What is the relationship between the base-emitter and collector-emitter voltages of a bipolar transistor?

Generally, the current that flows from the emitter to the collector or vice versa is the base current times the DC current gain (hFE). Note, however, that hFE varies with the collector-emitter voltage (VCE). For example, Table 1 shows the hFE specification for the 2SC2712. Figure 1 shows its IC–VCE curves. When IB = 0.5 mA, the collector current (IC) is 70 mA at a VCE of 3 V (point A). At this point, the DC current gain is calculated to be 140. However, the collector current (IC) is 40 mA at a VCE of 0.15 V (point B), at which the DC current gain is calculated to be 80.

In addition, hFE is also affected by ambient temperature as indicated by Figure 2.

Table 1  Electrical Characteristics (2SC2712)
Table 1  Electrical Characteristics (2SC2712)
Figure 1 I<sub>C</sub> – V<sub>CE</sub> curves (2SC2712)
Figure 1 IC – VCE curves (2SC2712)
Figure 2 h<sub>FE</sub> – I<sub>C</sub> curves (2SC2712)
Figure 2 hFE – IC curves (2SC2712)