# Transistor Collector Feedback Bias

In Figure, the base resistor RB is connected to the collector rather than to VCC, as itÂ was in the base bias arrangement discussed earlier. The collector voltage provides the biasÂ for the base-emitter junction. The negative feedback creates an âoffsettingâ effect thatÂ tends to keep the Q-point stable. If IC tries to increase, it drops more voltage across RC,Â thereby causing VC to decrease. When VC decreases, there is a decrease in voltage acrossÂ RB, which decreases IB. The decrease in IB produces less IC which, in turn, drops less voltage across RC and thus offsets the decrease in VC.

Analysis of a Collector-Feedback Bias CircuitÂ

By Ohmâs law, the base current can beÂ expressed as

Â

Q-Point Stability Over Temperature

Above Equation Â shows that the collector current isÂ dependent to some extent on Î²DC and VBE. This dependency, of course, can be minimizedÂ by making RC >>Â RB/Î²DC and VCC >>Â VBE. An important feature of collector-feedbackÂ bias is that it essentially eliminates the Î²DC and VBE dependency even if the stated conditions are met.

As you have learned, Î²DC varies directly with temperature, and VBE varies inverselyÂ with temperature. As the temperature goes up in a collector-feedback circuit, Î²DC goesÂ up and VBE goes down. The increase in Î²DC acts to increase IC. The decrease in VBEÂ acts to increase IB which, in turn also acts to increase IC. As IC tries to increase, theÂ voltage drop across RC also tries to increase. This tends to reduce the collector voltageÂ and therefore the voltage across RB, thus reducing IB and offsetting the attempted increase in IC and the attempted decrease in VC. The result is that the collector-feedbackÂ circuit maintains a relatively stable Q-point. The reverse action occurs when the temperature decreases.