Negative feedback is a fundamental concept in electronics, particularly in amplifier design, used to improve stability and performance. It works by taking a portion of the output signal and feeding it back to the input, out of phase with the original input signal. This process helps to correct deviations and stabilize the amplifier's operation. Let us analyze each statement to determine which one is false regarding the improvements brought about by negative feedback.

Negative Feedback and Bandwidth

One of the significant advantages of applying negative feedback to an amplifier is the increase in its bandwidth. While negative feedback reduces the overall gain of the amplifier, it extends the frequency range over which the amplifier operates effectively. The gain-bandwidth product often remains constant, meaning that a reduction in gain leads to an increase in bandwidth.

If the open-loop gain of an amplifier is \( A_{OL} \) and the feedback factor is \( \beta \), the closed-loop gain \( A_{CL} \) is given by:

\( A_{CL} = \frac{A_{OL}}{1 + A_{OL}\beta} \)

The upper cutoff frequency \( f_{H(CL)} \) with feedback is approximately:

\( f_{H(CL)} = f_{H(OL)}(1 + A_{OL}\beta) \)

Where \( f_{H(OL)} \) is the open-loop upper cutoff frequency. This clearly shows that bandwidth is increased. Therefore, the statement "Increased bandwidth" is true.

Noise Reduction with Negative Feedback

Negative feedback is highly effective in reducing internally generated noise within an amplifier circuit. By continuously comparing the output with the input and making corrections, any unwanted signals (like noise originating from the amplifier components) are also attenuated. However, it's important to note that negative feedback does not reduce noise that is already present in the input signal before it enters the amplifier. It primarily reduces noise and interference generated within the amplifier stages themselves. Therefore, stating that negative feedback leads to "Increased noise" is false; it actually decreases noise generated by the amplifier.

Distortion Mitigation by Negative Feedback

Distortion, especially non-linear distortion, is a major problem in amplifiers where the output signal is not a perfect replica of the input due to the non-linear characteristics of the active devices. Negative feedback significantly reduces all types of distortion (harmonic, intermodulation, etc.) because it forces the output to closely follow the input. Any deviation from the desired linear response is fed back and corrected, thereby linearizing the amplifier's operation.

The distortion with feedback \( D_{CL} \) is reduced by the factor \( (1 + A_{OL}\beta) \):

\( D_{CL} = \frac{D_{OL}}{1 + A_{OL}\beta} \)

Where \( D_{OL} \) is the open-loop distortion. This confirms that distortion is decreased. Therefore, the statement "Decreased distortion" is true.

Impedance Modification by Negative Feedback

Negative feedback also plays a crucial role in modifying the input and output impedances of an amplifier, making it more practical for various applications. Depending on the feedback topology (e.g., voltage-series, current-series, voltage-shunt, current-shunt), negative feedback can:

• Significantly increase the input impedance, which helps in minimizing the loading effect on the source.
• Significantly decrease the output impedance, which allows the amplifier to drive various loads more effectively without a significant drop in output voltage.

This characteristic makes the statement "High input impedance but lower output impedance" generally true for many common negative feedback configurations, especially those aimed at voltage amplification.

Conclusion

Based on the analysis of the effects of negative feedback:

• Increased bandwidth: True
• Increased noise: False (Negative feedback decreases internally generated noise)
• Decreased distortion: True
• High input impedance but lower output impedance: True

Therefore, the false statement among the given options is "Increased noise".