86 A Brief Review of Technologyis generating noise half thermally. Note that any resistor in thermal equilibrium must generate 4kTR of noise voltage. However, a conducting pn junction is not in thermal equilibrium, and power is added, so it is allowed to break the rules.4.9 Noise Sources in the Transistor ModelHaving discussed the various noise sources in a bipolar transistor, the model for these noise sources can now be added to the transistor model. The noise sources in a bipolar transistor can be shown as in Figure 4.12. These noise sources can also be added to the small-signal model as shown in Figure 188.8.131.52 Bipolar Transistor Design ConsiderationsFor highest speed, a bias current near peak fT is suggested. However, it can be noted from Figure 4.8 that the peaks are quite wide. fT drops by 10% of its peak value Figure 4.11 Noise model of base shot noise.Figure 4.12 Transistor with noise models: (a) with base series noise source and (b) with base paral-lel noise source.