Noise modeling in BJTs: How many fundamental noise sources are typically modeled for a bipolar junction transistor in small-signal analysis?

Introduction / Context:
Accurate noise modeling is crucial for low-noise amplifiers and precision analog design. For small-signal BJTs, an equivalent input-referred noise model captures the essential random processes with a minimal set of sources.

Given Data / Assumptions:

• Small-signal, midband operation.
• Equivalent noise model uses input-referred sources.
• Low-frequency 1/f noise may be considered separately.

Concept / Approach:
In the widely used two-source model, a BJT is represented by an input-referred noise voltage source (primarily due to base resistance thermal noise and correlated effects) in series with the input, and an input-referred noise current source (primarily due to base/collector shot noise) in parallel with the input. These two sources are sufficient to predict output noise with source resistance and circuit gain.

Step-by-Step Solution:

Verification / Alternative check:
Many texts present more detailed physical sources (thermal, shot, flicker), but the input-referred representation condenses them to two uncorrelated sources for most design calculations.

Why Other Options Are Wrong:

Common Pitfalls:

Final Answer: