Voltage Series Feedback Amplifier Overview

A voltage series feedback amplifier is a type of negative feedback amplifier where the feedback signal is sampled in parallel with the output (voltage sampling) and mixed in series with the input signal (series mixing). This configuration is widely used because it stabilizes voltage gain, reduces distortion, and improves bandwidth. Key characteristics of this configuration include its effects on input and output impedances.

• Voltage Sampling (Parallel Connection): The feedback network senses the output voltage, meaning it is connected in parallel with the output load.
• Series Mixing: The feedback voltage is connected in series with the input signal and the input impedance of the amplifier. This series connection directly impacts the apparent input resistance seen by the source.

Input Resistance in Voltage Series Feedback Amplifiers

When negative feedback is applied in a voltage series configuration, the input resistance of the amplifier is significantly affected. Specifically, the input resistance is increased. Let's understand why and look at the formula.

For a voltage series feedback amplifier, the input resistance with feedback, denoted as \( R_{if} \), is given by the formula:

\[R_{if} = R_i (1 + A_v\beta)\]

Where:

• \( R_{if} \) is the input resistance of the amplifier with feedback.
• \( R_i \) is the input resistance of the amplifier without feedback (i.e., the input resistance of the basic amplifier).
• \( A_v \) is the open-loop voltage gain of the basic amplifier (the gain without feedback).
• \( \beta \) is the feedback factor or feedback ratio, which represents the fraction of the output voltage fed back to the input.

The term \( (1 + A_v\beta) \) is often referred to as the desensitivity factor or amount of feedback. Since \( A_v \) and \( \beta \) are typically positive for negative feedback (when the feedback signal subtracts from the input), this factor is always greater than 1. This means that \( R_{if} \) will always be greater than \( R_i \).

Why Input Resistance Increases with Voltage Series Feedback

The increase in input resistance can be understood intuitively by considering the series mixing at the input. The feedback voltage \( V_f \) is connected in series with the applied input voltage \( V_s \). This feedback voltage opposes the input signal, effectively reducing the net voltage at the amplifier's input terminals.

Consider the input current \( I_i \). For a given input voltage \( V_s \), the feedback voltage \( V_f \) reduces the effective voltage \( V_{in\_amp} \) across \( R_i \). Since \( V_{in\_amp} = V_s - V_f \) (for negative feedback) and \( V_f \) is proportional to \( V_{out} \), the amplifier's input draws less current \( I_i \) for the same input voltage \( V_s \) compared to the case without feedback. According to Ohm's law, if the current drawn for a given voltage decreases, the effective resistance must increase.

This increased input resistance is beneficial as it reduces the loading effect on the signal source, allowing more of the signal voltage to reach the amplifier's input. This is a desirable characteristic for voltage amplifiers.