Collector-to-base bias:

Figure: Collector-to-base bias

This configuration uses negative feedback to avoid thermal runaway and stabilize the operating point. In this category of biasing, the base resistor R_B is associated to the collector in place of connecting it to the DC source V_cc. Thus any thermal runaway will induce a voltage drop across the R_C resistor which will throttle the transistor's base current.

From voltage law of Kirchhoff, the voltage V_Rb across the base resistor R_b is

By the Ebers-Moll model, I_c = βI_b, and thus:

From law of Ohm, the base current I_b = V_Rb/R_b, and thus

Therefore, the base current I_b is

I_b = V_cc - V_be/ (R_b+ (β+1) R_C)

If V_be is held stable and temperature get increases, after that the collector current I_c increases. Though, a larger I_c causes the voltage drop across resistor R_c to increase that in turn decreases the voltage V_Rb across the base resistor R_b. A lower base-resistor voltage drop decreases the base current I_b that results in less collector current I_c. Since an increase in collector current with temperature is opposed, the operating point is reserved stable.