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 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
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
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
Packaging Schemes 200 A, 100 V, 4.5 mω FET $35 580 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! β increases gets better with increasing temp β decreases (significantly) with increasing I C A. Kruger Power + Output Stages6
Transistor Maximum Operating Ratings A. Kruger Power + Output Stages7
The plot the function y = K x or K = xy is a hyperbola Maximum Power Hyperbola y K = xy x Power transistor generally have a maximum power dissipation specification P T (Watt) Since power = I V, we have I C P T = I C V CE P T = I C V CE V CE Collector Current Collector Emitter Voltage A. Kruger Power + Output Stages8
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 Stages9
Heatsink Calculations BJTs and MOSFETs absolute maximum operating parameters include a maximum junction temperature specification To stay within the specification, one has to extract the heat from the junction, and transfer the heat to the surrounding air. The material between the junction and the ambient air resists the flow of heat and this leads to the concept of thermal resistance. A heatsink is a piece of metal that lowers the thermal resistance have aids the flow of heat away from the junction. A. Kruger Power + Output Stages10
Electrical Equivalent Circuit: Heat Flow From Device to Ambient Sect 8.4.2 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 Stages11
Electrical Equivalent Circuit: Heat Flow From Device to Ambient This is the temperature at the semiconductor junction T dev T amb P D snkamb casesnk 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 Stages12
Some Heat Sinks o 0.1 C/W $260 1 o C/W $10 $3 10 o C/W A. Kruger Power + Output Stages13
MOSFET 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 snkamb casesnk 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 T D amb P D dev-case dev-case Tdev T amb casesnk casesnk snkamb snkamb 15.5 W With heat sink case-amb T dev P T D amb P T D dev dev-case 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 Stages14
Interdigitated BJT: Top view Cross-sectional view A. Kruger Power + Output Stages15
Typical High-Power MOSFET Characteristics Sect 8.2.2 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 Stages16
High-Power MOSFET Structures Special structures to lower contact resistance, channel resistance and aid heat removal VMOS DMOS HEXMOS / HEXFET A. Kruger Power + Output Stages17
Drain-to-Source Resistance Versus I D D R DS S Some MOSFETSs have R DS(ON) in milliohm range At I D = 30 A, V DS > 15 V P D = 30 15 = 450 W Note the large currents A. Kruger Power + Output Stages18
Sect 8.2.3 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 Stages19
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 Stages20
Class-A CE Amplifier Sect 8.3.1 Q-point was chosen for maximum output swing Consumes power even when there is no input signal (max) signal load power supply power 25% Don t design this into MP3 player/cell phone etc. A. Kruger Power + Output Stages21
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 1.1 V @ 4 A. Thus, P D = 4.4 W in saturation. A. Kruger Power + Output Stages22
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
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 Stages24
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 Stages25
Class-B Complementary Push-Pull Output Stage Sect 8.3.2 Note dual power supplies With v i = 0, both transistors off => no current consumption Note dual power supplies A. Kruger Power + Output Stages26
Class-B Complementary Push-Pull Output Stage Sect 8.3.2 On Note dual power supplies Off Note dual power supplies A. Kruger Power + Output Stages27
Class-B Complementary Push-Pull Output Stage Sect 8.3.2 Note dual power supplies Off On Note dual power supplies A. Kruger Power + Output Stages28
Voltage Transfer Characteristics: Complementary Push-Pull Output Stage Transistors off until v i reaches V BE(ON) Crossover distortion A. Kruger Power + Output Stages29
Effective Load Line: Ideal Class-B Output Stage A. Kruger Power + Output Stages30
Average Power Dissipation vs. Peak Output Voltage: Class-B Output Stage P Qn 2 VCC (max) 2 R (max) 78.5% L Remember, for Class A, η = 25% (fixed) A. Kruger Power + Output Stages31
Bipolar Class-AB Output Stage Sect 8.3.3 Supply bias voltages so that both transistors are turned on, with small quiescent current A. Kruger Power + Output Stages32
Class-AB Output Stage: Q-Point Established by Diodes Sect 8.5.1 How? Place diodes in same thermal environment as transistors A. Kruger Power + Output Stages33
Class-AB Output Stage: Darlington Pair Sect 8.5.4 Mimics a PNP Darlington Why not use PNP Darlington (no mimic)? A. Kruger Power + Output Stages34
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 Stages35
Matched and Complementary Transistors Push-pull stages work best if the complementary output transistors have matching characteristics. That is, β n = β p, V n(sat) = V p(sat), etc. or V TP = V TN, K n = K p, etc. This hard to achieve in discrete parts, but there are several so-called complemenary-pairs on the market. It is easier to achieve close matching in ICs A. Kruger Power + Output Stages36
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Amplifier IC LM386 250-700 mw amplifier (~ $1) Class-AB output Differential input Current mirror (biasing) Composite pnp transistor A. Kruger Power + Output Stages39
Amplifier IC A. Kruger Power + Output Stages40
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Recall: Thermal Model T dev T amb P D dev-case casesnk snkamb 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 Stages42
Power Derating Curve T T P dev dev D,max T T amb amb T P D P D T j, max case devcase dev-case dev-case 0 casesnk 0 snkamb devcase T j,max P T D,max case 1 slope A. Kruger Power + Output Stages43
Inductively Coupled Class-A Amplifier Short at DC (establish Q-point). Open at AC Sect 8.4.1 DC and AC load lines (max) 50% This is double that of a Class A with collector resistor A. Kruger Power + Output Stages44
Transformer-Coupled Common Emitter A transformer gives us flexibility to transform the load resistance to a value that is easier to work with. Sect 8.4.2 What do these dots mean? a is the turns ratio R L = a 2 R L (max) 50% A. Kruger Power + Output Stages45
Transformer-Coupled Emitter Follower Sect 8.4.2 R L = a 2 R L (max) 50% A. Kruger Power + Output Stages46
Characteristics of Class-AB Output Stage Input Signal Transistor Currents Load Current A. Kruger Power + Output Stages47