Analog & Digital Electronics

1 Analog & Digital ElectronicsCourse No: PH-218 Lec-14: Feedback AmplifierCourse Instructors: Dr. A. P. VAJPEYID epartment of Physics, Indian Institute of Technology Guwahati, India1 Marks distribution: Quiz-2 345678 MarksStudents20123123456789101112 Highest: 07 ( 1 student)Lowest: 00 ( 5 students)Feedback Amplifier Feedback amplifier contains two component namely feedbackcircuitand amplifier circuit. Feedback circuit is essentially a potential divider consisting ofresistances R1& R2 The purpose of feedback circuit is to return a fraction of theoutputvoltage to the input of the amplifier circuit Feedback amplifier 21212 RRRVV+== feedback fraction Both negative feedback and positive feedback are used in amplifiercircuits.

2 Negative feedback returns part of the output to oppose theinput, whereas in positive feedback the feedback signal aids the If Vf= 0 (there is no feedback)ioSoVVVVA==44 Feedback amplifier iSVV)(fsioVVAAVV ==ofVV =If feedback signal Vfis connected in series with the input, then Vi= VS- VfBut )(osoVVAV =)1(AAVVAsof +==soAVAV=+)1( Af: closed-loop gain of the amplifierA: Open-loop gain of the amplifier gain Negative feedback can reduce the gain of the amplifier, but it has manyadvantages, such as gain stabilization, reduction of nonlinear distortionand noise, control of input and output impedances, and extension of Negative feedback For negative feedback: A > 0 ; For positive feedback: A < 0 Feedback 5bandwidth.

3 Gain stabilization)1(AAAf +=2)1(1 AAdAf +=AdAAAdAff)1(1 +=Therefore percentage change in Af (due to variations in some circuit parameter) is reduced by (1+ A) times compared to without feedback. Advantages of Negative feedback Reduction of nonlinear distortion(a) Open loop gain(b) Closed loop gain6 Bandwidth extensionBasic Feedback Topologies Depending on the input signal (voltage or current) to be amplified and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used.

4 Voltage series feedback (Af= Vo/Vs) Voltage amplifier Voltage shunt feedback (Af= Vo/Is) Trans-resistance amplifier Current series feedback (Af= Io/Vs) -Trans-conductance amplifier7fos Current shunt feedback (Af= Io/Is) - Current amplifier Here voltage refers to connecting the output voltage as input to thefeedback network. Similarly current refers to connecting the output currentas input to the feedback network. Series refers to connecting the feedback signal in series with the inputvoltage; Shunt refers to connecting the feedback signal in shunt (parallel)with an input current topologies 8 Feedback topologies: Voltage Shunt Feedback Outputresistanceoftheamplifier and output resistanceof the feedback circuit are inparallel hence effectiveoutputresistanceofthefeedbackam plifier will reduce.

5 Similarlyoverallinputresistanceofthefeed backamplifier will reducedue toparallelconnectionofamplifier Since effective input resistance is small hence input should be a current. (for ideal voltage source input resistance is very high compare to internal source resistance, if not then, lot of voltage will be dropped at internal source resistance and voltage source won t be a ideal voltage source ) Effective output resistance is also small compare to the resistance of amplifier without feedback hence less voltage will drop at Ro-eff and most of the voltage occurs at RL.

6 Hence output ckt will behave like a voltage source. Thus voltage shunt feedback ckt behave like a current controlled voltage feedback topologies: Voltage Shunt Feedback Voltage Gain : )1()1()()(AAIVAVAAIVVIAVIIIAiIAVofoSooSo ffSo +==+== = = =101 AISf +Input Impedance :)1( ) ininArZAIIrIVIrIIIVIVZ iiiiioiiifiiSi +=+ =+ =+==Output Impedance :ooooiooofiooVrAVVrAIVIVIIIVZS += = = ===port,output fromport,input from|0out)1(outArIVZooo +==Feedback topologies: Voltage Series Feedback Output resistance of the amplifier and output resistance ofthe feedbackcircuit are in parallel henceeffective output resistanceof the feedbackamplifier will reduce.

7 Input resistance of the amplifier and feedback network are in serieshence effective input resistance will topologies: Voltage Series Feedback Voltage Gain : )1()1()()(AAVVAVAAVVVVAVVVVAiVAVofoSooSo ffSo +==+== = = =Input Impedance :)1()1( ininArIAVZIAVVIVVIVVIVZiSiSiiSoiSfiSS +=+=+=+=+==Output Impedance : + === + ===VAVVVVVVrVAVIIVZoiSoioiooooVS0|0out12 121 AVSf +IS +== +=ArIVZrVAVI ooooooo1outFeedback topologies: Current Series Feedback Output resistance of the amplifier and output resistance ofthe feedbackcircuit are in series hence effective output resistance of the feedback amplifierwill increase.

8 Input resistance of the amplifier and feedback network are in series henceeffective input resistance will increase. Thus Current series feedback circuit behave like a voltage controlled +Basic amplifierIS+ ViriVf= IoroAViI VSV + )1()1()()(AAVIAIAAVIIVAIVVVAiVAISofoSooS offSo +==+== = = =Voltage Gain :Feedback topologies: Current Series Feedback 14+ Feedback networkVf= Io1 AVS +Input Impedance :)1()1( ininArIAVZIAVVIVVIVVIVZiSiSiiSoiSfiSS +=+=+=+=+==Output Impedance :oiooooVrVAVIIVZS ===;|0out +==ArIVZooo1outooooSfirIAVIVVV +===+ 0 Feedback topologies: Current Shunt Feedback Effective output resistance of the feedback amplifier willincrease.

9 Effective input resistance will decrease. Thus current shunt feedback circuitbehave like a current controlledcurrent