The signal source switch of Fig. 3-9(*a*) is closed, and the transistor base current becomes

i_B = I_{BQ} + i_b = 40 + 20 \sin ωt μ\text{A}

The collector characteristics of the transistor are those displayed in Fig. 3-9(*b*). If V_{CC} = 12 \text{V} and R_{dc} = 1 kΩ, graphically determine (*a*) I_{CQ} and V_{CEQ}, (*b*) i_c and v_{ce}, and (c) h_{FE}(= β) at the Q point.

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(*a*) The dc load line has ordinate intercept V_{CC}/R_{dc} = 12 \text{mA} and abscissa intercept V_{CC} = 12 \text{V} and is constructed on Fig. 3-9(*b*). The Q point is the intersection of the load line with the characteristic curve i_B = I_{BQ} = 40 μ\text{A}. The collector quiescent quantities may be read from the axes as I_{CQ} = 4.9 \text{mA} and V_{CEQ} = 7.2 \text{V}.

(*b*) A time scale is constructed perpendicular to the load line at the Q point, and a scaled sketch of i_b = 20 \sin ωt μ\text{A} is drawn [see Fig. 3-9(*b*)] and translated through the load line to sketches of i_c and v_{ce}. As i_b swings ±20 μ\text{A} along the load line from points a to b, the ac components of collector current and voltage take on the values

i_c = 2.25 \sin ωt \text{mA} \quad \text{and} \quad v_{ce} = -2.37 \sin ωt \text{V}

The negative sign on v_{ce} signifies a 180° phase shift.

(*c*) From (*3.2*) with I_{CEO} = 0 [the i_B = 0 curve coincides with the v_{CE} axis in Fig. 3-9(*b*)],

β(≡ h_{FE}) ≡ \frac{α}{1 – α} ≡ \frac{I_C – I_{CEO}}{I_B} (*3.2*)

h_{FE} = \frac{I_{CQ}}{I_{BQ}} = \frac{4.9 × 10^{-3}}{40 × 10^{-6}} = 122.5

It is clear that amplifiers can be biased for operation at any point along the dc load line. Table 3-4 shows the various classes of amplifiers, based on the percentage of the signal cycle over which they operate in the linear or active region.

Table 3-4

Class | Percentage of Active-Region Signal Excursion |

A | 100 |

AB | between 50 and 100 |

B | 50 |

C | less than 50 |

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