a159298350984.pdf
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The document introduces feedback amplifiers, explaining the principle of feeding back a portion of the output signal to the input. It details two types of feedback: positive, which increases gain but also distortion and instability, and negative, which reduces gain but improves stability and decreases noise. Additionally, it discusses the effects of negative feedback on various amplifier characteristics, including distortion, gain stability, input/output impedances, and bandwidth.
a159298350984.pdf
- 3. Introduction of Feedback Amplifiers: The phenomenon of feeding a portion of the output signal back to the input circuit is known as feedback. The effect results in a dependence between the output and the input and an effective control can be obtained in the working of the circuit. Feedback is of two types. • Positive Feedback • Negative Feedback
- 4. Positive or regenerate feedback: • In positive feedback, the feedback energy (voltage or currents), is in phase with the input signal and thus aids it. Positive feedback increases gain of the amplifier also increases distortion, noise and instability. • Because of these disadvantages, positive feedback is seldom employed in amplifiers. But the positive feedback is used in oscillators.
- 6. Negative or Degenerate feedback: • In negative feedback, the feedback energy (voltage or current), is out of phase with the input signal and thus opposes it. • Negative feedback reduces gain of the amplifier. It also reduce distortion, noise and instability. • This feedback increases bandwidth and improves input and output impedances. • Due to these advantages, the negative feedback is frequently used in amplifiers.
- 8. Comparison Between Positive and Negative Feed Back: S.No. Negative Feedback Positive Feedback 1. 2. 3. 4. 5. 6. Feedback energy is out phase with their input signal Gain of the amplifier decreases Gain stability increases Noise and distortion decreases. Increase the band width Used in amplifiers Feedback energy is in phase with the input signal. Gain of the amplifier increases Gain stability decreases Noise and distribution increases. Decreases bandwidth Used in Oscillators
- 9. Principle of Feedback Amplifier: A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amplifier can be understood from the following figure.
- 11. In 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. This voltage is subtracted for negative feedback, from the signal voltage Vs. Now, Vi=Vs−Vf=Vs−βVo The quantity β = Vf/Vo is called as feedback ratio or feedback fraction.
- 12. 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 AVs−AβVo=Vo AVs=Vo(1+Aβ) Vo/Vs=A/(1+Aβ) Therefore, the gain of the amplifier with feedback is given by Af =A/(1+Aβ)
- 13. Effect of negative feedback on amplifier performance: The effect of negative feedback on an amplifier is considered in relation to gain, gain stability, distortion, noise, input/output impedance and bandwidth and gain-bandwidth product.
- 14. Gain: The gain of the amplifier with feedback is given by Af =A/(1+Aβ) Hence, gain decreases with feedback.
- 15. Gain Stability: An important advantage of negative voltage feedback is that the resultant gain of the amplifier can be made independent of transistor parameters or the supply voltage variations, Af=A/(1+Aβ) For negative voltage feedback in an amplifier to be effective, the designer deliberately makes the product Aβ much greater than unity. Therefore, in the above relation, ‘1’ can be neglected as compared to Aβ and the expression becomes
- 16. Af = A/(1+Aβ) = 1/β It may be seen that the gain now depends only upon feedback fraction, β, i.e., on the characteristics of feedback circuit. As feedback circuit is usually a voltage divider (a resistive network), therefore, it is unaffected by changes in temperature, variations in transistor parameters and frequency. Hence, the gain of the amplifier is extremely stable.
- 17. Distortion: A power amplifier will have non-linear distortion because of large signal variations. The negative feedback reduces the nonlinear distortion. It can be proved mathematically that: Df = D/(1+Aβ) Where D = distortion in amplifier without feedback Df = distortion in amplifier with negative feedback It is clear that by applying negative feedback, the distortion is reduced by a factor (1+Aβ)
- 18. Noise : There are numbers of sources of noise in an amplifier. The noise N can be reduced by the factor of (1+Aβ), in a similar manner to non-linear distortion, so that the noise with feedback is given by However, if it is necessary to increase the gain to its original level by the addition of another stage, it is quite possible that the overall system will be noisier that it was at the start. If the increase in gain can be accomplished by the adjustment of circuit parameters, a definite reduction in noise will result from the use of negative feedback. Nf = N/(1+Aβ)
- 19. Input / Output Impedance : The input and output impedances will also improve by a factor of (1+Aβ), based on feedback connection type.
- 20. Bandwidth and Gain-bandwidth Product: Bandwidth and Gain-bandwidth Product Each of higher and lower cut-off frequencies will improve by a factor of (1+Aβ). However, gain-bandwidth product remains constant. fhf= fh (1+Aβ) flf= fl /(1+Aβ)
- 21. An important piece of information that can be obtained from a frequency response curve is the bandwidth of the amplifier. This refers to the ‘band’ of frequencies for which the amplifier has a useful gain. Outside this useful band, the gain of the amplifier is considered to be insufficient compared with the gain at the centre of the bandwidth. The bandwidth specified for the voltage amplifiers is the range of frequencies for which the amplifiers gain is greater than 0.707 of the maximum gain Alternatively, decibels are used to indicate gain, the ratio of output to input voltage. The useful bandwidth would be described as extending to those frequencies at which the gain is -3db down compared to the gain at the mid-band frequency.
- 22. Diagram of an emitter follower Feedback in Emitter Follower Amplifier:
- 23. Operation: For the emitter follower, the input voltage is applied at base and the resulting a.c. emitter current produces an output voltage (IeRE) across the emitter resistance. This voltage opposes the input voltage, thus providing negative feedback (Voltage series). It is called emitter follower because the output voltage follows the input voltage. The major characteristics of the emitter follower are: The voltage gain of an emitter follower is close to 1. Relatively high current gain and power gain. High input impedance and low output impedance. Input and output ac voltages are in phase.
- 24. Classification of Basic Amplifiers: Amplifiers can be classified broadly as, • Voltage amplifiers. • Current amplifiers. • Transconductance amplifiers. • Transresistance amplifiers.
- 25. Voltage Amplifier: Equivalent circuit of voltage amplifier. Ri >> Rs and Ro << RL
- 26. Current Amplifier: Ri << Rs and Ro >> RL Equivalent circuit for current amplifier
- 27. Transconductance Amplifier: Equivalent circuit for transconductance amplifier Ri >> Rs and Ro >> RL
- 28. Transresistance Amplifier: Ri << Rs and Ro << RL Equivalent circuit for transresistance amplifier
- 29. Sl. No. Type Input Output Ri Ro 1 Voltage Amplifier Voltage Voltage High Low 2 Current Amplifier Current Current Low High 3 Transconductance Amplifier Voltage Current High High 4 Transresistance Amplifier Current Voltage Low Low Summary:
- 30. Classification of Feedback Amplifiers: There are four types of feedback, • Voltage series feedback. • Voltage shunt feedback. • Current shunt feedback. • Current series feedback
- 31. Voltage series feedback. Voltage shunt Feedback Rif = Ri (1+Aβ) Rof = Ro / (1+Aβ) Rif = Ri / (1+Aβ) Rof = Ro / (1+Aβ)
- 32. Current Shunt Feedback Current Series Feedback Rif = Ri / (1+Aβ) Rof = Ro (1+Aβ) Rif = Ri (1+Aβ) Rof = Ro (1+Aβ)
- 33. Effect of feedback on Input Resistance: Voltage shunt Feedback Current Shunt Feedback Voltage series feedback. Current series Feedback Rif = Ri / (1+Aβ) Rif = Ri / (1+Aβ) Rif = Ri (1+Aβ) Rif = Ri (1+Aβ)
- 34. Effect of feedback on Output Resistance: Voltage shunt Feedback Current Shunt Feedback Voltage series feedback. Current series Feedback Rof = Ro / (1+Aβ) Rof = Ro / (1+Aβ) Rof = Ro (1+Aβ) Rof = Ro (1+Aβ)
- 35. Sl. No. Type Rif Rof 1 Voltage Shunt Feedback Amplifier 2 Current Shunt Feedback Amplifier 3 Voltage Series Feedback Amplifier 4 Current Series Feedback Amplifier Summary: Rif = Ri / (1+Aβ) Rif = Ri / (1+Aβ) Rif = Ri (1+Aβ) Rif = Ri (1+Aβ) Rof = Ro / (1+Aβ) Rof = Ro / (1+Aβ) Rof = Ro (1+Aβ) Rof = Ro (1+Aβ)
- 37. Current Series Feedback Voltage Shunt Feedback
- 38. GATE Questions and Solutions
- 75. 14. Match the following 1. 2. 3. 4.
- 76. Sol: Trick to remember The first term shows how the feedback is taken Voltage is measured in parallel (by voltmeter) - Hence 1st term voltage means, the parallel connection at the output Current is measured in series - Hence 1st term current means series connection at the output. Series connection increases resistance. Parallel connection decreases resistance . Answer: 1