Operating Point Stabilisation

Negative feedback enhances a transistor amplifier's stability by countering fluctuations in temperature, voltage, and amplification. It works by reversing a portion of the output voltage and feeding it back to the input. In negative current feedback, the emitter resistor's amplified current generates the reverse-phase voltage for feedback. Conversely, negative voltage feedback sends a portion of the amplified collector voltage back to the transistor's base. While negative feedback boosts stability and linearizes the characteristic curve, it also reduces amplification. The larger the feedback-to-output voltage ratio, the greater the amplification decrease. Understanding the stabilising effect involves considering a constant control voltage at the amplifier's output, divided between the base-emitter line and the emitter resistor. If the emitter current increases due to temperature or voltage changes, the voltage at the emitter resistor also rises, reducing the control voltage's effectiveness. This counters the original collector current increase. The same principle applies with alternating voltage, but connecting a capacitor in parallel with the emitter resistor cancels negative feedback for alternating voltage, not direct current feedback.

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