Amplification by a Transistor

The grapho-analytical procedure is based on dynamic or load characteristics.

Fig. 4.17. Grapho-analytic study of a transistor's operation at load, using its output and input characteristics

On a family of output characteristics (Fig. 4.17a), the load line (LL) is constructed using specified or chosen values of the E_C supply voltage and load resistance R_C .

Because the output circuit satisfies the equa­tion:

Е_C = U_CE + I_CR_C ,

the load line is constructed, using its inter­sections with the coordinate axes.

The shaded area in Fig. 4.17a is what may be called the available power triangle. Its hypo­tenuse is the operating region AB, and its sides are, respectively, the double amplitude of cur­rent, 2I _Cm, and of voltage 2U_CEm. The area of the triangle is four times the available power, 2I_CmU_CEm.

The operating point Q₁ also defines the d.c. base voltage U_BEQ1. Knowing U_BEQ1 and assuming approximately that the direct component of base current in the dynamic operation is I_BQ1, it is an easy matter to calculate the resistance of the swamping resistor R_B (Fig.4.17c)via which a direct voltage will be applied from the E_C source to the base:

.

The stage current, voltage, and power gains can then be found by the usual equations:

K_I = I_Cm /I_Bm; K_U= U_CEm / U_BEm ; K_p=K_IK_U.

Figure 4.18 shows a shaded area-it represents the operating region for a CE circuit.

Fig. 4.18. Feasible region of a transistor

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