Feedback Amplifier

Introduction

A feedback amplifier is an electronic circuit that uses feedback to control the gain and performance of the amplifier. Feedback is the process of taking a portion of the output signal and feeding it back to the input of the amplifier. This feedback signal is then combined with the input signal to produce the desired output. Feedback amplifiers are widely used in various applications such as audio amplifiers, operational amplifiers, and power amplifiers.

Importance of Feedback Amplifier

Feedback amplifiers play a crucial role in electronic systems as they provide stability, accuracy, and control over the amplifier's performance. They help in reducing distortion, improving linearity, increasing bandwidth, and reducing noise. By using feedback, the amplifier can be designed to have specific characteristics and meet certain requirements.

Fundamentals of Feedback Amplifier

To understand feedback amplifiers, it is essential to understand the concept of feedback and its impact on the amplifier's performance. Feedback can be either positive or negative. In positive feedback, a portion of the output signal is fed back to the input in phase with the input signal, which leads to instability and oscillation. In negative feedback, a portion of the output signal is fed back to the input out of phase with the input signal, which improves stability and performance.

Negative Feedback

Negative feedback is the most commonly used type of feedback in amplifiers. It involves feeding a portion of the output signal back to the input in such a way that it opposes the input signal. This helps in reducing distortion, increasing linearity, and improving the overall performance of the amplifier.

Definition and Explanation

Negative feedback is a technique in which a fraction of the output signal is combined with the input signal in such a way that it reduces the overall gain of the amplifier. This is achieved by subtracting the feedback signal from the input signal, resulting in a reduced output signal.

Benefits of Negative Feedback

Negative feedback offers several benefits in amplifier design:

1. Reduced distortion: By reducing the gain of the amplifier, negative feedback helps in reducing distortion and improving linearity.

2. Increased bandwidth: Negative feedback helps in increasing the bandwidth of the amplifier by reducing the gain at high frequencies.

3. Improved stability: Negative feedback improves the stability of the amplifier by reducing the effects of variations in component values and temperature.

4. Reduced noise: Negative feedback helps in reducing the noise in the amplifier by reducing the gain at high frequencies where noise is more prominent.

Types of Negative Feedback

There are four main types of negative feedback used in amplifiers:

1. Voltage-Series Feedback

In voltage-series feedback, a portion of the output voltage is fed back to the input in series with the input voltage. This type of feedback is commonly used in voltage amplifiers and operational amplifiers.

1. Voltage Shunt Feedback

In voltage shunt feedback, a portion of the output voltage is fed back to the input in parallel with the input voltage. This type of feedback is commonly used in voltage amplifiers and power amplifiers.

1. Current Series Feedback

In current series feedback, a portion of the output current is fed back to the input in series with the input current. This type of feedback is commonly used in current amplifiers and transconductance amplifiers.

1. Current Shunt Feedback

In current shunt feedback, a portion of the output current is fed back to the input in parallel with the input current. This type of feedback is commonly used in current amplifiers and transconductance amplifiers.

Voltage-Series Feedback

Definition and Explanation

Voltage-series feedback is a type of negative feedback in which a portion of the output voltage is fed back to the input in series with the input voltage. This type of feedback is commonly used in voltage amplifiers and operational amplifiers.

Characteristics and Properties

Voltage-series feedback has the following characteristics and properties:

1. Reduced voltage gain: Voltage-series feedback reduces the voltage gain of the amplifier. The amount of feedback determines the reduction in gain.

2. Increased input impedance: Voltage-series feedback increases the input impedance of the amplifier, which helps in reducing the loading effect on the source.

3. Reduced output impedance: Voltage-series feedback reduces the output impedance of the amplifier, which helps in driving the load.

4. Improved linearity: Voltage-series feedback improves the linearity of the amplifier by reducing distortion.

Analysis of Voltage-Series Feedback Amplifier

To analyze a voltage-series feedback amplifier, the following steps are followed:

1. Determine the open-loop gain of the amplifier without feedback.

2. Determine the feedback factor, which is the ratio of the feedback voltage to the output voltage.

3. Calculate the closed-loop gain using the formula:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

where:

• $$A_{CL}$$ is the closed-loop gain
• $$A_{OL}$$ is the open-loop gain
• $$B$$ is the feedback factor

1. Calculate the input and output impedances of the amplifier with feedback.

Example Problems and Solutions

Problem: Calculate the closed-loop gain of a voltage-series feedback amplifier with an open-loop gain of 1000 and a feedback factor of 0.1.

Solution:

Using the formula for closed-loop gain:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

Substituting the given values:

$$A_{CL} = \frac{1000}{1 + 1000 \cdot 0.1}$$

$$A_{CL} = \frac{1000}{1 + 100}$$

$$A_{CL} = \frac{1000}{101}$$

$$A_{CL} \approx 9.90$$

Therefore, the closed-loop gain of the voltage-series feedback amplifier is approximately 9.90.

Real-world Applications

Voltage-series feedback amplifiers are widely used in various applications, including:

• Audio amplifiers: Voltage-series feedback is used in audio amplifiers to improve linearity and reduce distortion.

• Operational amplifiers: Operational amplifiers use voltage-series feedback to achieve high gain, high input impedance, and low output impedance.

• Instrumentation amplifiers: Instrumentation amplifiers use voltage-series feedback to provide high common-mode rejection ratio (CMRR) and high input impedance.

Voltage Shunt Feedback

Definition and Explanation

Voltage shunt feedback is a type of negative feedback in which a portion of the output voltage is fed back to the input in parallel with the input voltage. This type of feedback is commonly used in voltage amplifiers and power amplifiers.

Characteristics and Properties

Voltage shunt feedback has the following characteristics and properties:

1. Reduced voltage gain: Voltage shunt feedback reduces the voltage gain of the amplifier. The amount of feedback determines the reduction in gain.

2. Increased input impedance: Voltage shunt feedback increases the input impedance of the amplifier, which helps in reducing the loading effect on the source.

3. Reduced output impedance: Voltage shunt feedback reduces the output impedance of the amplifier, which helps in driving the load.

4. Improved linearity: Voltage shunt feedback improves the linearity of the amplifier by reducing distortion.

Analysis of Voltage Shunt Feedback Amplifier

To analyze a voltage shunt feedback amplifier, the following steps are followed:

1. Determine the open-loop gain of the amplifier without feedback.

2. Determine the feedback factor, which is the ratio of the feedback voltage to the output voltage.

3. Calculate the closed-loop gain using the formula:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

where:

• $$A_{CL}$$ is the closed-loop gain
• $$A_{OL}$$ is the open-loop gain
• $$B$$ is the feedback factor

1. Calculate the input and output impedances of the amplifier with feedback.

Example Problems and Solutions

Problem: Calculate the closed-loop gain of a voltage shunt feedback amplifier with an open-loop gain of 1000 and a feedback factor of 0.1.

Solution:

Using the formula for closed-loop gain:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

Substituting the given values:

$$A_{CL} = \frac{1000}{1 + 1000 \cdot 0.1}$$

$$A_{CL} = \frac{1000}{1 + 100}$$

$$A_{CL} = \frac{1000}{101}$$

$$A_{CL} \approx 9.90$$

Therefore, the closed-loop gain of the voltage shunt feedback amplifier is approximately 9.90.

Real-world Applications

Voltage shunt feedback amplifiers are widely used in various applications, including:

• Voltage amplifiers: Voltage shunt feedback is used in voltage amplifiers to improve linearity and reduce distortion.

• Power amplifiers: Power amplifiers use voltage shunt feedback to provide high power output and low distortion.

• Oscillators: Oscillators use voltage shunt feedback to achieve stable and accurate oscillations.

Current Series Feedback

Definition and Explanation

Current series feedback is a type of negative feedback in which a portion of the output current is fed back to the input in series with the input current. This type of feedback is commonly used in current amplifiers and transconductance amplifiers.

Characteristics and Properties

Current series feedback has the following characteristics and properties:

1. Reduced current gain: Current series feedback reduces the current gain of the amplifier. The amount of feedback determines the reduction in gain.

2. Increased input impedance: Current series feedback increases the input impedance of the amplifier, which helps in reducing the loading effect on the source.

3. Reduced output impedance: Current series feedback reduces the output impedance of the amplifier, which helps in driving the load.

4. Improved linearity: Current series feedback improves the linearity of the amplifier by reducing distortion.

Analysis of Current Series Feedback Amplifier

To analyze a current series feedback amplifier, the following steps are followed:

1. Determine the open-loop gain of the amplifier without feedback.

2. Determine the feedback factor, which is the ratio of the feedback current to the output current.

3. Calculate the closed-loop gain using the formula:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

where:

• $$A_{CL}$$ is the closed-loop gain
• $$A_{OL}$$ is the open-loop gain
• $$B$$ is the feedback factor

1. Calculate the input and output impedances of the amplifier with feedback.

Example Problems and Solutions

Problem: Calculate the closed-loop gain of a current series feedback amplifier with an open-loop gain of 1000 and a feedback factor of 0.1.

Solution:

Using the formula for closed-loop gain:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

Substituting the given values:

$$A_{CL} = \frac{1000}{1 + 1000 \cdot 0.1}$$

$$A_{CL} = \frac{1000}{1 + 100}$$

$$A_{CL} = \frac{1000}{101}$$

$$A_{CL} \approx 9.90$$

Therefore, the closed-loop gain of the current series feedback amplifier is approximately 9.90.

Real-world Applications

Current series feedback amplifiers are widely used in various applications, including:

• Current amplifiers: Current series feedback is used in current amplifiers to improve linearity and reduce distortion.

• Transconductance amplifiers: Transconductance amplifiers use current series feedback to provide high transconductance and low distortion.

• Transimpedance amplifiers: Transimpedance amplifiers use current series feedback to convert current signals to voltage signals.

Current Shunt Feedback

Definition and Explanation

Current shunt feedback is a type of negative feedback in which a portion of the output current is fed back to the input in parallel with the input current. This type of feedback is commonly used in current amplifiers and transconductance amplifiers.

Characteristics and Properties

Current shunt feedback has the following characteristics and properties:

1. Reduced current gain: Current shunt feedback reduces the current gain of the amplifier. The amount of feedback determines the reduction in gain.

2. Increased input impedance: Current shunt feedback increases the input impedance of the amplifier, which helps in reducing the loading effect on the source.

3. Reduced output impedance: Current shunt feedback reduces the output impedance of the amplifier, which helps in driving the load.

4. Improved linearity: Current shunt feedback improves the linearity of the amplifier by reducing distortion.

Analysis of Current Shunt Feedback Amplifier

To analyze a current shunt feedback amplifier, the following steps are followed:

1. Determine the open-loop gain of the amplifier without feedback.

2. Determine the feedback factor, which is the ratio of the feedback current to the output current.

3. Calculate the closed-loop gain using the formula:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

where:

• $$A_{CL}$$ is the closed-loop gain
• $$A_{OL}$$ is the open-loop gain
• $$B$$ is the feedback factor

1. Calculate the input and output impedances of the amplifier with feedback.

Example Problems and Solutions

Problem: Calculate the closed-loop gain of a current shunt feedback amplifier with an open-loop gain of 1000 and a feedback factor of 0.1.

Solution:

Using the formula for closed-loop gain:

$$A_{CL} = \frac{A_{OL}}{1 + A_{OL} \cdot B}$$

Substituting the given values:

$$A_{CL} = \frac{1000}{1 + 1000 \cdot 0.1}$$

$$A_{CL} = \frac{1000}{1 + 100}$$

$$A_{CL} = \frac{1000}{101}$$

$$A_{CL} \approx 9.90$$

Therefore, the closed-loop gain of the current shunt feedback amplifier is approximately 9.90.

Real-world Applications

Current shunt feedback amplifiers are widely used in various applications, including:

• Current amplifiers: Current shunt feedback is used in current amplifiers to improve linearity and reduce distortion.

• Transconductance amplifiers: Transconductance amplifiers use current shunt feedback to provide high transconductance and low distortion.

• Transimpedance amplifiers: Transimpedance amplifiers use current shunt feedback to convert current signals to voltage signals.

Advantages and Disadvantages of Feedback Amplifier

Advantages

• Improved stability: Feedback amplifiers are more stable compared to non-feedback amplifiers as they can compensate for variations in component values and temperature.

• Reduced distortion: Feedback amplifiers help in reducing distortion and improving linearity by reducing the gain at high frequencies.

• Increased bandwidth: Feedback amplifiers have a wider bandwidth compared to non-feedback amplifiers as they can reduce the gain at high frequencies.

• Improved accuracy: Feedback amplifiers provide accurate amplification of the input signal by reducing the effects of non-linearities and variations in component values.

Disadvantages

• Reduced gain: Feedback amplifiers have a reduced gain compared to non-feedback amplifiers due to the negative feedback.

• Increased complexity: Feedback amplifiers are more complex compared to non-feedback amplifiers as they require additional components for feedback.

• Risk of instability: Improper design or implementation of feedback amplifiers can lead to instability and oscillations.

Conclusion

Feedback amplifiers are an essential part of analog electronics as they provide stability, accuracy, and control over the amplifier's performance. Negative feedback is the most commonly used type of feedback in amplifiers, offering several benefits such as reduced distortion, increased bandwidth, improved stability, and reduced noise. There are four main types of negative feedback used in amplifiers: voltage-series feedback, voltage shunt feedback, current series feedback, and current shunt feedback. Each type has its own characteristics, properties, and applications. Feedback amplifiers have advantages such as improved stability, reduced distortion, increased bandwidth, and improved accuracy, but they also have disadvantages such as reduced gain, increased complexity, and the risk of instability.

Summary

A feedback amplifier is an electronic circuit that uses feedback to control the gain and performance of the amplifier. Negative feedback is the most commonly used type of feedback in amplifiers, offering several benefits such as reduced distortion, increased bandwidth, improved stability, and reduced noise. There are four main types of negative feedback used in amplifiers: voltage-series feedback, voltage shunt feedback, current series feedback, and current shunt feedback. Each type has its own characteristics, properties, and applications. Feedback amplifiers have advantages such as improved stability, reduced distortion, increased bandwidth, and improved accuracy, but they also have disadvantages such as reduced gain, increased complexity, and the risk of instability.

Analogy

An analogy to understand feedback amplifiers is to think of a thermostat in a room. The thermostat measures the temperature and compares it to the desired temperature set by the user. If the temperature is too high, the thermostat turns off the heating system. If the temperature is too low, the thermostat turns on the heating system. This feedback loop helps maintain the desired temperature in the room. Similarly, in a feedback amplifier, the output signal is compared to the desired signal, and the feedback loop adjusts the amplifier to achieve the desired output.