Transistor Collector Feedback Bias

In Figure, the base resistor R_B is connected to the collector rather than to V_CC, as it was in the base bias arrangement discussed earlier. The collector voltage provides the bias for the base-emitter junction. The negative feedback creates an “offsetting” effect that tends to keep the Q-point stable. If I_C tries to increase, it drops more voltage across R_C, thereby causing V_C to decrease. When V_C decreases, there is a decrease in voltage across R_B, which decreases I_B. The decrease in I_B produces less I_C which, in turn, drops less voltage across R_C and thus offsets the decrease in V_C.

Analysis of a Collector-Feedback Bias Circuit 

By Ohm’s law, the base current can be expressed as

Q-Point Stability Over Temperature

Above Equation  shows that the collector current is dependent to some extent on β_DC and V_BE. This dependency, of course, can be minimized by making R_C >> R_B/β_DC and V_CC >> V_BE. An important feature of collector-feedback bias is that it essentially eliminates the β_DC and V_BE dependency even if the stated conditions are met.

As you have learned, β_DC varies directly with temperature, and V_BE varies inversely with temperature. As the temperature goes up in a collector-feedback circuit, β_DC goes up and V_BE goes down. The increase in β_DC acts to increase I_C. The decrease in V_BE acts to increase I_B which, in turn also acts to increase I_C. As I_C tries to increase, the voltage drop across R_C also tries to increase. This tends to reduce the collector voltage and therefore the voltage across R_B, thus reducing I_B and offsetting the attempted increase in I_C and the attempted decrease in V_C. The result is that the collector-feedback circuit maintains a relatively stable Q-point. The reverse action occurs when the temperature decreases.