電子電路基礎英文課件：ch6 Feedback Amplifiers

1、A feedback amplifier is a close loop:Basic amplifier without feedback (also termed as open loop)Feedback amplifierSampling of output signalsSuperimposition of feedback signal and input signalidifxxx consists of returning part of the output signal of a system to the input side, and summing or subtrac

2、tion with the original input signal to form a net input signal to the amplifier. Feedback network existing in DC networks introduces DC feedbackImprove quiescent pointFeedback network existing in AC networks introduces AC feedbackImprove AC characteristics of an amplifierFeedback types:The feedback

3、signal is of opposite polarity to the input signal causing the net input signal be reduced compared to the original input signalTypical application: to improve circuit featuresThe feedback signal is of same polarity to the input signal causing the net input signal be increasedTypical application: Os

4、cillator circuitBe identified by analyzing instantaneous polarities for a network. Be identified by analyzing DC and AC equivalent network Example 6.1.1：For feedback networks illustrated in Figure 6.1.2, determine whether the feedback is positive feedback or negative feedback, and determine whether

5、the feedback is DC feedback or AC feedback. DC networkAC networkInstantaneous polarity (1) voltage-series(2) voltage-parallel (3) current-series(4) current-parallelVoltage feedback Current feedback How to identify voltage feedback or current feedback?ooxuooxiSeries mixing Parallel mixing ,iidfiidfx

6、xxu uu,iidfiidfx xxi iiHow to identify series feedback or parallel feedback?Example 6.2.1：A feedback amplifier is illustrated in Figure 6.2.3(a), determine the polarity of AC feedback and if it is negative feedback, identify its specific type AC networkTo identify voltage feedback and current feedba

7、ck, a simple test is to short-circuit the load to see whether the feedback signal vanishes. In parallel mixing, the feedback signal and the input signals are connected at the same input terminal.Example 6.2.2：A feedback circuit is shown in Figure 6.2.4(a), determine the polarity of AC feedback. If i

8、t is a negative feedback, identify its specific type. In series mixing, the feedback signal and the input signal are connected at two input terminals separately, and combined by voltage signals.AC networkExample 6.2.3：A feedback circuit is shown in Figure 6.2.5(a), determine the polarity of AC feedb

9、ack. If it is a negative feedback, identify its specific type. Example 6.2.4：A feedback circuit is shown in Figure 6.2.6(a), determine the polarity of AC feedback. If it is a negative feedback, identify its specific type . AC networkidifxxxoidxAxThe net input signal to the basic amplifier: The Negat

10、ive Feedback Amplifier (close loop)foxFxThe open-loop gain of the basic amplifier: The feedback coefficient: The gain with feedback (the closed-loop gain): 111oooofiidfidididxxxxAAxxxxAFxAF xAFFor an amplifier with strong negative feedback ( ) ,1AF1fAFThe closed-loop input resistance: Parallel mixin

11、g Series mixing (1)idfiididifiiiiuuuuAFuRAF RiiiNegative series feedback increases input resistance The closed-loop input resistance: Negative parallel feedback reduces input resistance 1iiiiifiidfididuuuRRiiiiAFiAFVoltage feedback 0ioofoxuRi()(1)oooooouAFuAF uiRR 1ooofouRRiAFnegative voltage feedba

12、ck reduces the output resistance For strong negative feedback with (1)AF 0ofRCurrent feedback (1)oofoovRAF Ri()ooooviAFiR negative current feedback increases the output resistance For strong negative feedback with (1)AF ofR 22(1)(1)(1)fAF dAAFdAdAdAAFAF1fAAAF1(1)ffdAdAAAFANegative feedback improves

13、the stability of the closed-loop gain. For an amplifier with strong negative feedback,1fAFNegative feedback improves the reduction of nonlinear distortion to times that of open-loop amplifiers. (1)AFThe product of the gain and bandwidth for the first order low-pass or high-pass amplifier is a consta

14、nt, which equals to the bandwidth with 0dB gain Negative feedback reduces the gain while extends the bandwidth of the closed-loop amplifier times(1)AFGain Reduced gain, but improve gain stabilityInput impedance *- series: increase input impedance *-parallel: decrease input impedanceOutput impedance

15、voltage-*: decrease output impedance current-*: increase output impedanceExtends bandwidthReduce nonlinear operation1fAFwhen 1AF (1)oofRRAF(1)ofoRAF R1iifRRAF(1)ifiRAF R1fAAAF(1)fBAF BifR 0ifR 0ofRofR Determine the feedback type(AC network)Determine the feedback Determine the feedback gainDetermine

16、otherunknowns F1fAF,vfifofARRwhen 1AF General method Divide the close-loop amplifier into two parts: a basic open-loop amplifier and a feedback network. For the open-loop amplifier, calculate , , . For the feedback network, calculate . CalculateSpecial method for strong negative feedback Approximate

17、 method By “virtue short” and “virtue open”, commonly used for operational amplifiersAiRoRF,fA,vfifofARR1fofAFxxfoiAxxifxx0idxFor series feedback ifvv0idv “virtue short” For parallel feedback ifii0idi “virtue open” Example 6.6：The AC network of a feedback amplifier is illustrated in Figure 6.4.1(a),

18、 identify the type of the negative feedback. Suppose it is a strong negative feedback, calculated the voltage gain、input resistance and output resistance of the amplifier denoted in the circuit. 11ffoofxvRFxvRR111ffRAFR oofvfiixvAAxv11fvfRAR 0ofRifR It is a voltage-series negative feedback amplifier

19、 Feedback networkExample 6.7：The AC network of a feedback amplifier is illustrated in Figure 6.4.2(a), identify the type of the negative feedback. Suppose it is a strong negative feedback, calculated the voltage gain、input resistance and output resistance of the amplifier denoted in the circuit. It

20、is a current-parallel negative feedback amplifier Feedback network2ffEoeEFxiRFxiRR 11FfERAFR 22oecfiiixiiAxii2222|1cCLCLCLoFvffss issEiRRRRRRvRAAvR iRRR ofR 0ifR Example 6.8：The AC network of a feedback amplifier is illustrated in Figure 6.4.3(a), identify the type of the negative feedback. Suppose

21、it is a strong negative feedback, calculated the voltage gain、input resistance and output resistance of the amplifier denoted in the circuit. ifR It is a current-series negative feedback amplifier Feedback network2ffEooxvFRxi211fEAFR2oocfiiixiiAxvv2|cCLCLovfCLfiiEiRRRRvARRAvvR ofR Example 6.9：The AC network of a feedback amplifier is illustrated in Figure 6.4.4(a), identify the type of the negative feedback. Suppose it is a strong