DC Levels and Loop Equations

Now look at Fig. 2.3. In designing BJT circuits, it is often important to ascertain DC voltage levels at the base (V_B), emitter (V_E) and collector (V_C). For this figure, these DC levels are determined by applying the following loop equations:

V_B=V_CC - I_BR_B

where V_CC in the figure is 10V.

$$V_E=V_B - V_{BE} \approx V_B- 0.7 = I_ER_E$$ (19)

V_CC - I_BR_B - 0.7 - I_ER_E = 0

Since $\beta \gg 1$, $I_E \approx I_C$, we can use the following expression for the collector voltage:

$$V_C=V_{CC} - I_CR_C \approx V_{CC} - I_ER_C$$ (21)