Which of the following statements is INCORRECT regarding the input impedance and bandwidth of a feedback amplifier?

Explanation:

Feedback Amplifiers

Definition: A feedback amplifier is an electronic amplifier that uses feedback to control its gain, bandwidth, and other performance characteristics. Feedback can be positive or negative, and it involves returning a portion of the amplifier's output signal to its input through a feedback network.

Working Principle: In a feedback amplifier, a fraction of the output signal is fed back to the input. Depending on the type of feedback, this signal can either reinforce (positive feedback) or oppose (negative feedback) the input signal. Negative feedback is commonly used to stabilize the amplifier, improve linearity, increase bandwidth, and modify input/output impedance.

Correct Option Analysis:

The correct option is:

Option 3: Positive feedback increases gain, but bandwidth remains almost unchanged.

This statement is incorrect. Positive feedback indeed increases the gain of the amplifier, but it does not leave the bandwidth unchanged. Instead, positive feedback tends to reduce the bandwidth of the amplifier. This is because the amplified signal, when fed back in phase with the input signal, increases the overall loop gain, which can lead to instability and can amplify noise or undesired frequencies. Therefore, the bandwidth is adversely affected when positive feedback is applied.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: If the feedback signal is returned to the input in series opposition with the applied signal, the input impedance increases.

This statement is correct. In series feedback (negative feedback), the feedback signal is added in opposition to the input signal, and this configuration increases the input impedance of the amplifier. The opposition of the feedback signal reduces the effective input current, which in turn increases the input impedance.

Option 2: If the feedback signal is returned to the input in shunt with the applied signal, the input impedance decreases.

This statement is correct. In shunt feedback (negative feedback), the feedback signal is applied in parallel with the input signal, and this configuration decreases the input impedance of the amplifier. The parallel connection effectively reduces the overall resistance seen at the input.

Option 4: If the feedback signal is subtracted from the applied signal, the amplifier's bandwidth increases.

This statement is correct. Negative feedback, where the feedback signal is subtracted from the input signal, increases the amplifier's bandwidth. This is because negative feedback reduces the gain at low frequencies, which allows the amplifier to maintain a more consistent gain across a wider range of frequencies.

Conclusion:

Understanding the effects of feedback on amplifier characteristics such as gain, input/output impedance, and bandwidth is crucial. Negative feedback is widely used in amplifiers to improve stability, linearity, and bandwidth, while positive feedback is generally used in oscillators or specific applications where gain enhancement is desired but at the cost of reduced bandwidth and potential instability. The incorrect statement in this case is option 3, as positive feedback not only increases gain but also reduces the bandwidth of the amplifier.