Understanding Current Shunt Feedback Effects on Amplifier Resistance

Feedback is a fundamental concept in amplifier design, where a portion of the output signal is fed back to the input. This process significantly alters the amplifier's characteristics, including gain, bandwidth, distortion, and importantly, input and output resistances.

There are four main types of negative feedback configurations, classified based on how the feedback signal is sampled from the output and how it is mixed with the input signal:

• Voltage-series feedback (Series mixing, Voltage sampling)
• Voltage-shunt feedback (Shunt mixing, Voltage sampling)
• Current-series feedback (Series mixing, Current sampling)
• Current-shunt feedback (Shunt mixing, Current sampling)

The question asks about the effect of current shunt feedback.

Effect of Current Shunt Feedback

In a current shunt feedback configuration, the feedback signal is proportional to the output current (current sampling) and is connected in parallel (shunt) with the input signal (shunt mixing).

Let's analyze how this specific feedback type affects the input and output resistances.

Input Resistance with Current Shunt Feedback

Shunt mixing at the input means the feedback network is connected in parallel with the amplifier's input terminals. When a signal source with some internal resistance is connected, the feedback current effectively shunts (or bypasses) some of the input signal current away from the amplifier's input. This reduction in current entering the amplifier for a given input voltage means the effective input resistance seen by the source is decreased.

The input resistance of the amplifier with feedback (\(R_{inF}\)) is related to the amplifier's open-loop input resistance (\(R_{in}\)) and the feedback factor. For shunt input mixing, the input resistance is decreased by the factor \((1 + A\beta)\), where \(A\) is the open-loop gain and \(\beta\) is the feedback factor (assuming a specific circuit configuration and proper derivation).

The general relationship for shunt mixing at the input under negative feedback is:

\(R_{inF} = \frac{R_{in}}{1 + \text{Loop Gain}}\)

Where the loop gain is \(A\beta\) in simple cases. Thus, \(R_{inF}\) is typically less than \(R_{in}\).

Output Resistance with Current Shunt Feedback

Current sampling at the output means the feedback network senses the output current flowing into the load. The feedback signal generated is proportional to this output current and is used to oppose changes in it. If the load resistance changes, causing the output current to attempt to change, the feedback system acts to stabilize the output current. This characteristic of maintaining a relatively constant output current despite changes in load or output voltage is indicative of a high output resistance, similar to an ideal current source.

The output resistance of the amplifier with feedback (\(R_{outF}\)) is related to the amplifier's open-loop output resistance (\(R_{out}\)) and the feedback factor. For current sampling at the output, the output resistance is increased by the factor \((1 + A\beta)\).

The general relationship for current sampling at the output under negative feedback is:

\(R_{outF} = R_{out} \times (1 + \text{Loop Gain})\)

Where the loop gain is \(A\beta\) in simple cases. Thus, \(R_{outF}\) is typically greater than \(R_{out}\).

Summary of Effects on Resistance

Based on the analysis of input shunt mixing and output current sampling:

• Input Resistance: Decreases
• Output Resistance: Increases

This combination of decreased input resistance and increased output resistance makes the current shunt feedback configuration suitable for applications where the amplifier needs to accept a current input (ideally low input impedance for current sensing) and provide a current output (ideally high output impedance for driving a load).

Revision Table: Amplifier Feedback Configurations

Additional Information: Benefits of Negative Feedback

While the effect on input and output resistance is significant, negative feedback in amplifiers provides several other crucial benefits:

• It makes the amplifier gain less sensitive to variations in component parameters (like transistor \(h_{FE}\)) and more dependent on the stable external feedback network components (usually resistors).
• It reduces non-linear distortion by making the amplifier's response more linear.
• It increases the bandwidth of the amplifier, allowing it to amplify signals over a wider range of frequencies.
• It reduces the impact of unwanted noise signals that originate within the amplifier stages.

Choosing the correct feedback configuration depends on the desired amplifier characteristics and the nature of the signal source and load.