<< Graphics`Graphics`

dB[z_] = 20 Log[10, Abs[z]];      

    dg[z_] = Im[Log[z]]/Degree;

example 6.9  p.113

gw = k/(1 + s) /. {k ->  1, s ->   I wT }

LogLinearPlot[dB[gw], {wT, 0.01, 100}, GridLines ->  Automatic, 
      PlotRange ->  {{0.01, 100}, {-40, 20}}]

LogLinearPlot[dg[gw], {wT, 0.01, 100}, GridLines ->  Automatic, 
      PlotRange ->  {{0.01, 100}, {-90, 45}}, 
      PlotPoints ->  500]

memo for additional study

gs1 = 1/(1 + s)
gs2 = (1 + 10 s)/(1 + s)
gs3 = gs1 gs2

gw1 = gs1 /. s ->  I w
gw2 = gs2 /. s ->  I w
gw3 = gs3 /. s ->  I w

LogLinearPlot[{dB[gw1], dB[gw2], dB[gw3]}, {w, 0.001, 100}, 
      GridLines ->  Automatic, 
      PlotRange ->  {{0.001, 100}, {-60, 40}}, 
      PlotPoints ->  500, 
      PlotStyle ->  {Hue[0.9], Hue[0.7], Hue[0.5]}]

LogLinearPlot[{dg[gw1], dg[gw2], dg[gw3]}, {w, 0.001, 100}, 
      GridLines ->  Automatic, 
      PlotRange ->  {{0.001, 100}, {-90, 60}}, 
      PlotPoints ->  500, 
      PlotStyle ->  {Hue[0.9], Hue[0.7], Hue[0.5]}]