newcommand{subfigimg}[3][,]{%
  setbox1=hbox{includegraphics[#1]{#3}}% Store image in box
  leavevmoderlap{usebox1}% Print image
  rlap{hspace*{5pt}raisebox{.5baselineskip}{sffamilyfootnotesize{#2}}}% Print label
  phantom{usebox1}% Insert appropriate spacing
}

newcommand{modsubfigimg}[2][,]{%
  setbox1=hbox{#1}% Store image in box
  leavevmoderlap{usebox1}% Print image
  rlap{hspace*{5pt}raisebox{.5baselineskip}{small{#2}}}% Print label
  phantom{usebox1}% Insert appropriate spacing
}

begin{minipage}[b]{.5textwidth}
tikzsetnextfilename{PD-PWM_carrier_band}%
    modsubfigimg[(a)]{input{gfx/PDPWM_carrier_band.tikz}}
end{minipage}%
begin{minipage}[b]{.5textwidth}
tikzsetnextfilename{PS-PWM_carrier_band}%
    modsubfigimg[(b)]{input{gfx/PSPWM_carrier_band.tikz}}
end{minipage}