Transistors
- Bias Compensation
- Methods of Transistor Biasing
- Transistor Biasing
- Transistor as an Amplifier
- Operating Point
- Transistor Load Line Analysis
- Transistor Regions of Operation
- Transistor Configurations
- Transistors - Overview
Amplifiers
- Noise in Amplifiers
- Emitter Follower & Darlington Amplifier
- Negative Feedback Amplifiers
- Feedback Amplifiers
- Types of Tuned Amplifiers
- Tuned Amplifiers
- Class AB and C Power Amplifiers
- Class B Power Amplifier
- Push-Pull Class A Power Amplifier
- Transformer Coupled Class A Power Amplifier
- Class A Power Amplifiers
- Classification of Power Amplifiers
- Power Amplifiers
- Direct Coupled Amplifier
- Transformer Coupled Amplifier
- RC Coupling Amplifier
- Multi-Stage Transistor Amplifier
- Based on Configurations
- Classification of Amplifiers
- Basic Amplifier
Amplifiers Useful Resources
Selected Reading
- Who is Who
- Computer Glossary
- HR Interview Questions
- Effective Resume Writing
- Questions and Answers
- UPSC IAS Exams Notes
Amppfiers Feedback
An amppfier circuit simply increases the signal strength. But while amppfying, it just increases the strength of its input signal whether it contains information or some noise along with information. This noise or some disturbance is introduced in the amppfiers because of their strong tendency to introduce hum due to sudden temperature changes or stray electric and magnetic fields. Therefore, every high gain amppfier tends to give noise along with signal in its output, which is very undesirable.
The noise level in the amppfier circuits can be considerably reduced by using negative feedback done by injecting a fraction of output in phase opposition to the input signal.
Principle of Feedback Amppfier
A feedback amppfier generally consists of two parts. They are the amppfier and the feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amppfier can be understood from the following figure.
![Feedback](/amppfiers/images/feedback.jpg)
From the above figure, the gain of the amppfier is represented as A. the gain of the amppfier is the ratio of output voltage Vo to the input voltage Vi. the feedback network extracts a voltage Vf = β Vo from the output Vo of the amppfier.
This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. Now,
$$V_i = V_s + V_f = V_s + eta V_o$$
$$V_i = V_s - V_f = V_s - eta V_o$$
The quantity β = Vf/Vo is called as feedback ratio or feedback fraction.
Let us consider the case of negative feedback. The output Vo must be equal to the input voltage (Vs - βVo) multipped by the gain A of the amppfier.
Hence,
$$(V_s - eta V_o)A = V_o$$
Or
$$A V_s - A eta V_o = V_o$$
Or
$$A V_s = V_o (1 + A eta)$$
Therefore,
$$frac{V_o}{V_s} = frac{A}{1 + A eta}$$
Let Af be the overall gain (gain with the feedback) of the amppfier. This is defined as the ratio of output voltage Vo to the appped signal voltage Vs, i.e.,
$$A_f = frac{Output : voltage}{Input : signal : voltage} = frac{V_o}{V_s}$$
So, from the above two equations, we can understand that,
The equation of gain of the feedback amppfier, with negative feedback is given by
$$A_f = frac{A}{1 + A eta}$$
The equation of gain of the feedback amppfier, with positive feedback is given by
$$A_f = frac{A}{1 - A eta}$$
These are the standard equations to calculate the gain of feedback amppfiers.
Types of Feedbacks
The process of injecting a fraction of output energy of some device back to the input is known as Feedback. It has been found that feedback is very useful in reducing noise and making the amppfier operation stable.
Depending upon whether the feedback signal aids or opposes the input signal, there are two types of feedbacks used.
Positive Feedback
The feedback in which the feedback energy i.e., either voltage or current is in phase with the input signal and thus aids it is called as Positive feedback.
Both the input signal and feedback signal introduces a phase shift of 180o thus making a 360o resultant phase shift around the loop, to be finally in phase with the input signal.
Though the positive feedback increases the gain of the amppfier, it has the disadvantages such as
Increasing distortion
Instabipty
It is because of these disadvantages the positive feedback is not recommended for the amppfiers. If the positive feedback is sufficiently large, it leads to oscillations, by which oscillator circuits are formed. This concept will be discussed in OSCILLATORS tutorial.
Negative Feedback
The feedback in which the feedback energy i.e., either voltage or current is out of phase with the input and thus opposes it, is called as negative feedback.
In negative feedback, the amppfier introduces a phase shift of 180o into the circuit while the feedback network is so designed that it produces no phase shift or zero phase shift. Thus the resultant feedback voltage Vf is 180o out of phase with the input signal Vin.
Though the gain of negative feedback amppfier is reduced, there are many advantages of negative feedback such as
Stabipty of gain is improved
Reduction in distortion
Reduction in noise
Increase in input impedance
Decrease in output impedance
Increase in the range of uniform apppcation
It is because of these advantages negative feedback is frequently employed in amppfiers.
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