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Understanding Negative Feedback in Audio Amplifiers

Learn how negative feedback stabilizes and improves audio amplifier performance, types of feedback, analyzing circuits, and advantages of negative feedback.

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Understanding Negative Feedback in Audio Amplifiers

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  1. Review • 0、introduction • 1、what is feedback? • 2、why we need feedback in our circuit? • 3、how many types of feedback? • 4、how many types of negative feedback? • 5、how to judge all kinds of negative feedback? • 6、advantages of negative feedback? • 7、how to analyse the negative feedback circuit? • 8、conclusion----- assignment

  2. introduction • An audio-frequency amplifier can be designed to have a certain current, voltage or power gain together with particular values of input and output impedance. • The amplifier will add noise and distortion to the signals handled by it. The components employed in the amplifier, both passive (resistors, capacitors, etc.) and active (transistors and f.e.t.s), will vary in value with both time and change in temperature and will have manufacturing tolerances.

  3. introduction • The gain of the amplifier may therefore vary with • time, with change in ambient temperature, and when • a component has to be replaced by another of the • Same type and(nominal)value. • any fluctuations in the power supply may also cause • the gain of the amplifier to alter.

  4. feedback • 1、what is feedback? • 2、For many applications a more or less constant gain iS necessary，and this can be obtained if negative feedback (n.f.b.) iS applied to the amplifier, at the • expense, however,of a reduction in gain.

  5. why we need feedback in our circuit? • 1、make our circuit stabilization: • Quescent operation point; • Stability of gain: current gain, voltage gain; • Decrease non-linearity distortion • Reduces noise • Change input and output impedance • Reduces amplitude／frequency distortion • 2、generate oscillation (oscilloscope)

  6. Types of feedback • 0、d.c feedback • 1、a.c. feedback • 2、positive feedback • 3、negative feedback • 4、voltage feedback • 5、current feedback • 6、Series feedback • 7、parallel feedback

  7. chapter 4 negative feedback • 4.2 types of negative feedback • 4.2.1.voltage-voltage feedback, • 4.2.2.voltage-current feedback, • 4.2.3.current-current feedback. • 4.2.4.current-voltage feedback.

  8. The negative feedback • An n.f.b. amplifier has a fraction of its output signal feed back into its input terminals in antiphase with the input signal. • four types： • voltage-voltage feedback, • voltage-current feedback, • current-current feedback. • current-voltage feedback.

  9. Voltage-Voltage Feedback(Series-voltage Feedback)

  10. Voltage gain with n.f.b.

  11. EXAMPLE 4.1 • A voltage amplifier has a voltage gain of 100 before n.f.b.is applied.Calculate its voltage gain if 3/100 of the output voltage is fed back to the input in antiphase with the input signal. • Solution: • From equation(4.1),

  12. The voltage gain of a transistor amplifier without n.f.b.iS • The effective load resistance RL(eff)seen by the transistor in an emitter follower circuit iS the resultant of the emitter resistor R3 in parallel with the external load resistance RL.The voltage gain of an emitter follower is

  13. Voltage-Current Feedback(Series-current Feedback)

  14. When voltage-current feedback is applied to an amplifier the fed-back voltage is proportional to the current-flowing in the load.

  15. EXAMPLE 4.5 • An amplifier of the type shown in Fig.4.8b has the following data:R2=4.7 kΩ,R3=1 kΩand gm=5 mS.Calculate the voltage gain of the circuit. • Solution • From equation(4.6)

  16. Current-Current Feedback(Shunt-Current Feedback)

  17. Current-Voltage Feedback（Shunt-voltage Feedback)

  18. EXAMPLE 4.6 • Calculate the current gain of the circuit shown in Fig.4.12a if the current gain of the transistor is 120. • Solution • The feedback factor is (Ans.) • Therefore • Current gain with feedback • (Ans.)

  19. Negative Feedback

  20. 4.3 advantages of negative feedback • 4.3.1. stability of gain • 4.3.2.amplitude/frequency distortion • 4.3.3.non-linearity distortion • 4.3.4.noise • 4.3.4.input and output impedance • 4.4 instability in negative feedback amplifiers • 4.5 operational amplifiers • 4.5.1.operational amplifier parameters • 4.5.2.inverting and non-inverting amplifiers • 4.5.3.frequency compensation

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