UCSC PHYS 160 - Lecture 9 Notes (16 pages)

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Lecture 9 Notes



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Lecture 9 Notes

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Pages:
16
School:
University of California, Santa Cruz
Course:
Phys 160 - Practical Electronics

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Physics 160 Lecture 9 R Johnson Bootstrap Clever technique to raise the input impedance of the bias network of an emitter follower or commonemitter itt amplifier lifi It relies on the fact that the gain of the emitter relative to the base is very close to unity Do not try to apply a bootstrap from a node with gain far from unity April 29 2014 Physics 160 2 Emitter Follower with Simple Bias Network 15 00V 764 5uA R1 10k 4 980mA High source impedance of 10kohm RS V2 10k 15Vdc Q1 C1 0V 0 1uF Extra resistor serves no purpose at the moment 7 218V 29 08uA Q2N3904 C2 5 745mA 0V 7 355V 735 5 A 735 5uA RB 4 7k 10u V 0V 5 009mA 5 009mA 0A 1Vac 0Vdc R2 10k VS RE RLoad 1 3k 10Meg 0A 0V 0 Input impedance 9 2k for 150 f 3dB April 29 2014 Physics 160 1 170 Hz 2 9 2k 0 1 F 3 No Source Impedance No Bootstrap f3dB 170 170 H Hz as expected t d April 29 2014 Physics 160 4 With Source Impedance 9 3 0 48 10 9 3 The low impedance of the bias network is killing more than half of our signal April 29 2014 Physics 160 5 Follower with Bootstrap Input impedance is boosted up to 135 k 15 00V Note the same trick will work with the common emitter amplifier 764 5uA 764 5uA R1 10k High source impedance of 10kohm 4 980mA RS V2 10k 15Vdc Q1 C1 0V 0 1uF 7 218V re 5 5 29 08uA Q2N3904 C2 5 745mA 0V 0 7 355V 355 735 5uA R2 10k VS 10u C3 0 4u 0A 1Vac 0Vdc RB 4 7k V 0V 5 009mA RE RLoad 1 3k 10Meg 0A 0V Above 80Hz the output feeds back through C3 and keeps the voltage changes h across RB close l tto zero making RB look like a very high impedance April 29 2014 1 80 Hz 2 0 4 F 5k 0 Physics 160 6 Ratio of Voltages Across RB RE 1300 0 996 RE rE 1305 At signal frequencies the effective resistance of RB is RB April 29 2014 Physics 160 1 4 7 k 780 k 1 0 994 7 Bootstrapped Follower with 10k Source R 135 0 93 10 135 From this simulation we infer a signal input impedance of 135 kohm dominated by the transistor base now not by the bias network April 29 2014 Physics 160 8 High Power Output Stage Class A Amplifier When



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