BJT Circuits & Limitations LTspice

BJT Circuits & Limitations LTspice Acnowledgements: Neamen, Donald: Microelectronics Circuit Analysis and Design, 3 rd Edition LTspice material by Devon Rosner 13 (6.101 TA), Engineer, Linear Technology 6.101 Spring 2015 Lecture 3 1

BJT Configurations Voltage Gain Current Gain Power Gain Common Emitter X X X Common Collector X X Common Base X X Common emitter: hgh input impedance, for general amplification of voltage, current and power from low power, high impedance sources. Common collector: aka "emitter follower" for high input impedance and current gain without voltage gain, as in an amplifier output stage. Common base: low input impedance for low impedance sources, for high frequency response. Grounding the base short circuits the Miller capacitance from collector to base and makes possible much higher frequency response. 6.101 Spring 2015 Lecture 3 2

General Configuration Common Emitter Common Collector Common Base 6.101 Spring 2015 Lecture 3 3

Transistor Configurations TRANSISTOR AMPLIFIER CONFIGURATIONS +15V +15V +15V R L R L R 2 R 2 R 2 + + + V in - + R 1 R E + V OUT - + V in - + R 1 R E + + V OUT - + V in - + R 1 + R E + V OUT - [a] Common Emitter Amplifier [b] Common Collector [Emitter Follower] Amplifier [c] Common Base Amplifier 6.101 Spring 2015 Lecture 3 4

Base Current Resistor Divider 68K i b I C F 3.7 ma 50 4.0 ma 100 4.2 ma 200 4.3 ma 300 I C =0.6 ma 33K i b Make small compared to the current through R 2 See handout: Transistor bias stability 6.101 Spring 2015 Lecture 3 5

Commom Emitter Hybrid π TRANSISTOR AMPLIFIER CONFIGURATIONS WITH HYBRID- EQUIVALENT CIRCUITS COMMON EMITTER AMPLIFER C R s + v in _ + R B I B R L +15V I C 2N3904 + v out _ g g 0 m v g m v m I V r CQ TH V TH 26mv b i b r R s R B R L + v in e _ c + v out _ A if R s =R B // R S v v v R' S out in r ; oibr i R' r b s then A L v or R' r s or o g m L L g m R L 6.101 Spring 2015 Lecture 3 6

Common Emitter with Emitter Degeneration 6.101 Spring 2015 Lecture 3 7 E L v E o s E o s L o E o s b L b o in out v R R then A R r R if R r R R R r R i R i v v A / ; 1 ' ; 1 ' 1 ' Input resistance (β+1)r E Voltage gain reduced by (1+g m R E ) Voltage gain less dependent on β (linearity) R s =R B // R S

AC Coupled vs DC Coupled Amplifiers AC Coupling Advantage: easy cascading with DC blocking capacitor, bias stability and stage independent Disadvantage: lot s of R s and C s, no DC gain, need large C for low freqency DC coupling Same gain at DC Fewer R s C s 6.101 Spring 2015 Lecture 3 8

Gain vs Frequency 6.101 Spring 2015 Lecture 3 9

Expanded Hybrid π r x 6.101 Spring 2015 Lecture 3 10

Miller Effect* Common Emitter C M C [ 1 gm( RC RL )] * Agarwal & Lang Foundations of Analog & Digital Electronics Circuits p 861 6.101 Spring 2015 Lecture 3 11

Low Pass Filter LPF log scale A V (db) R C V 1 V 2 0-3dB slope = -6 db / octave slope = -20 db / decade A A v v V V 2 1 j X C R j X 1 src 1 C 1 j C 1 R j C 1 j RC 1 0 o Degrees PHASE LAG f HI or f -3dB log f High frequency cutoff f hi 1 2RC -45 o -90 o log f f HI or f -3dB 6.101 Spring 2015 Lecture 2 12

g m q I C kt 0 h fe (datasheet) C C ob Hybrid π Parameters (datasheet) g m 2(C C ) f T (transit frequency datasheet) C g m 2 f T r C r x (low frequency):datasheet or estimate 50 100 (high frequency):estimate 25 6.101 Spring 2015 Lecture 3 13

β Use max for worst case c u 6.101 Spring 2015 Lecture 3 14

2N3904 CE configuration, V CC +15v 6.101 Spring 2015 Lecture 3 15

h fe and High Frequency Limits Small signal current gain versus frequency, h fe, of a BJT biased in a common emitter configuration: i b v r be v be jc h fe gmv i b be gmr 1 jr C 1 jr C For h fe =1 = f T, (transit frequency ) h T gm 2C where C ( c c ) For 2N3904*, I C =1ma, V CE =10V, c π =25pF, c μ =2pF f T 0.04mho 2 27 pf for a gain of 240MHz g m R L 100 f h 2 r 1 g m R L c 1 2 2.5K(100)2 pf 320kHz Miller effect reduces high frequency limit! *Lundberg, Kent: Become One with the Transistor p29 6.101 Spring 2015 Lecture 3 16

Common Base Configuration 6.101 Spring 2015 Lecture 3 17

Common Collector (Emitter Follower) 6.101 Spring 2015 Lecture 3 18 1 ; 1 ' ; 1 ' 1 1 ' 1 v E o s E o s E o E o s b E b o in out v A then R r R if R r R R R r R i R i v v A Buffer with unity gain High input resistance driving low output resistance (current gain). R s =R B // R S mv V V I g r g TH TH CQ m m 26 0 v g v m

Common Collector Emitter Follower Biasing R 2 A +15V 7.5 ma 2N3904 Β = 100, i B = 7.5ma/100 = 75µa Using Thevenin equivalent, R B = R 1 R 2, V B = 15 R R1 R 1 2 R 1 B 1.0 k 7.5 ma +15V V B = I B R B + 0.6V + 7.5V V B = [75 µa x 10k] + 0.6V + 7.5V V B = 750 mv + 0.6V + 7.5V V B = 8.9V I B R B V B 7.5 ma 2N3904 7.5 V [15 R 1 ] [R 1 + R 2 ] = 8.9V 15 R 1 = 8.9 x [R 1 + R 2 ] [15 8.9] R 1 = 8.9 R 2 R 1 = 1.44 R 2 [R 1 x R 2 ] [R 1 + R 2 ] = 10 kω [1.44R 2 x R 2 ] [1.44 R 2 + R 2 ] = 10kΩ R 2 = 16.9 kω (use 16 kω) R 1 = 1.44 R 2 = 24.4 kω (use 24 kω) 6.101 Spring 2015 Lecture 3 19

Common Collector Emitter Follower Biasing +15V With R1 = 24kΩ, R2 = 16 kω, the current through the voltage divider is 15 [40 kω] = 375 µa. R 2 I Divider 7.5 ma The 75 µa base current is 20% of 375 µa. 8.1 V R 1 A B 1.0 k 2N3904 7.5 ma With R1 = 2 kω, will need a divider current that is ~ 4.1 ma. (75 µa is only ~2% of 4.1 ma, which is negligible) The voltage drop across R2 will be [15 V 8.1 V] = 6.9 V; R2 = 1.7 kω But input impedance will be low = ~890Ω Use bootstrapping configuration = 24.4 kω (use 24 kω) 6.101 Spring 2015 Lecture 3 20

Low Frequency Hybrid Equation Chart High gain applications Moderate input resistance High output resistance Unity gain, low output resistance High input resist. High gain, better high frequency response Low input resistance 6.101 Spring 2015 Lecture 3 21

Introduction to LTspice Acknowledgment: LTspice material by Devon Rosner (6.101 TA), Engineer, Linear Technology 6.101 Spring 2015 Lecture 4 22

SPICE Simulation Program with Integrated Circuit Emphasis Developed in 1973 by Laurence Nagel at UC Berkeley s Electronics Research Laboratory Dependent on user defined device models 6.101 Spring 2015 Lecture 4 23

Netlists 6.101 Spring 2015 Lecture 4 24

LTspice Developed in 1998 by Mike Engelhardt at Linear Technology Corporation GUI, simulator and schematic > netlist for SPICE Free with lot s of models! 6.101 Spring 2015 Lecture 4 25

LTspice Available Windows and MAC OS X 10.7+ Windows version easier to use. For MAC users install Windows version under VMware or use WINE (short for Wine Is Not an Emulator) 6.101 Spring 2015 Lecture 4 26

Getting Started 6.101 Spring 2015 Lecture 4 27

Component to Menu Item 6.101 Spring 2015 Lecture 4 28

Net Labels Labeling nets avoid drawing wires. Use nets for power supply voltages, ground, circuit wide signals. 6.101 Spring 2015 Lecture 4 29

Adding Other Components 6.101 Spring 2015 Lecture 4 30

Op Amps 6.101 Spring 2015 Lecture 4 31

Editing Components 6.101 Spring 2015 Lecture 4 32

Component Values 1 MEG = 1 megohm resistor 1 = 1 farad capacitor (not 1F) 6.101 Spring 2015 Lecture 4 33

Basic Voltage Source Basic voltage source: used for DC supply voltage or bias voltage. 6.101 Spring 2015 Lecture 4 34

Advanced Voltage Sources Voltage source can produce arbitrary test signals using a PWL (Piece Wise Linearly) test file. 6.101 Spring 2015 Lecture 4 35

Device Models Many device models provided by Ltspice (BJT, MOSFETS, diodes. 6.101 Spring 2015 Lecture 4 36

Simulation: Transient Voltage/Current vs Time 6.101 Spring 2015 Lecture 4 37

Simulation Stop Time 6.101 Spring 2015 Lecture 4 38

Parameters in Simulation 6.101 Spring 2015 Lecture 4 39

What Should My Circuit Do? Before running simulation you should have an ideal of how the circuit behaves. Simulation is a verification tool circuit solver. 6.101 Spring 2015 Lecture 4 40

Analyze this circuit 6.101 Spring 2015 Lecture 4 41

Low Pass Filter Use low pass filter (LPF). Sallen Key filter is a second order filter. (A second order filter attenuates by a factor of one fourth for every doubling of frequency.) For low frequencies, capacitors are open circuit op amp feeds signal through. For high frequencies, capactiors act as shorts. f c 1 2RC R = 10K C = 1n (1nf) f c = 16 khz 6.117 IAP 2015 Lecture 3 42

Transient Simulation 6.101 Spring 2015 Lecture 4 43

Low Pass Filter Simulation 6.101 Spring 2015 Lecture 4 44

AC Simulation 6.101 Spring 2015 Lecture 4 45

AC Simulation 6.101 Spring 2015 Lecture 4 46

Math In LTspice 6.101 Spring 2015 Lecture 4 47

Transient Analysis 6.101 Spring 2015 Lecture 4 48

More Fun 6.101 Spring 2015 Lecture 4 49

AC Simulation 6.101 Spring 2015 Lecture 4 50

Finishing Touches 6.101 Spring 2015 Lecture 4 51

Finishing Touches 6.101 Spring 2015 Lecture 4 52