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INTRODUCTION
A practical amplifier has a gain of nearly one million i.e. its output is one million times the input. Consequently, even a casual disturbance at the  input will appear  in the amplified form in the output. There is a strong tendency in amplifiers to introduce hum due to sudden temperature changes or stray electric and magnetic fields.
Therefore, every high gain amplifier tends to give noise along with signal in its output.
The noise in the output of an amplifier is undesirable and must be kept to as small a level as possible. The noise level in amplifiers can be reduced considerably by the use of negative feedback i.e. by injecting a fraction of output in phase opposition to the input signal. The object of this chapter is to consider the effects and methods of providing negative feedback in transistor amplifiers.

5.1 Feedback
The process of injecting a fraction of output energy of some device back to the input is known as feedback. The principle of feedback is probably as old as the invention of first machine but it is only some 50 years ago that feedback has come into use in connection with electronic circuits. It has been found very useful in reducing noise in amplifiers and making amplifier operation stable. Depending upon whether the feedback energy aids or opposes the input signal, there are two basic types of feedback in amplifiers viz positive feedback and negative feedback.

(i) Positive feedback. When the feedback energy (voltage or current) is in phase with the input signal and thus aids it, it is called positive feedback. This is illustrated in Fig. 1.1. Both amplifier and feedback network introduce a phase shift of 180°. The result is a 360° phase shift around the loop, causing the feedback voltage Vf to be in phase with the input signal Vin.

 

                                   Feedback Amplifiers & Oscillators Add Content | Analog and Digital Electronics - Electrical Engineering (EE)

The positive feedback increases the gain of the amplifier. However, it has the disadvantages of increased distortion and instability. Therefore, positive feedback is seldom employed in amplifiers. One important use of positive feedback is in oscillators.  As we shall see in the next chapter, if positive feedback is sufficiently large, it leads to oscillations. As a matter of fact, an oscillator is a device that converts d.c. power into a.c. power of any desired frequency.

(ii) Negative feedback. When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it,  it is called negative feedback. This is illustrated in Fig. 1.2. As you can see, the amplifier introduces a phase shift of 180° into the circuit while the feedback network is so designed that it introduces no phase shift (i.e., 0° phase shift). The result is that the feedback voltage Vf is 180° out of phase with the input signal Vin.

             Feedback Amplifiers & Oscillators Add Content | Analog and Digital Electronics - Electrical Engineering (EE)

Negative feedback reduces the gain of the amplifier. However,  the advantages  of negative   feedback are: reduction in distortion, stability in gain, increased bandwidth and improved input and output impedances. It is due to these advantages that negative feedback  is frequently employed  in amplifiers.

The document Feedback Amplifiers & Oscillators Add Content | Analog and Digital Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Analog and Digital Electronics.
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FAQs on Feedback Amplifiers & Oscillators Add Content - Analog and Digital Electronics - Electrical Engineering (EE)

1. What is a feedback amplifier?
Ans. A feedback amplifier is an electronic circuit that uses a portion of the output signal and feeds it back to the input in order to control and modify the overall gain and response of the amplifier.
2. What are the advantages of using feedback amplifiers?
Ans. Feedback amplifiers offer several advantages, including increased gain stability, improved linearity, reduced distortion, wider bandwidth, and better control over the circuit's overall response.
3. How does negative feedback work in amplifiers?
Ans. Negative feedback in amplifiers involves taking a portion of the output signal, inverting it, and feeding it back to the input. This helps reduce distortion, improve linearity, and increase the overall stability and performance of the amplifier.
4. What are the different types of oscillators?
Ans. There are several types of oscillators, including the RC oscillator, LC oscillator, crystal oscillator, and voltage-controlled oscillator (VCO). Each type utilizes different components and techniques to generate and sustain an oscillating signal.
5. How does an oscillator work?
Ans. An oscillator works by utilizing positive feedback to sustain an oscillating signal. The positive feedback loop within the oscillator circuit amplifies and regenerates the output signal, creating a continuous oscillation. The circuit's components, such as resistors, capacitors, inductors, or crystals, determine the frequency and characteristics of the oscillation.
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