Collector to Base Bias

Collector to Base Bias

Figure shows the dc bias with voltage feedback. It is also called as collector to base bias circuit. It is an improvement over fixed bias method. In this, biasing resistor is connected between collector and base of the transistor to provide feedback path.

Circuit analysis:

Base circuit:

Consider the base circuit and applying voltage law then we get,

Only the difference between the equation for I_B and that obtained for fixed bias configuration is βR_C, so the feedback path results in a reflection of the resistance R_C to the input circuit.

Collector circuit:

Applying KVL to the collector circuit,

V_CC – (I_C + I_B) R_C – V_CE = 0

V_CE = V_CC – (I_C + I_B) R_C

If there is a change in β due to piece to piece variation between transistors or if there is a change in β and I _CO due to the change in temperature. So collector current tends to increase. As a result, voltage drop across R_C increases. Due to reduction in V_CE, I_B reduces. The result is that the circuit tends to maintain a stable value of collector current, keeping the Q point fixed.

In this circuit, R_B appears directly across input and output. A part of output is feedback to the input. And increase in collector current decreases the base current. So negative feedback exists in the circuit. It is also called as voltage feedback bias circuit.