Unit-5 BIASING AND STABILISATION:BJT biasing, DC equivalent model, criteria for fixing operating point, methods of Bias stabilization, Stabilization factors,Compensation techniques, Thermal run away, Thermal stability.

Biasing of BJT Transistors must be biased because They must operate in the active region, and Their small-signal model parameters are set by the bias conditions.

DC Analysis vs. Small-Signal Analysis Firstly, DC analysis is performed to determine the DC operating point and to obtain the small- signal model parameters. Secondly, independent sources are set to zero and the small-signal model is used.

Simplified Notation Hereafter, the voltage source that supplies power to the circuit is replaced by a horizontal bar labeled V CC, and input signal is simplified as one node labeled v in.

Example of Bad Biasing The microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. Unfortunately, there is no DC bias current running through the transistor to set the transconductance.

Biasing with Base Resistor Assuming a constant value for V BE, one can solve for both I B and I C and determine the terminal voltages of the transistor. However, the bias point is sensitive to variations.

Self-Biasing Technique This bias technique utilizes the collector voltage to provide the necessary V x and I B. One important characteristic of this approach is that the collector has a higher potential than the base, thus guaranteeing active-mode operation of the BJT

Self-Biasing Design Guidelines (1) provides insensitivity to. (2) provides insensitivity to variation in VBE

Summary of Biasing Techniques