Page 54 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications
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ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7




          underlying  device  technology.  The  PAE  can  be   network.  Moreover,  as  one  moves  from  LmmW
          expressed in terms of several factors.               bands at 60 GHz to the UmmW bands at 140 and
                                                               220 GHz,  the  optimum  conduction  angle  moves
                              1       V K    Q o
                 PAE = η (1 − ) (1 −     ) (    )    (3)       away from class B bias towards the class A bias. The
                              G      V DD  Q o +Q t
                                                               maximum possible PAE drops from more than 40%
          The  PAE  depends  on  the  drain  efficiency,  ,  the   to  30%  at  140  GHz.  When  the  PA  design  targets
          operating  gain  of  the  PA  (G),  the  knee  (VK)  and   220 GHz, the maximum PAE becomes around 17%.
          supply  voltage  (VDD)  of  the  device,  and  the  loss
          factor for matching the load line of the device to the
          load impedance, which is expressed above in terms
          of a impedance transformation quality factor, (Qt),
          and   passive   element   quality   factor   (Qo).
          Consequently,  the  knee  voltage  relative  to  the
          supply voltage imposes a penalty on the available
          PAE.  Additionally,  the  impedance  transformation
          between  the  load  line  of  the  transistor  and  the
          output  impedance,  e.g.  50  Ohms,  reduces  the
          maximum efficiency.

          The drain efficiency is determined by the biasing of
          the PA as well as the harmonic tuning at the load.
          With only matching of the load line of the device to
          the  load  and  no  additional  voltage  waveform     Fig.  3  -  PAE  as  a  function  of  power  amplifier  conduction
                                                                angle  for  upper  millimeter-wave  frequencies.  (fmax/fT  =
          shaping at the PA output, the gate bias determines    400 GHz, VK = 0.7, VDD = 2.5, Q = 10).
          the  drain  efficiency  as  a  class  of  operation.  The
          conduction angle, , of the drain current captures   We can also compare output power requirements in
          the maximum drain efficiency. When the device is     the previous section for 20 dBm and 10 dBm output
          conducting during the entire period  ( = 2π), the   power. At 60 GHz, the lower power PA is capable of
          transistor  is  operating  in  class  A  with  maximum   7%  better  efficiency.  However,  once  we  reach
          drain efficiency of 50%. If the bias is reduced such   220 GHz, the benefit of the reduced output power is
          that the transistor conducts half the time ( = π/2),   smaller. The difference in the PAE achievable with
          the drain current is class B and the maximum drain   different power levels is attributed to the change in
          efficiency  increases  to  78%.    Unfortunately,  the   the  impedance matching networks and additional
          reduced  conduction  in  class  B  also  reduces  that   losses.  Consequently,  the  dominant  performance
          transistor gain. Conduction angles between class A   limitation on PAE for UmmW PAs is the available
          and B are referred to as AB.                         gain  to  realize  high  efficiency  at  the  moderate
                                                               output  powers  described  in  Section  2.  We  will
          The available power gain produces a limitation in    investigate  approaches to improve the gain while
          PAE at bands near the maximum cutoff frequency of    optimizing  the  PAE  factors  in  (3)  in  the  next
          the  transistor,  fmax.  For  PAs  operating  above   two sections.
          100 GHz,  fmax  is  often  not  much  larger  than  the
          frequency of operation based on currently available   4.   UMMW SEMICONDUCTOR
          device technologies. The available gain is, therefore,     TECHNOLOGY COMPARISON
          limited and the class of operation can be chosen as   We can study approximate parameters of available
          part of the optimization process.
                                                               processes to understand the PAE limit in (3) with
          Fig. 3 indicates the theoretical PAE as a function of   different  trade-offs  in  terms  of  available  gain,
          the conduction angle. Note that several parameters   voltage  handling  requirements,  and  load-line
          in (3) are functions of  including the shape factor,   matching conditions for a given matching or output
          but also the gain and impedance matching. As the    power condition.
          reduces  from  class  A  to  class  B  and  beyond  into   We assume that the passive elements have similar
          class C, the PAE increases and then collapses as the   quality  factor.  Table  1  illustrates  sample
          gain  drops.  Notably,  several  factors  in  the  PAE   characteristics of different transistor technologies
          change  as  a  function  of  the  .  The  load-line   that  are  available  for  operation  above  100  GHz
          impedance  increases  the  loss  of  the  matching   in III-V and SiGe/SOI CMOS technologies. Si CMOS





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