It is usual to employ a single source, that of the output circuit, to energize a transistor stage EC. For the transistor to operate normally, it is essential that a d. с voltage of a few tenths of a volt
(UBE = 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 EC is dropped across R1, and the smaller part, serving as the bias voltage UBE is dropped across R2 which Bias Supply and Temperature Compensation for Transistors is placed in shunt with the transistor input.
The d. с. blocking capacitor Cb serves to pass the a. c. voltage to be amplified to the transistor input. More often, Cb is chosen to have a capacitance of several microfarads or tens of microfarads. For this reason, Cb 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 RE sets up negative feedback in terms of direct current. Should a rise in temperature cause the currents in Bias Supply and Temperature Compensation for Transistors the transistor to rise, the increase in IE0 will raise the emitter voltage UE, and lower the base bias voltage UBE in proportion, thus leading to a fall in the currents. The resistor RE is placed in shunt with a sufficiently high-valued capacitor CE so as to prevent it from setting up negative feedback in terms of alternating current.
Neglecting UBE in comparison with the other voltages, the resistance values for the emitter compensation scheme can be calculated, using the following approximate equations:
R1 ≈ (ЕC – UE)/(IB + I1); R2 ≈ UE/I1; RE = UE/IE .
A Bias Supply and Temperature Compensation for Transistors further requirement is to choose the value of UE in view of the likely increase in EC. The divider current is usually anywhere between three to five times IB0.