55:041 Electronic Circuits

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1 55:041 Electronic Circuits Output Stages and Power Amplifiers Sections of Chapter 8 A. Kruger Power + Output Stages1

2 Power Amplifiers, Power FETS & BJTs Audio (stereo) MP3 Players Motor controllers Servo controllers (robotics) Drivers for transducers Sonar Buzzers Laser diodes (CDs, DVDs) Considerations Low output resistance (to deliver power) Low total harmonic distortion Power dissipation by output transistors A. Kruger Power + Output Stages2

3 Packaging Schemes Can/tab is normally the collector (BJT) Heat sink E & B (BJT) TO-3 One of these may also be collector (BJT) A. Kruger Power + Output Stages3

4 Packaging Schemes Tab is normally the Collector (BJT) or Drain (MOSFET) Schottky Diode E & B (BJT) or G & S (MOSFET) DPAK TO-252 MOSFET ChipFET A. Kruger Power + Output Stages4

5 Packaging Schemes 200 A, 100 V, 4.5 mω FET $ A, 200 V, 3.5 mω FET $260 A. Kruger Power + Output Stages5

Typical DC β for 2N3055 (Power BJT) Sect 8.2.1 Note that at 10 A, β drops to 10!

6 Typical DC β for 2N3055 (Power BJT) Sect Note that at 10 A, β drops to 10! β increases gets better with increasing temp β decreases (significantly) with increasing I C A. Kruger Power + Output Stages6

Linear scale Log scale Max P T, hyperbola SOA Slight non-uniformities in semiconductor material causes non-uniformities in current density.

7 Safe Operating Area (SOA) for BJT Max I, determined by, for example, how much current bonding wires can handle Max V normally determined by avalanche breakdown of reverse biased CB junction. Linear scale Log scale Max P T, hyperbola SOA Slight non-uniformities in semiconductor material causes non-uniformities in current density. This produces regions of increased heating, which lowers the resistance of semiconductor, which increases current, which generates more heat,... This can cause material to melt. This is called second breakdown. A. Kruger Power + Output Stages7

8 Interdigitated BJT: Top view Cross-sectional view A. Kruger Power + Output Stages8

9 Typical High-Power MOSFET Characteristics Sect g m decreases with increasing temperature But, variation in g m is less than variation in β Power MOSFETs no second breakdown Faster switching times A. Kruger Power + Output Stages9

10 High-Power MOSFET Structures Special structures to lower contact resistance, channel resistance and aid heat removal VMOS DMOS HEXMOS / HEXFET A. Kruger Power + Output Stages10

11 Drain-to-Source Resistance Versus I D Some MOSFETSs have R DS(ON) in milliohm range At I D = 30 A, V DS > 15 V P D = = 450 W Note the large currents A. Kruger Power + Output Stages11

12 Sect Comparing Power BJTs and MOSFETS BJT Large base current I C = 10 A, β = 10, needs I B = 1 A Second breakdown Thermal runaway MOSFET No gate current, can be driven by standard logic circuit No second breakdown No thermal runaway MOSFETs are rapidly replacing power BJTs A. Kruger Power + Output Stages12

Electrical Equivalent Circuit: Heat Flow From Device to Ambient Sect 8.4.

The heat is analogous to current. We model this as a current source.

13 Electrical Equivalent Circuit: Heat Flow From Device to Ambient Sect Model the power (heat) the device generates internally with a current source. The heat is analogous to current. We model this as a current source. Model the thermal resistance as resistors. Units: o C/W Model the temperature and temperature differences as voltages. Ambient temperature A. Kruger Power + Output Stages13

14 Electrical Equivalent Circuit: Heat Flow From Device to Ambient This is the temperature at the semiconductor junction T dev T P ( θ + θ + θ ) = amb + D snk amb case snk dev-case Think of as a voltage R between case and device (junction) Think of as a current Think of as resistances R between case and heat sink R between heat sink and ambient A. Kruger Power + Output Stages14

15 Some Heat Sinks o θ = 0.1 C/W $260 θ = 1 o C/W $10 $3 θ = 10 o C/W A. Kruger Power + Output Stages15

16 MOSFET θ θ dev-case case-amb T j,max = 1.75 = 50 = T dev o o Example 8.2 C/W C/W = 150 o C R between case and ambient, no heat sink Maximum allowable junction temperature Heat sink θ θ snk amb case snk = 5 = 1 o o C/W C/W Ambient T amb = 30 o C Find the max. power the device can dissipate with/without the heat sink T dev => P D = ( θ + θ + θ ) dev-case ( θ + θ ) = T + P θ amb D dev-case T dev T case snk amb case snk + snk amb snk amb = 15.5 W With heat sink θ case-amb T dev => = Tamb + PD P D = T dev ( θ + θ ) dev-case ( θ + θ ) dev-case T amb case-amb case-amb = 2.32 W With no heat sink the thermal resistance is much larger A. Kruger Power + Output Stages16

17 Recall: Thermal Model T dev ( θ + θ ) = T + P θ amb D dev-case case snk + snk amb Rated power on data sheets: the power at which the device reaches its maximum temperature while the case remains at ambient, T j = 25 o C. This is the same a using an infinite heat sink so that the thermal resistance between case and heat sink, and heats sink and ambient is zero. A. Kruger Power + Output Stages17

18 Power Derating Curve T T P dev dev D,max = T j,max θ T dev case ( θdev-case + θcase snk + snk amb ) ( + 0 0) = T + P θ amb D = T + P θ amb D dev -case + case θ dev case = T j,max P T D,max case = 1 slope A. Kruger Power + Output Stages18

19 I C /I D Versus Time Characteristics Sect 8.3 Class-A (always on) Class-B (on during one half cycle) Class-AB (hybrid) Class-C (on less than one half cycle) A. Kruger Power + Output Stages19

20 Class-A CE Amplifier Sect Q-point was chosen for maximum output swing Consumes power even when there is no input signal Don t design this into MP3 player/cell phone etc. signal load power η(max) = = supply power 25% Derivation of this will be on a future exam. See page 572. A. Kruger Power + Output Stages20

21 Class-A CE Amplifier Sect Q-point was chosen for maximum output swing Consumes power even when there is no input signal Don t design this into MP3 player/cell phone etc. signal load power η(max) = = supply power 25% Derivation of this will be on a future exam. See page 575. A. Kruger Power + Output Stages21

22 BJT Saturation Voltage In both circuit, as I B increases, I C increases and V CE decreases. However, it reaches a point where V CE will not decrease, regardless how much I B is. This is the saturation voltage V CE(sat) For low power BJTs, V CE(sat) is often 200 mv~ 600 mv. However, saturation voltage can be higher. For example, for the PN2222 it is ~ 1 V For power transistors this can be significant. For example, for the 2N3055 BJT, V CE(sat) can be as high as A. Thus, P D = 4.4 W in saturation. A. Kruger Power + Output Stages22

23 I C /I D Versus Time Characteristics Sect 8.3 Class-A (always on) Class-B (on during one half cycle) Class-AB (hybrid) Class-C (on less than one half cycle) A. Kruger Power + Output Stages23

24 Inductively Coupled Class-A Amplifier Short at DC (establish Q-points). Open at AC Sect DC and AC load lines η(max) = 50% This is double that of a Class A with collector resistor A. Kruger Power + Output Stages24

25 Transformer-Coupled Common Emitter A transformer gives us flexibility to transform the load resistance to a value that is easier to work with Sect What do these dots mean? a is the turns ratio R L = a 2 R L η(max) = 50% A. Kruger Power + Output Stages25

Transformer-Coupled Emitter Follower Sect 8.4.

26 Transformer-Coupled Emitter Follower Sect R L = a 2 R L η(max) = 50% A. Kruger Power + Output Stages26

27 Class-A Characteristics Power hungry efficiency is 25%, or 50% but then inductors/transformers are needed (costly, heavy) Simple Inherently low distortion A. Kruger Power + Output Stages27

28 I C /I D Versus Time Characteristics Sect 8.3 Class-A (always on) Class-B (on during one half cycle) Class-AB (hybrid) Class-C (on less than one half cycle) A. Kruger Power + Output Stages28

29 Class-B Complementary Push-Pull Output Stage Sect Note dual power supplies With v i = 0, both transistors off => no current consumption Note dual power supplies A. Kruger Power + Output Stages29

30 Class-B Complementary Push-Pull Output Stage Sect Note dual power supplies On Off Note dual power supplies A. Kruger Power + Output Stages30

Class-B Complementary Push-Pull Output Stage Sect 8.3.

31 Class-B Complementary Push-Pull Output Stage Sect Note dual power supplies Off On Note dual power supplies A. Kruger Power + Output Stages31

32 Class-B Complementary Push-Pull Output Stage Sect Note dual power supplies On With v i = 0, both transistors off => no current consumption On Note dual power supplies Both transistors are followers => No voltage gain, and v o < v i A. Kruger Power + Output Stages32

33 Voltage Transfer Characteristics: Complementary Push-Pull Output Stage Transistors off until v i reaches V BE(ON) Crossover distortion A. Kruger Power + Output Stages33

34 Effective Load Line: Ideal Class-B Output Stage A. Kruger Power + Output Stages34

35 Average Power Dissipation vs. Peak Output Voltage: Class-B Output Stage P Qn 2 VCC (max) = 2 π R η(max) 78.5% L Remember, for Class A, η = 25% (fixed) A. Kruger Power + Output Stages35

36 Bipolar Class-AB Output Stage Sect Supply bias voltages so that both transistors are turned on, with small quiescent current A. Kruger Power + Output Stages36

37 Characteristics of Class-AB Output Stage Input Signal Transistor Currents Load Current A. Kruger Power + Output Stages37

38 MOSFET Class-AB Output Stage Supply bias voltages so that both FETs are turned on, with small quiescent current MOSFETs V TO s vary much more than BJT s V BE See example 8.8 A. Kruger Power + Output Stages38

39 Class-AB Output Stage: Q-Point Established by Diodes Sect How? Place diodes in same thermal environment as transistors A. Kruger Power + Output Stages39

40 Class-AB Output Stage: V BE Multiplier Bias Neglect I bq1 Sect V I V I ( R ) 1 R2 BB = R + R = V R V = BE1 2 ( R ) BE1 BB 1 + R2 R2 V BB R R = 1 VBE 2 V BE I 1 = VT ln I C1 S1 Because of the log relationship, V BE1 is practically constant, say 0.7 V V BB R = 1 R2 A. Kruger Power + Output Stages40

Class-AB Output Stage: Input Buffer BJTs Sect 8.5.

41 Class-AB Output Stage: Input Buffer BJTs Sect Follower Short circuit protection + thermal stability - remember thermal runaway? Voltage gain ~ 1 Current gain can be large A. Kruger Power + Output Stages41

Class-AB Output Stage: Darlington Pair Sect 8.5.

42 Class-AB Output Stage: Darlington Pair Sect Mimics a PNP Darlington Why not use PNP Darlington (no mimic)? A. Kruger Power + Output Stages42

43 Design Application: Output Stage With MOSFETS See Sect 8.6 A. Kruger Power + Output Stages43

44 Amplifier IC LM mw amplifier (~ $1) Class-AB output Differential input Current mirror (biasing) Composite pnp transistor A. Kruger Power + Output Stages44

45 Amplifier IC A. Kruger Power + Output Stages45

46 A. Kruger Power + Output Stages46

ECE:3410 Electronic Circuits

ECE:3410 Electronic Circuits Output Stages and Power Amplifiers Sections of Chapter 8 A. Kruger Power + Output Stages1 Power Amplifiers, Power FETS & BJTs Audio (stereo) MP3 Players Motor controllers Servo