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By: Dr Thangjam Tejkumar Singh Assistant Professor Department of Physics,

Feedback in Amplifiers. By: Dr Thangjam Tejkumar Singh Assistant Professor Department of Physics, Don Bosco College Maram, Manipur. Outline. Introduction The general feedback structure Some properties of negative feedback The four basic feedback topologies

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By: Dr Thangjam Tejkumar Singh Assistant Professor Department of Physics,

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  1. Feedback in Amplifiers By: Dr Thangjam Tejkumar Singh Assistant Professor Department of Physics, Don Bosco College Maram, Manipur.

  2. Outline • Introduction • The general feedback structure • Some properties of negative feedback • The four basic feedback topologies • The series-shunt feedback amplifier • The series-series feedback amplifier • The shunt-shunt feedback amplifier, and • The shunt-series feedback amplifier

  3. Introduction • Definition of feedback • It’s impossible to think of electronic circuits without some forms of feedback. Namely: • Negative feedback (and its characteristics) • Desensitize the gain • Reduce nonlinear distortion • Reduce the effect of noise • Control the input and output impedance • Extend the bandwidth of the amplifier The basic idea of negative feedback is to trade off gain for other desirable properties. • Positive feedback (Increase in gain of an amplifier) • Will cause the amplifier oscillation.

  4. The General Feedback Structure This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

  5. The General Feedback Equation Closed loop and open loop Closed loop gain (Gain of amplifier with –ve feedback) Feedback factor β Loop gain Aβ Amount of feedback (1+ Aβ)

  6. Some Properties of Negative Feedback • Gain desensitivity • Bandwidth extension • Noise reduction • Reduction in nonlinear distortion

  7. The Four Basic Feedback Topologies • Voltage amplifier---series-shunt feedback (voltage mixing and voltage sampling) • Current amplifier---shunt-series feedback (Current mixing and current sampling) • Transconducatnce amplifier---series-series feedback (Voltage mixing and current sampling) • Transresistance amplifier---shunt-shunt feedback (Current mixing and voltage sampling)

  8. The Series-Shunt Feedback Topologies Block diagram of voltage-mixing voltage-sampling (series–shunt) topology

  9. The Amplifier with Series-Shunt Feedback voltage-mixing voltage-sampling (series–shunt) topology (real circuit)

  10. The Shunt-Series Feedback Topologies Block diagram of current-mixing current-sampling (shunt–series) topology

  11. The Amplifier with Shunt-Series Feedback current-mixing current-sampling (shunt–series) topology

  12. The Series-Series Feedback Topologies voltage-mixing current-sampling (series–series) topology

  13. The Amplifier with Series-Series Feedback voltage-mixing current-sampling (series–series) topology

  14. The Shunt-Shunt Feedback Topologies current-mixing voltage-sampling (shunt–shunt) topology

  15. The OP Amplifier withShunt-Shunt Feedback current-mixing voltage-sampling (shunt–shunt) topology

  16. Input and Output Resistance with Feedback Input resistance In this case, the negative feedback increases the input resistance by a factor equal to the amount of feedback. Output resistance In this case, the negative feedback reduces the output resistance by a factor equal to the amount of feedback.

  17. The Practical Situation(of Series-Shunt Feedback Amplifier) • Block diagram of a practical series–shunt feedback amplifier. • Feedback network is not ideal and load the basic amplifier thus affect the values of gain, input resistance and output resistance.

  18. Thank You

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