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 the output swings high the NPN transistor sources the output current to the load When it swings low the NPN transistor turns off and current from the load flows through the emitter resistor to the 30 V supply 0 6 V 0V With no signal signal the quiescent current is 30V 8 ohms 4 amps e g audio speaker April 29 2014 Physics 160 The quiescent power is 45 V 4 A 180 W in just the output stage 9 Push Pull Output Stage Bias currents are near zero with no signal applied applied Class B Amplifier Q1 NPN emitter follower VCC 10Vdc Q2N3904 V3 VOFF 0 VAMPL 2 FREQ 1k V V Q2 10Vdc PNP emitter follower 0 The small 2N3904 and 2N3096 transistors in this simulation are only for illustration A real power amp would use big power transistors April 29 2014 R1 Q2N3906 VEE Physics 160 1k load 0 For positive output swings the NPN transistor sources the current t for f negative ti swings i the PNP transistor sinks the current 10 Severe Cross Over Distortion Major j crossover distortion April 29 2014 Physics 160 11 Reducing Cross Over Distortion Class AB Amplifier Diodes ensure that at least one transistor is always l on b butt att the th expense of using more power even with no input p signal g 510 1uA For stable bias you wantt the th current through the diodes to be much larger than the base current R3 18k Q1 145 1uA Q2N3904 Q VCC 10Vdc D1 D1N914 R4 2 397 9uA D2 D1N914 R5 2 These improve temperature stability 25 44mA V3 VOFF 0 VAMPL 2 FREQ 1k VEE 65 98uA R1 32 88uA 10Vdc 200 Q3 0 112 0uA 112 0uA Q2N3906 25 41mA 0 R2 18k April 29 2014 V Physics 160 25 01mA Size these resistors according to the drive current needed 12 Cross Over Distortion Gone We will see that another way of greatly reducing cross over cross over distortion is by negative feedback April 29 2014 Physics 160 13 Differential Amplifier Extremely E t l common and d iimportant t t circuit i it element l t When transmitting a small signal over a significant distance noise can be rejected to a very high degree by encoding the signal into a difference of two voltages voltages This typically translates into sending the signal as a current loop e g the LVDS standard Si Signal l current R V Diff amp receiver output The two wires are typically twisted to avoid picking up magnetic noise in the current loop All voltage g noise that is common to the two wires g gets rejected j by y the differential amplifier The termination resistor R is chosen to match the characteristic impedance of the transmission line typically 100 ohms for twisted wires i Thi This avoids id reflections fl ti April 29 2014 Physics 160 14 Differential Amplifier Emitter follower E itt f ll ffor output drive capability low output impedance 15 00V 945 8uA Differential Amplifier p RC 7 5k 1 706mA Q4 V1 15Vdc 7 907V Differentialmode input 11 11uA Q2N3904 1 717mA 952 0uA v 3 604mA 3 604mA 0V 934 7uA Q1 Q2 6 143uA v vout 7 229V 7 229V 6 498uA 0V Q2N3904 V Q2N3904 7 229uA RLoad Vdm 1Vac 0Vdc 659 6mV 6 143uA RE1 RE2 958 1uA 661 3mV 1Meg 941 2uA 100 1 710mA 100 RE 13k 0V Common mode input Vcm 755 5mV 0Vac 0Vdc 12 64uA 1 899mA RT 0V tail current current 7 5k V2 0 GDM RC 1 7500 12 30 2 RE re 100 25 GCM RC 2 RT RE rE 1 7500 2 7500 15Vdc Diff mode gain 30 0 3 609mA 15 00V April 29 2014 vout 30 v v Physics 160 Common mode gain 0 5 CMRR 60 15 0 5 Diff Amp with Current Source in Tail 15 00V 1 012mA RC Emitter Follower 7 5k 1 668mA 1 668mA Q4 Diff Amp 7 410V 10 83uA Q2N3904 V1 Non Inverting Input 15Vdc 1 020mA 1 001mA Q1 3 700mA 0V Q2 6 523uA 1 679mA Inverting Input 6 734V 6 935uA 0V Q2N3904 V Q2N3904 6 734uA RLoad Vdm 1Vac 0Vdc 661 5mV 6 523uA RE1 RE2 1 027mA 663 3mV 1 008mA 100 1Meg 1 672mA 100 RE 13k 0V 0Vac 0Vdc 2 035mA 13 46uA Q3 R2 0V 1 023mA V2 12k 12 27V 12 39uA Q2N3904 Current Source 1 010mA 15Vdc 0 R3 2 7k 4 729mA 12 95V 2 047mA RT …


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