Subject: Logic Design

Topic: Biasing of BJT

Difficulty: Hard

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 IB and that obtained for fixed bias configuration is βRC, so the feedback path results in a reflection of the resistance RC 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 ICO due to the change in temperature. So collector current tends to increase. As a result, voltage drop across RC increases. Due to reduction in VCE, IB reduces. The result is that the circuit tends to maintain a stable value of collector current, keeping the Q point fixed.

In this circuit, RB 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.