Noise modeling in BJTs: How many fundamental noise sources are typically modeled for a bipolar junction transistor in small-signal analysis?
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A3
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B2
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C1
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D4
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E5
Answer
Correct Answer: 2
Explanation
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:
Represent base spreading resistance thermal noise as an equivalent voltage source v_n.Represent base/collector junction shot noise as an equivalent current source i_n at the input.Use standard noise analysis to compute total output noise given source resistance and transfer functions.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:
1: inadequate to capture both voltage and current noise contributions.3 or 4: overcounts relative to the standard compact two-source representation used in design.Common Pitfalls:
Confusing physical processes (shot, thermal, flicker) with the simplified input-referred model (two sources).Final Answer:
2