Noise Statement Analysis

This section analyzes the given statements regarding electronic noise to identify the incorrect ones.

Statement 1: Shot Noise vs. Thermal Noise Spectral Density

Statement: The shot noise has a uniform spectral density like thermal noise.

Analysis: This statement is correct. Both shot noise and thermal noise are considered 'white' noise sources, meaning their power spectral density is approximately uniform over a wide frequency range.

• Shot noise arises from the discrete nature of charge carriers.
• Thermal noise arises from the random thermal agitation of charge carriers.
• Both exhibit a constant spectral density ($S_I(f) = \text{constant}$) up to high frequencies.

Statement 2: Shot Noise Proportionality to Output Current

Statement: For the amplifying devices, the shot noise is inversely proportional to the output current.

Analysis: This statement is incorrect. Shot noise is generally directly related to the DC current flowing through the device, not inversely proportional.

• For many semiconductor devices like diodes and BJTs, the mean-square shot noise current is proportional to the DC current ($I$). For instance, the shot noise current ($I_{n, \text{shot}}$) is often given by $ \langle I_{n, \text{shot}}^2 \rangle = 2qI \Delta f $, where $q$ is the elementary charge and $\Delta f$ is the bandwidth.
• Therefore, shot noise typically increases with (or is proportional to the square root of) the DC current, rather than decreasing.

Statement 3: Partition Noise Comparison (Diode vs. Transistor)

Statement: The partition noise in a diode will be higher than that in a transistor.

Analysis: This statement is incorrect. Partition noise occurs when current splits between multiple paths, which is more significant in transistors than in simple diodes.

• Diodes: Current flows primarily in one direction across the junction. Significant current splitting doesn't occur, making partition noise negligible.
• Transistors (BJTs): The emitter current ($I_E$) splits into base current ($I_B$) and collector current ($I_C$). This splitting gives rise to partition noise.
• The random fluctuation in the ratio $I_C / I_E$ constitutes partition noise. Hence, partition noise is a relevant consideration for transistors but not typically for simple diodes.

Conclusion

Based on the analysis:

• Statement 1 is correct.
• Statement 2 is incorrect.
• Statement 3 is incorrect.

The incorrect statements are 2 and 3.