Feedback in amplifier
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The document discusses the concept of feedback in amplifiers, detailing the types of feedback (positive and negative) and their respective effects on amplification. It explains the functions of amplifiers, the importance of feedback in enhancing stability and reducing distortion, as well as outlining the advantages of negative feedback over positive feedback in various applications. Additionally, it provides an overview of different feedback configurations, such as voltage-series and current-shunt feedback, and their implications on input and output impedances.
Feedback in amplifier
- 1. Physics BSc III Sem V Shri Shivaji Science College, Amravati Dr. Vaishali V. Deshmukh
- 2. FEEDBACK IN AMPLIFIERS Oscillators and Multivibrators 2 1
- 3. Concept of Feedback What is feedback in an amplifier Topologies of Feedback Voltage and current feedback Types of Feedback Positive and negative feedback 3 What is an amplifier Basic Amplifier
- 4. What is an Amplifier? 4 ▰ A device which accept an input signal and produces an output signal proportional to the input, is called an amplifier. VS : is the signal voltage, RS : is the internal resistance of the source, V1 : is the actual input voltage to the amplifier, I1 : is the input current to the amplifier, V2 : is the output voltage across the load RL, I2 : is the output current flowing through the load RL.
- 5. 5 ✓The important function of an amplifier is the amplification. ✓Circuit that increases the amplitude of the weak signals. ✓The important parameters of an amplifier are input impedance, output impedance, current gain and voltage gain. ✓A good design of an amplifier circuit must possess high input impedance, low output impedance and high current gain. ✓Constitute an essential part of radio, television and communication devices. ✓In discrete circuits, bipolar JFET, FET is used as amplifying element. ✓We find the use of amplifiers in medical devices, scientific equipment, automation, military tools, communication devices, and even in household equipment.
- 6. Limitations of basic amplifier ▰ The instability of a.c. gain is due to the change in supply, change in h-parameters, age of the device. ▰ The input and output impedance are not according to the requirements of the basic amplifier. ▰ Distortion is large. ▰ Noise is more. ▰ Bandwidth is not sufficiently large. 6
- 7. 7 FEEDBACK AMPLIFIER Feedback is the process of taking a proportion of an amplifier´s output signal and feeding it back into the input. Feedback can be arranged to increase or decrease the input signal. An amplifier that has a feedback connection established between the output to the input is known as the feedback amplifier.
- 8. Positive The positive feedback can be defined as when the feedback current otherwise voltage is applied for increasing the i/p voltage, then it is named as positive feedback. Direct feedback is another name of this positive feedback. Because positive feedback generates unnecessary distortion; it is not often used in amplifiers. But, it amplifies the original signal power and can be used in oscillator circuits. Types of feedback Negative The negative feedback can be defined as if the feedback current otherwise voltage can be applied for reducing the amplifier i/p, then it is called as negative feedback. Inverse feedback is another name of this negative feedback. This kind of feedback is regularly used in amplifier circuits. 8
- 10. Fig2: Negative feedback 10
- 11. Advantages of negative feedback ▰ Stability of gain is improved ▰ Reduction in distortion ▰ Reduction in noise ▰ Increase in input impedance ▰ Decrease in output impedance ▰ Increase in the range of uniform application 11 Though the gain of negative feedback amplifier is reduced, there are many advantages of negative feedback such as, It is because of these advantages negative feedback is frequently employed in amplifiers.
- 12. Comparison between Negative & Positive feedback ▰ When the feedback voltage or current is in phase with the input signal and thus aids it, it is called positive feedback. ▰ Positive feedback increases the gain of the amplifier. ▰ Noise in the output signal increases ▰ Distortion increases ▰ Bandwidth decreases ▰ It is used in oscillators ▰ When the feedback voltage or current is out of phase with the input signal and thus opposes it, it is called positive feedback. ▰ Negative feedback reduces the gain of the amplifier. ▰ Noise in the output signal decreases ▰ Distortion decreases ▰ It is used in amplifiers 12 Positive Feedback Negative Feedback
- 13. Principle of Feedback Amplifier 13 A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. A feedback network is a linear two-port network that contains resistors, inductors, capacitors. Its function is to fed back some portion of output to the input. From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier 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 amplifier.
- 14. 14 This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. Now, Vi = Vs + Vf = Vs + βVo Vi = Vs – Vf = Vs − βVo 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) multiplied by the gain A of the amplifier. Hence, (Vs−βVo) A=Vo Or AVs−AβVo = Vo Or AVs = Vo (1+Aβ) Therefore, Vo/Vs=A/ (1+Aβ)
- 15. 15 Let Af be the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio of output voltage Vo to the applied signal voltage Vs, i.e. Af = Output voltage /Input signal voltage = Vo/Vs So, from the above two equations, we can understand that, The equation of gain of the feedback amplifier, with negative feedback is given by Af = A/ (1+Aβ) The equation of gain of the feedback amplifier, with positive feedback is given by Af = A/ (1−Aβ) Case1: when β is positive i.e. when feedback voltage is in phase with input voltage( +Ve FB) then |1-Aβ | < 1, Af >A. Therefore, positive feedback increases the amplifier gain. Case2: when β is negative i.e. when feedback voltage is out of phase with input voltage( -Ve FB) then |1-Aβ | >1, Af < A.Therefore, negative feedback reduces the amplifier gain. The term Aβ is called loop gain.
- 16. 16 Voltage-Series Feedback Negative Voltage Feedback: In this method, the voltage feedback to the input of amplifier is proportional to the output voltage. In the voltage series feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as shunt- driven series-fed feedback, i.e., a parallel-series circuit. The figure shows the block diagram of voltage series feedback, by which it is evident that the feedback circuit is placed in shunt with the output but in series with the input. As the feedback circuit is connected in shunt with the output and the input as well, both the output impedance and the input impedance are decreased.
- 17. 17 Voltage-Shunt Feedback Negative Voltage Feedback: In this method, the voltage feedback to the input of amplifier is proportional to the output voltage. In the voltage shunt feedback circuit, a fraction of the output voltage is applied in parallel with the input voltage through the feedback network. This is also known as shunt-driven shunt-fed feedback i.e., a parallel-parallel proto type. The figure shows the block diagram of voltage shunt feedback, by which it is evident that the feedback circuit is placed in shunt with the output and also with the input. As the feedback circuit is connected in shunt with the output and the input as well, both the output impedance and the input impedance are decreased.
- 18. 18 Current-Series Feedback Negative Current Feedback: In this method, the voltage feedback to the input of amplifier is proportional to the output current. In the current series feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as series- driven series-fed feedback i.e., a series-series circuit. The figure shows the block diagram of current series feedback, by which it is evident that the feedback circuit is placed in series with the output and also with the input. As the feedback circuit is connected in series with the output and the input as well, both the output impedance and the input impedance are increased.
- 19. 19 Current-Shunt Feedback Negative Current Feedback: In this method, the voltage feedback to the input of amplifier is proportional to the output current. In the current shunt feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as series driven shunt-fed feedback i.e., a series-parallel circuit. The figure shows the block diagram of current shunt feedback, by which it is evident that the feedback circuit is placed in series with the output but in parallel with the input. As the feedback circuit is connected in series with the output, the output impedance is increased and due to the parallel connection with the input, the input impedance is decreased.
- 20. 20 Characteristics Types of Feedback Voltage-Series Voltage-Shunt Current-Series Current-Shunt Voltage Gain Decreases Decreases Decreases Decreases Bandwidth Increases Increases Increases Increases Input resistance Increases Decreases Increases Decreases Output resistance Decreases Decreases Increases Increases Harmonic distortion Decreases Decreases Decreases Decreases Noise Decreases Decreases Decreases Decreases
- 21. Reference Books 21 1. Principle of Electronics by V.K. Mehata 2. Basic Electronics (Solid State) by B.L. Theraja 3. Handbook of Electronics by Gupta Kumar
- 22. “Once you stop learning, you start dying 22 22 Albert Einstein
- 23. 23 THANKS! Any questions? You can find me at [email protected]