Bias Supply and Temperature Compensation for Transistors

It is usual to employ a single source, that of the output circuit, to energize a transistor stage E_C. For the transistor to operate normally, it is essential that a d. с voltage of a few tenths of a volt

(U_BE = 0.7V), known as the base bias voltage, be maintained between the emitter and base.

Figure 4.16,shows how the bias voltage is derived with the aid of a divider R₁R₂ in a CE stage. Here, the greater proportion of E_C is dropped across R₁, and the smaller part, serving as the bias voltage U_BE is dropped across R₂ which is placed in shunt with the transistor input.

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The d. с. blocking capacitor C_b serves to pass the a. c. voltage to be amplified to the transistor input. More often, C_b is chosen to have a capacitance of several microfarads or tens of microfarads. For this reason, C_b in low-frequen­cy circuits is usually a small-sized electrolytic capacitor.

Fig. 4.16. Base biasing circuit and temperature compensation of a transistor

The scheme shown in Fig. 4.16 be called the emitter temperature-stabilized scheme. The resistor R_E sets up negative feedback in terms of direct current. Should a rise in temperature cause the currents in the transistor to rise, the increase in I_E0 will raise the emitter voltage U_E, and lower the base bias voltage U_BE in proportion, thus leading to a fall in the currents. The resistor R_E is placed in shunt with a sufficiently high-valued capacitor C_E so as to prevent it from setting up negative feedback in terms of alternating current.

Neglecting U_BE in comparison with the other voltages, the resistance values for the emitter compensation scheme can be calculated, using the following approximate equations:

R₁ ≈ (Е_C – U_E)/(I_B + I₁); R₂ ≈ U_E/I₁; R_E = U_E/I_E .

A further requirement is to choose the value of U_E in view of the likely increase in E_C. The divider current is usually anywhere between three to five times I_B0.

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