Chapter 8 Feedback 1 Desensitize The Gain 2 Reduce Nonlinear Distortions 3 Reduce The Effect of Noise 4 Control The Input And Output Impedances 5 Extend The Bandwidth Of The Amplifier The General Feedback Structure A xs xi xo A xi xi xs xf xf xo Af xf xo A xs 1 A xo A 1 A feedabck factor loop gain amount of feedabck xf A x s 1 A The General Feedback Structure MATLAB SIMULINK Anyone interested to put together an Introduction to MATLAB Simulink PowerPoint Presentation plus some examples The General Feedback Structure Exercise 8 1 4 A f 10 A 10 a b Af c Amount Feedback 20 log 1 A Amount Feedback 60 R1 R1 R2 A d 1 A Vs 1 Vo A f Vs Vf Vo 1 given Vo 10 Vf 0 999 Vi Vs Vf 4 Vi 10 10 A Af 1 A Find R1 R1 R2 0 1 e 4 A 0 8 10 0 1 R2 R1 9 10 9 998 10 100 0 02 A f A 1 A A f 9 998 The General Feedback Structure Exercise 8 1 Some Properties of Negative Feedback Gain Desensitivity Af A 1 A deriving dAf dA 1 A dividing by 2 dAf 1 Af 1 A Af A 1 A dA A The percentage change in Af due to variations in some circuit parameter is smaller than the pecentage cahnge in A by the amount of feedback For this reason the amount of feedback 1 A is also known as the desensitivity factor Some Properties of Negative Feedback Bandwidth Extension High frequency response with a single pole A s AM 1 s H AM denotes the midband gain and H the upper 3 dB frequency Af s A s 1 A s AM 1 AM Af s 1 Hf s H 1 AM H 1 AM Lf L 1 AM Some Properties of Negative Feedback Noise Reduction Reduction of Nonlinear Distortion Read and discuss in class The Four Basic Feedback Topologies Voltage Amplifiers V V Current Amplifiers I I Transconductance Amplifiers I V Transresistance Amplifiers V I The Four Basic Feedback Topologies The four basic feedback topologies a voltage sampling series mixing series shunt topology b current sampling shunt mixing shunt series topology c current sampling series mixing series series topology d voltage sampling shunt mixing shunt shunt topology The Four Basic Feedback Topologies Voltage Amplifiers VCVS Input Resistance High Output Resistance Low Feedback sample the output voltage Voltage sampling series mixing Current Amplifiers Transconductance Amplifiers Transresistance Amplifiers The Shunt Series Feedback Amplifier The Ideal Situation The series shunt feedback amplifier a ideal structure b equivalent circuit Z of s Af Rif Vo A Vs 1 A Vs Vs Ii Vi Vs Ri Vi Vi A Vi Ri Vi Ri Rif Ri 1 A Zif s Zi s 1 A s s Z o s 1 A s s The Shunt Series Feedback Amplifier The Practical Situation Derivation of the A circuit and circuit for the series shunt feedback amplifier a Block diagram of a practical series shunt feedback amplifier b The circuit in a with the feedback network represented by its h parameters c c The circuit in b after neglecting h21 The Shunt Series Feedback Amplifier Summary For Finding the A Circuit for a given series shunt feedback amplifier The Shunt Series Feedback Amplifier Example 8 1 RL R1 R2 A Vo Vi RL R1 R2 RL R1 R2 RL R1 R2 Af Rof Ro Vf R1 Vo R1 R2 Vo A Vf 1 A Ro 1 A RL R1 R2 ro RL R1 R2 ro RL R1 R2 RL R1 R2 Rid ro Rid Rs Rif R1 R2 R1 R2 Ri 1 A Ri Rs Rid Rin Rif Rs R1 R2 R1 R2 The Shunt Series Feedback Amplifier Exercise 8 4 The Series Series Feedback Amplifier The Ideal Case The Series Series Feedback Amplifier The Ideal Practical Case The Series Series Feedback Amplifier For Finding the A Circuit for a given series series feedback amplifier The Series Series Feedback Amplifier Example 8 2 The Series Series Feedback Amplifier Example 8 2 The Series Series Feedback Amplifier Example 8 2 Gain of the first stage RC1 r 2 1 RC1 r 2 Vc1 Vi1 RE1 RF RE2 re1 RE1 RF RE2 Since Q1 is biased at 0 6mA re1 41 7ohms Q2 is biased at 1mA r 2 h fe 100 g m2 40 2 5kohms substituting 1 0 99 RC1 RF Vc1 Vi1 14 92 V V 9kohms RE1 100ohms 640ohms RE2 100ohms The Series Series Feedback Amplifier Example 8 2 The Shunt Shunt and Shunt Series Feedback Amplifiers Shunt Configuration The Shunt Shunt and Shunt Series Feedback Amplifiers The Shunt Shunt and Shunt Series Feedback Amplifiers Example 8 3 The Shunt Series Feedback Amplifier The Shunt Series Feedback Amplifier The Shunt Series Feedback Amplifier Example 8 4 The Shunt Series Feedback Amplifier Example 8 4 Determining Loop Gain The Stability Problem and Margins Polar Plot Af s Nyquist Closed Loop Transfer Function Root Locus Bode A s 1 A s s The Nyquist Plot intersects the negative real axis at 180 If this intersection occurs to the left of the point 1 0 we know that the magnitude of the loop gain at this frequency is greater than the unity and the system will be unstable If the intersection occurs to the right of the point 1 0 the system will be stable It follows that if the Nyquist encircles the point 1 0 the amplifier will be unstable The Stability Problem and Margins Nyquist Closed Loop Transfer Function Root Locus Magnitude and Phase Bode Gain Margin Phase Margin If at the frequency of unity loop gain magnitude the phase lag is in excess of 180 degrees the amplifier is unstable The Nyquist Plot w 100 99 9 100 j 1 s w j w f w 1 50 4 6 G w 3 2 s w 9 s w 30 s w 40 5 Im G w 0 0 5 2 1 0 1 2 Re G w 3 4 5 6 Effect of Feedback On The Amplifier Poles Stability Study Using Bode Plots w 1 11 2 G w K 2 K j w j w 1 j w 2 j 1 Bode1 w 20 log G w 20 20 Open Loop Bode DiagramBode1 w Bode1 w 0 0 20 20 0 0 5 1 w T w G w 1 5 0 5 1 1 5 2 w 2 T w 0 G w Bode2 w 20 log T w 1 G w 1 G w 10 0 Bode2 w 10 Closed Loop Bode Diagram 20 0 5 1 1 5 w 2 Frequency Compensation Spice Simulation Examples