Feedback Amplifiers

A Feedback amplifier is a type of electronic Circuit Where the output of the Signal is fed back to the input. This feedback loop allows the Amplifier to adjust its performance according to the output received which helps to improve the characteristics of the signal. Feedback Amplifiers are used in various electronic circuits such as Operational amplifiers, audio amplifiers, and various other electronic circuits.

In this Article, We will be going through Feedback Amplifiers, We will start our Article with an introduction of the Feedback Amplifier, then we will go through its types and Topologies, At last we will conclude our Article with its Advantages, Disadvantages and Some FAQs.

An electronic circuit that sends feedback from its output to its input to control and enhance performance is called a feedback amplifier. It works by taking a sample of the output signal, comparing it to the input signal, and then modifying the amplification process as necessary. Next, we will see the Block Diagram and Working of Feedback Amplifier.

Block Diagram and Working of Feedback Amplifier

Given Below is the Block Diagram of the Feedback Amplifier

The above block diagram consists of an amplifier and a feedback circuit.

Gain of amplifier is represented by A and feedback ratio is given by [Tex]\frac{Vo}{Vs} [/Tex]

The voltage feedback is given by ,[Tex] V_f= βV_o [/Tex].

For negative feedback :

[Tex]V_i=V_s−V_f=V_s−βV_o [/Tex]

For positive feedback :

[Tex]V_i=V_s+V_f=V_s+βV_o [/Tex]

Let’s consider the case of negative feedback,

The output voltage [Tex]V_o [/Tex] should be equal to [Tex]V_s−βV_o [/Tex] times gain of the amplifier “A”

[Tex]AV_s−AβV_o=V_o [/Tex]

[Tex]AV_s=V_o (1+Aβ) [/Tex]

[Tex]\frac{V_o}{V_s}=\frac{A}{1+A\beta\ } [/Tex]

Where [Tex]V_s [/Tex] is applied signal voltage.

The overall gain with feedback from the amplifier is,

[Tex]Af=\frac{V_o}{V_s} [/Tex]

Where [Tex]V_o [/Tex] is the output voltage and

[Tex]V_s [/Tex] is the input signal voltage.

The gain feedback for a positive feedback amplifier,

[Tex]Af=\frac{A}{1-A\beta\ } [/Tex]

And for a negative feedback amplifier,

[Tex]Af=\frac{A}{1+A\beta\ } [/Tex]

These are the standard equations to calculate the gain feedback of amplifiers depending on their type.

Feedback amplifiers can be categorized into two main types based on the type of feedback they use :

• Positive feedback Amplifiers
• Negative feedback Amplifiers

Negative Feedback

Given Below is the Block Diagram of the Negative Feedback Amplifier

• Sending a portion of the output signal back to the input with an inverted phase is known as negative feedback in a feedback amplifier.
• Feedback of this sort is frequently applied to stabilize amplifiers, lower distortion, and enhance performance in general. It aids in keeping the input signal amplified precisely and under control.

Positive Feedback

Given Below is the Block Diagram of the Positive Feedback System

• In Positive Feedback, a portion of the output signal is sent back to the input with the same phase in positive feedback.
• Applications like oscillators and signal regeneration may benefit from this kind of feedback, which raises the amplifier’s gain. It does, Yet, they tend to amplify even minor changes in the input signal, requiring careful monitoring to prevent instability.

Unity feedback system

In a feedback amplifier system, a unity feedback occurs when the input signal receives the output feedback at a gain of 1.

According to the definition , [Tex]gain (β) = 1 [/Tex]

Therefore, for positive feedback system :

[Tex]\frac{Vo}{Vs}=\frac{A}{1+A} [/Tex]

And for negative feedback system:

[Tex]\frac{Vo}{Vs}=\frac{A}{1-A} [/Tex]

Both voltage and current can be fed back to the input I/P in either series or parallel .The four ways of connecting the feedback to the signals are:

• Voltage Series Feedback Amplifier
• Voltage Shunt Feedback Amplifier
• Current Shunt Feedback Amplifier
• Current Series Feedback Amplifier

Voltage Series Feedback Amplifier

Given Below is the Block Diagram of the Voltage Series Feedback Amplifier

• The input voltage and the feedback are applied in series in a voltage series feedback amplifier. This kind of feedback is used to increase voltage gain, reduce distortion, and improve consistency.
• Feeds back some of the output voltage in series to improve precision. This controls gain, reduces distortion, and enhances the signal’s precision overall.
  

Voltage Shunt Feedback Amplifier

Given Below is the Block Diagram of the Voltage Shunt Feedback Amplifier

• In a voltage shunt feedback amplifier, the input voltage and the feedback are connected in parallel, or shunt . This type of feedback is used to increase voltage gain, enhance linearity, and decrease output impedance.
• Enhances stability by in parallel applying a portion of the output voltage to the input. As a result, performance is improved under a range of situations by increasing linearity and decreasing distortion.

Current Shunt Feedback Amplifier

Given Below is the Block Diagram of the Current Shunt Feedback Amplifier

• The feedback and input current are applied in series in a current series feedback amplifier. It is common practice for using this type of feedback to increase stability, input impedance, and bandwidth.
• Increases stability by running in parallel some of the output current. This improves linearity, lowers distortion, and gives the amplifier’s characteristics more control for reliable performance in a range of scenarios.

Current Series Feedback Amplifier

Given Below is the Block Diagram of the Current Series Feedback Amplifier

• The feedback in a current shunt feedback amplifier is linked in parallel, or shunt, to the input current. This arrangement is frequently used to increase bandwidth, stability, and input impedance.
• Increases control by series-firing a portion of the output current. This controls gain, guaranteeing precise amplification and reducing distortion for enhanced integrity of signals across a range for applications.

• Feedback improves general stability by reducing sensitivity to shifts in component parameters.
• Negative feedback boosts the amplifier’s bandwidth through extending its frequency response.
• By reducing distortion caused by harmonics, negative feedback generates a signal that is cleaner and easier to reproduce.
• Feedback makes it possible to precisely control the amplifier’s gain, which facilitates achieving the intended performance.
• By maintaining an increasingly linear relationship between input and output, negative feedback improves linearity.

• Feedback makes the circuit more complicated and difficult to fix and design.
• Feedback has the potential to amplify noise in system due to which SNR reduces.
• The cost of production is high due to complexity of circuit.
• Feedback amplifiers may have a small phase margin, which increases stability problems.
• Excessive feedback may lead to a reduction in overall gain.
  

Feedback amplifiers have various real life applications such as :

• Used in amplifiers for speakers and headphones to ensure clear and high-quality sound.
• Utilized in medical equipment like ECG ,machines to improve signal accuracy and reliability.
• Integrated in control systems for automation and robotics to regulate processes.
• Used in video amplifiers to improve the quality of signals transmitted to TVs and monitors.
• Used in testing equipment to amplify and analyse signals during circuit testing and debugging.

The Feedback Amplifier are important electronic circuit, which is used to shape the signal. They have mainly two types which are positive and negative feedback. Negative feedback stabilizes and improves amplifier precision and reliability, enhancing characteristics like bandwidth, distortion reduction, and gain control. Positive feedback, while potentially increasing gain, requires careful handling due to the risk of distortion and instability. Shunt and series configurations are common feedback types, with shunt reducing distortion and adding stability, while series ensures accurate amplification.

How does negative feedback stabilize gain?

Negative feedback lessens sensitivity to changes in component characteristics, stabilizing gain in amplifiers. The feedback loop counteracts the rise in gain through the generation of a reverse signal as gain increases. On the other hand, the feedback increases the signal if the gain drops. Because of the dynamic regulation, the amplifier is less prone to variations in part tolerances or operating conditions and maintains a steady and consistent gain.

How does negative feedback increase the bandwidth of an amplifier?

An amplifier’s bandwidth (BW) in the absence of feedback is the difference between the three-dB frequencies f1 and f2. The gain-bandwidth product is equal to A x BW if A is the gain. Since the gain bandwidth product must stay constant in both scenarios, the amplifier gain is decreased by the negative feedback. Consequently, the bandwidth increases to make up for the loss of gain.

Why voltage-series feedback is most commonly used in cascaded amplifiers?

Voltage series feedback is most frequently used in cascaded amplifiers because it can satisfy the two main requirements of cascaded amplifiers, which are low output impedance and high input impedance.