Presentation on theme: "Review 0、introduction 1、what is feedback?"— Presentation transcript:
1 Review0、introduction1、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 introductionAn 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 The gain of the amplifier may therefore vary with introductionThe gain of the amplifier may therefore vary withtime, with change in ambient temperature, and whena component has to be replaced by another of theSame type and(nominal)value.any fluctuations in the power supply may also causethe gain of the amplifier to alter.
4 expense, however,of a reduction in gain. feedback1、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 theexpense, however,of a reduction in gain.
6 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 distortionReduces noiseChange input and output impedanceReduces amplitude／frequency distortion2、generate oscillation (oscilloscope)
10 voltage-voltage feedback, voltage-current feedback, The negative feedbackAn 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.
13 EXAMPLE 4.1A 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),
14 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
22 The feedback factor is (Ans.) Therefore Current gain with feedback EXAMPLE 4.6Calculate the current gain of the circuit shown in Fig.4.12a if the current gain of the transistor is 120.SolutionThe feedback factor is (Ans.)ThereforeCurrent gain with feedback(Ans.)