing balanced gyrators having single-ended transconductors of the type shown in  FIG. 6  together with the cmfb circuits  26  and varying the size of the transistors  38  until the desired performance is achieved.  
       FIG. 10  illustrates a transceiver in which a polyphase channel filter CF in the receiver section Rx comprises a Gm-C filter based on the fifth order bandpass filter shown in  FIG. 8 . More particularly the polyphase channel filter CF comprises two fifth order bandpass filters, one for each of the quadrature related phases, with the addition of cross branch balanced gyrators coupling corresponding capacitors, that is C 1 , C 1 ; C 2 , C 2  and so on, to create extra susceptance.  
      An antenna  50  is coupled to a low noise amplifier (LNA)  52  in the receiver section Rx. An output of the LNA  52  is coupled by way of a signal divider  54  to first inputs of quadrature related mixers  56 ,  58 . A local oscillator signal generated by a signal generator  60  is applied to a second input of the mixer  56  and, by way of a ninety degree phase shifter  62 , to a second input of the mixer  58 . Quadrature related outputs  1 , Q, respectively, from the mixers  56 ,  58  are applied to the polyphase channel filter CF which passes the wanted quadrature related signals to respective analogue-to-digital converters  62 ,  64 . The digital outputs from the A-to-D converters  62 ,  64  are applied to a digital demodulator  66  which provides an output signal on a terminal  68 .  
      The transmitter Tx comprises a digital modulator  70  which includes a digital-to-analogue converter (not shown) providing an analogue signal to a mixer  72  for frequency up-conversion to the required transmission frequency. A power amplifier  74  amplifies the frequency up-converted signal and supplies it to the antenna  50 .  
      The transceiver including the channel filter CF may be fabricated as an integrated circuit using known low voltage CMOS processes.  
      In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.  
     INDUSTRIAL APPLICABILITY  
      Electronic circuits comprising a gyrator, such as gyrator filters and integrated transceivers including gyrators.