Below is a ~2900-character, exam-ready answer comparing all four types of feedback amplifiers with respect to gain (Aᵥ), bandwidth, input resistance (Rᵢ), and output resistance (Rₒ).
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Feedback amplifiers are classified based on (i) the quantity sampled at the output (voltage or current) and (ii) the method of mixing the feedback signal at the input (series or shunt). Accordingly, there are four basic types of negative feedback amplifiers: Voltage–Series, Voltage–Shunt, Current–Series, and Current–Shunt. Negative feedback has a significant effect on amplifier parameters such as gain, bandwidth, input resistance, and output resistance.
1. Voltage–Series Feedback Amplifier (Series–Shunt)
In this type, a fraction of the output voltage is sampled and fed back in series with the input signal. This configuration is commonly used as a voltage amplifier. Due to negative feedback, the overall voltage gain (Aᵥ) decreases, but the bandwidth increases because feedback flattens the frequency response. Since the feedback is applied in series at the input, the input resistance (Rᵢ) increases, making it suitable for high-impedance signal sources. Sampling the output voltage in shunt causes the output resistance (Rₒ) to decrease, which improves voltage regulation.
2. Voltage–Shunt Feedback Amplifier (Shunt–Shunt)
Here, the output voltage is sampled and the feedback is applied in shunt with the input. This configuration is also known as a transresistance (voltage-controlled current) amplifier. As with all negative feedback systems, the gain decreases while the bandwidth increases. Because the feedback is mixed in shunt at the input, the input resistance decreases. Since the output voltage is sampled, the output resistance decreases. This type is suitable where low input and low output resistances are desired.
3. Current–Series Feedback Amplifier (Series–Series)
In a current–series feedback amplifier, a fraction of the output current is sampled and fed back in series with the input. This configuration is known as a transconductance amplifier. The application of negative feedback causes the gain to reduce and the bandwidth to increase. Series mixing at the input leads to an increase in input resistance. Since the output current is sampled in series, the output resistance increases, which is desirable in current-source applications.
4. Current–Shunt Feedback Amplifier (Shunt–Series)
In this type, the output current is sampled and the feedback is applied in shunt at the input. It is commonly used as a current amplifier. Negative feedback reduces the gain and increases the bandwidth. Shunt mixing at the input results in a decrease in input resistance, while series sampling of the output current causes the output resistance to increase.
In summary, negative feedback always reduces gain and increases bandwidth, while the effect on input and output resistances depends on whether the feedback is applied in series or shunt at the input and whether voltage or