Which techniques stabilize bias against temperature variations in BJTs?

• A. Temperature compensation alone is sufficient.
• B. Emitter degeneration, negative feedback, and temperature compensation.
• C. Increasing supply voltage to counter drift.
• D. Using PNP transistors exclusively.

Answer: (B)

Temperature changes cause the base-emitter voltage of a BJT to drift, which would push the operating point unless the circuit counteracts it. The most effective approach combines three ideas that work together.

Emitter degeneration adds a resistor in the emitter so that as current increases, the resulting voltage drop reduces the effective base-emitter drive. This built-in negative feedback stabilizes the collector current against variations in transistor parameters and temperature, making the bias less sensitive to Vbe shifts.

Negative feedback in the biasing network or circuit path further reduces sensitivity to exact device values. By feeding a portion of the output back to the input, the bias point becomes governed more by the feedback conditions than by the individual transistor’s characteristics, which helps keep the operating point stable as temperature changes.

Temperature compensation deliberately uses components that track temperature in a similar way to the transistor’s Vbe, such as diodes or thermally matched transistors, to offset the Vbe drift. This counteracts the inherent bias drift caused by temperature and helps maintain a steadier bias.

Relying on temperature compensation alone doesn’t fully fix bias drift without feedback to limit current, and simply increasing supply voltage doesn’t stabilize the bias point. Using PNP transistors alone doesn’t address the underlying temperature-driven changes in Vbe and gain. The combination of emitter degeneration, negative feedback, and temperature compensation provides a robust way to keep the bias stable across temperatures.