56
Sam Palermo Analog & Mixed-Signal Center Texas A&M University ECEN326: Electronic Circuits Fall 2017 Lecture 8: Output Stages and Power Amplifiers

ECEN326: Electronic Circuits Fall 2017ece.tamu.edu/~spalermo/ecen326/lecture08_ee326_output_stages.pdf · ECEN326: Electronic Circuits Fall 2017 Lecture 8: Output Stages and Power

  • Upload
    buicong

  • View
    215

  • Download
    0

Embed Size (px)

Citation preview

Sam PalermoAnalog & Mixed-Signal Center

Texas A&M University

ECEN326: Electronic CircuitsFall 2017

Lecture 8: Output Stages and Power Amplifiers

Announcements

• Homework 8 due 12/6• Exam 3 12/11

• 8:00-10:00 • Closed book w/ one standard note sheet• 8.5”x11” front & back• Bring your calculator• Emphasis will be on feedback and output stages• Sample Exam 3 is posted on the website

• Reading• Razavi Chapter 14

2

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes3

Why Special Output Stages or Power Amplifiers?

• Power amplifiers are necessary to efficiently drive a load with high power

• Examples• A cellphone may need 1W of power at the antenna• Audio systems require tens to hundreds of Watts

• As ordinary voltage/current amplifiers are not suited for efficiently supporting these power levels, specialized output stages are necessary

4

Power Amplifier Characteristics

• Can drive a small “heavy” load resistance• Example: Speaker resistance

can be 4-16

• Delivers large current levels• Often in the hundreds of mA

• Requires large voltage swings• Draws a large amount of

power from the supplies• Dissipates a large amount of

power, and thus can get hot5

mAVRVI

VV

WR

VP

L

PP

P

L

Pout

50084

currentpeak a and4

gepeak volta ain results This

112

Sinusoidal

speaker8an 1W toDelivering :Example2

Power Amplifier Performance Metrics

• Linearity or Distortion• Audio amplifiers must have low distortion to

reproduce music with high fidelity

• Power Efficiency

• Maximum Voltage Rating• May require high power supplies• Transistors must have sufficiently high

breakdown voltages6

Supply fromDrawn Power Load toDeliveredPower

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes7

Emitter Follower Large-Signal Behavior: Positive Half-Cycle

As Vin increases Vout also follows and Q1 provides more current No major issue with the input signal positive half-cycle

8CH 13 Output Stages and Power Amplifiers

Here VBE is assumed to be 0.8V due to the high current levels

Emitter Follower Large-Signal Behavior: Negative Half-Cycle

However as Vin decreases, Vout also decreases, shutting off Q1 and resulting in a constant Vout

The output signal clips at a minimum value of –I1RL

9CH 13 Output Stages and Power Amplifiers

Example: Emitter Follower

10CH 13 Output Stages and Power Amplifiers

mVA

mAmVmVV

mVmAmAA

mAmVV

RIIIIVV

VVVAI

out

in

LCS

CTin

outBEin

S

258105325.0ln9.25387

38785.32325.0105325.0ln9.25

ln

105 Assume

15

15

111

15

• At this current level, the VBE has dropped to 645mV• For voltage levels much below 400mV, the output

will saturate at –IlRL = -260mV

• What are the input and output voltages when IC1drops to 1% of I1?

Linearity of an Emitter Follower

As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics

Overall, the fixed I1 limits the output swing

11CH 13 Output Stages and Power Amplifiers

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes12

Push-Pull Stage

As Vin increases, Q1 is on and pushes a current into RL. As Vin decreases, Q2 is on and pulls a current out of RL. For positive Vin, Q1 shifts the output down and for negative Vin,

Q2 shifts the output up.13CH 13 Output Stages and Power Amplifiers

Overall I/O Characteristics of Push-Pull Stage

However, for small Vin, there is a dead zone (both Q1 and Q2 are off) in the I/O characteristic, resulting in gross nonlinearity

The dead zone is from -|VBE2| ≤ Vin ≤ VBE114CH 13 Output Stages and Power Amplifiers

22

12

11

,,0

,

BEinBEin

BEinBE

BEinBEin

out

VVVVVVV

VVVVV

Small-Signal Gain of Push-Pull Stage

The push-pull stage exhibits a gain that tends to unity when either Q1 or Q2 is on.

When Vin is very small, the gain drops to zero.15CH 13 Output Stages and Power Amplifiers

eL

Lv rR

RA

Sinusoidal Response of Push-Pull Stage

For large Vin, the output follows the input with a fixed DC offset, however as Vin becomes small the output drops to zero and causes “Crossover Distortion.”

16CH 13 Output Stages and Power Amplifiers

Here VBE is assumed to be 0.6V for small currents near crossover

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes17

Improved Push-Pull Stage

With a battery of VB inserted between the bases of Q1 and Q2, the dead zone is eliminated.

18CH 13 Output Stages and Power Amplifiers

21

211

111

22

Thus,or on, want point we at this

when off turnson? always isr transistooneleast at that so be shouldWhat

BEBEB

BEBEinB

BE

BEin

B

VVV

VVVVVVVQ

VVQV

Implementation of VB

Since VB=VBE1+|VBE2|, a natural choice would be two series diodes I1 in figure (b) is used to bias the diodes and Q1.

19CH 13 Output Stages and Power Amplifiers

Example: Current Flow I

1 1 2in B BI I I I

Iin

20CH 13 Output Stages and Power Amplifiers

0 whenoccurs thisThus,

0 that and that implieswhich

, when occurs This? assuming , isWhen

21

21

211

out

out

CC

BB

in

V

III

IIII

outI

Example: Current Flow II

21CH 13 Output Stages and Power Amplifiers

• The offset between input and output is minimized when the input is in the middle of the diodes

0 when 0then , and

hdesign wit balanced a Assumingthen

, and If

2121

2211

outin

BB

inout

BEDBED

VIIIII

VVVVVV

Addition of CE Stage

A CE stage (Q4) is added to provide voltage gain from the input to the bases of Q1 and Q2

This push-pull circuit is often used in high-power output stages22CH 13 Output Stages and Power Amplifiers

Bias Point Analysis

VX=0Vout=0

23CH 13 Output Stages and Power Amplifiers

circuit theof half top theredrawcan weand ,0diodes theof middle in the voltagethen the

and , , thatassuming and 0With

221143

X

BEDBEDCC

out

V

VVVVIIV

31,

1,1

1,

11

1,

31 lnln

CDS

QSC

QS

CTBE

DS

CTD

III

I

IIVV

II

VV

For bias point analysis, the circuit can be simplified to the one on the right, which resembles a current mirror

For a well-defined IS ratio, D1 can be realized as a diode-connected transistor

Small-Signal Analysis - I

24CH 13 Output Stages and Power Amplifiers

followersemitter parallel 2 as viewedbecan stage second the,0 and Assuming

amplifierstage-twoaasit can treat wecircuit,theofgain thederive order toIn

DA

N

out

in

N

in

out

rVvv

vv

vv

21

21 1mm

L

L

eeL

L

N

out

ggR

RrrR

Rvv

Small-Signal Analysis - II

25CH 13 Output Stages and Power Amplifiers

LmmN

Nmin

N

RrrggrrR

Rgvv

212121

4 where

asexpressed becan stagefirst theofgain The

Lmmmin

out

mmL

LLmmm

N

out

in

N

in

out

Rggrrgvv

ggR

RRrrggrrgvv

vv

vv

21214

21

2121214 1

isgain overallThe

Output Resistance Analysis

This image cannot currently be displayed.

3 4

1 2 1 2 1 2

||1( )( || )

O Oout

m m m m

r rRg g g g r r

If β is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance.

26CH 13 Output Stages and Power Amplifiers

• For the output resistance, we do need to consider the finite rO of Q3 & Q4

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes27

Example: Biasing

28CH 13 Output Stages and Power Amplifiers

21 assume and

1002 ,8with 8.0 StageOutput

5Emitter Common :SpecsDesign

CC

pnpnpnL

v

v

II

RA

A

mAII

VmAggII

gg

ggR

RvvA

IgI

CC

mmCC

mm

mmL

L

N

outOSv

Cm

C

5.6 and

25041with

21

8.01

and findcan weconditions andgain stageoutput theFrom 1.

21

12121

21

21

,

2,12,1

2,1

Example: Biasing

29CH 13 Output Stages and Power Amplifiers

AIVmAgRrr

RrrggrrgRgAI

Cm

L

LmmmNmCEv

C

19552.78 and ,199 ,398 Using

findcan wecondition gain Emitter Common theFrom 2.

44

21

21212144,

4

However, this design procedure neglects any output swing specification!

Problem of Base Current

195 µA of base current in Q1 can only support 19.5 mA of collector current, insufficient for high current operation (hundreds of mA).

30CH 13 Output Stages and Power Amplifiers

mA5.19

8

mVV pout 156, Note, this assumes that IC3 is the same as IC4, which may not exactly be the case due to the different 1,2

Modification of the PNP Emitter Follower

31CH 13 Output Stages and Power Amplifiers

• Another output stage issue is that the Q2 PNP transistor typically have low current gain () and low fT

• Using a composite transistor consisting of an emitter follower PNP (Q3) and a common emitter NPN (Q2) allows for improved performance

Here Vin is the first stage output (VN)

Modified PNP Emitter Follower Gain & Output Resistance

32CH 13 Output Stages and Power Amplifiers

Small-Signal Model

3

32

322

23

3233

11

1

current base theformscurrent collector theAs

0

nodeoutput at the KCL a Writing

rg

R

Rvv

ggQQ

Rvvvggvvg

rvv

m

L

L

in

out

mm

L

outoutinmmoutinm

inout

32

332

11

11

1

mm

out gr

gR

Output resistance has been reduced by 2+1 factor

Modified PNP Emitter Follower Input Resistance

33CH 13 Output Stages and Power Amplifiers

Small-Signal Model

Lin

mL

Lvin

inin

invinoutinin

RrR

gR

Rr

Ar

ivR

rvAv

rvvi

1

1111

233

32

33

33

Input resistance has been increased by ~(2+1) factor

Additional Bias Current

Q2 is often a large transistor to support the output current levels, resulting in large base capacitance

I1 is added to the base of Q2 to provide an additional bias current to Q3 so the capacitance at the base of Q2 can be charged/discharged quickly

34CH 13 Output Stages and Power Amplifiers

Example: Minimum Vin

Min Vin≈0Vout≈|VEB2|

Min Vin≈VBE2Vout≈|VEB3|+VBE2

35CH 13 Output Stages and Power Amplifiers

• There is a minimum Vin for which the effective emitter follower transistors remain in active mode

• Here we conservatively assume for active mode we need a min. VBC=0VSimple PNP Emitter Follower Modified PNP Emitter Follower

• The modified PNP emitter follower has a reduced input range by VBE2

HiFi Design

36CH 13 Output Stages and Power Amplifiers

Lmmmin

out Rggrrgvv

21214

• As gm1 and gm2 vary dramatically during large signal operation, the circuit displays nonlinear distortion

2

1

21

2111

RR

RRRKv

v

in

out

• Placing the output stage in a negative feedback loop allows for a more constant gain, and thus better linearity

Short-Circuit Protection

37CH 13 Output Stages and Power Amplifiers

• Excessive output stage current can result if the output is shorted to ground, damaging the circuit

• Qs and r are used to “steal” some base current away from Q1 when the output is accidentally shorted to ground, preventing short-circuit damage

• Drawbacks• r increases output resistance• Voltage drop across r reduces

maximum output swing

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes38

Emitter Follower Power Rating

Maximum power dissipated across Q1 occurs in the absence of a signal.

39CH 13 Output Stages and Power Amplifiers

time.period by the dividing and period complete aover product voltage-currenttheintegratedby calculatedbecan by dissipatedpower average The 1Q

22

2cos12

1

sinsinsin1

sinsin1

1

1

2

1

2

0 1

22

10 1

0 1

0

1P

CCRVI

L

PCC

L

PCC

T

L

P

L

PCCPCC

T

PCCT

L

Pav

TCECav

VVIR

VVI

tR

VVIT

dtR

tVR

tVVtVIVIT

dttVVR

tVIT

P

dtVIT

P

LP

CCav

PCCav

VIP

VVIP

1max,

1 2

0 0

Example: Current Source Power Dissipation

40CH 13 Output Stages and Power Amplifiers

• The power dissipated by the I1 current source is also important, as it will ultimately need to be realized at the transistor level also

EEI

EEPT

TEEP

TEEoutI

VIP

dtVItVIT

dtVtVIT

dtVVIT

P

1

10 1

0 10 1

1

1

sin1

sin11

• Note that the I1 power is positive, as VEE is a negative supply

0

Push-Pull Stage Power Rating

41CH 13 Output Stages and Power Amplifiers

• Assuming a symmetric design, the power ratings of Q1 and Q2 are the same and can be computed by integrating over a half-period.

Push-Pull Stage Power Rating

42CH 13 Output Stages and Power Amplifiers

4

4

2cos12

1sin1

sinsin1

sinsin11

2

20

220

20

22

20

20

PCC

L

Pav

L

P

L

PCC

T

L

PT

L

PCC

T

L

P

L

PCC

T

L

PPCC

TCCEav

VVRVP

RV

RVV

dttR

VT

tdtR

VVT

dttRVt

RVV

T

dttRVtVV

TdtIV

TP

L

CCav

CCP

RVP

VV

2

2

max,

2 with occurspower Maximum

0

Example: Push-Pull Pav

4CCP P

avL

VV VPR

If Vp = 4VCC/π → Pav=0

Impossible since Vp cannot goabove supply (VCC)

43CH 13 Output Stages and Power Amplifiers

Heat Sink

Heat sink, provides large surface area to dissipate heat from the chip.

44CH 13 Output Stages and Power Amplifiers

Thermal Runaway

45CH 13 Output Stages and Power Amplifiers

• For a constant IC, VBE has a negative-to-absolute-temperature (NTAT) coefficient of approximately -2mV/K

• Conversely, if the push-pull output stage is biased with a constant voltage, the current will increase with temperature

• This increased current results in more heat (higher temperature), and thus more current

• This positive feedback loop is called thermal runaway and can result in transistor damage

Thermal Runaway Mitigation

46CH 13 Output Stages and Power Amplifiers

• Assuming that Q3 and Q4 have relatively constant bias currents with temperature (using good current mirror techniques), the diode voltages drop with temperature

2,1,

21

2,1,

21

2,1,

21

2,1,

21

2,1,

21

2,

2

1,

121

2,1,

21

2,

2

1,

121

lnln

same theare pairs voltage theseKVL, a From

lnlnln

Voltages Transistor

lnlnln

VoltagesDiode

QSQS

CC

DSDS

DD

QSQS

CCT

DSDS

DDT

QSQS

CCT

QS

CT

QS

CTBEBE

BE

DSDS

DDT

DS

DT

DS

DTDD

IIII

IIII

IIIIV

IIIIV

IIIIV

IIV

IIVVV

VIIIIV

IIV

IIVVV

Using diode biasing prevents thermal runaway since the currents in Q1and Q2 will track those of D1 and D2 as long as their Is’s track with temperature.

Agenda

• General Output Stage Considerations

• Emitter Follower as a Power Amplifier

• Push-Pull Stage

• Improved Push-Pull Stage

• Large-Signal Considerations

• Heat Dissipation

• Efficiency and Power Amplifier Classes47

Power Amplifier Efficiency

• Power amplifier efficiency is critical because PAs draw a significant amount of power from the supply voltages

• Power amplifier efficiency is the ratio of the average power delivered to the load over the total power consumed from the supplies

48

cktout

outPP

P

Supply fromDrawn Power LoadtoDeliveredPower

Emitter Follower Efficiency

49

22

2

ispower circuit average total theThus

with is by consumedpower average The

2

is by consumedpower average The2

isloadthetodeliveredpower average The

111

11

1

1

1

2

1

1

PCCCC

PCCckt

CCEECCEEI

PCCQ

L

Pout

VVIVIVVIP

VVVIVIPI

VVIP

QR

VP

CC

PEF

L

P

PCC

L

P

L

P

EF

VV

RVI

VVIR

VR

V

4 with

22

2

2 is EfficiencyFollower Emitter The

1

1

2

2

• The emitter follower output stage only has a peak efficiency of 25% as VPgoes to VCC

• Note, this can’t actually be achieved, as we need some VCE and voltage across I1

Emitter Follower Efficiency Example

50

• What if the emitter follower is designed to deliver Vp, but only operates with Vp=VCC/2?

151

42

8

8

and with 4

24

2

becomespower circuit average totalThe82

2

isloadthetodeliveredpower average theNow

222

2

1

22

1

2

2

L

P

L

P

L

P

L

P

EF

PCCL

P

L

P

L

PPCCckt

L

P

L

P

out

RV

RV

RV

RV

VVRVI

RV

RVVVIP

RV

R

V

P

• Now the efficiency drops to only 6.7%

Push-Pull Stage Efficiency

51

4

ofpower averagean consume and Both 2

isloadthetodeliveredpower average The

2,1

21

2

PCC

L

PQ

L

Pout

VVRVP

QQR

VP

CC

PPP

PCC

L

P

L

P

L

P

PP

VV

VVRV

RV

RV

4

42

2

2 is Efficiency Stage Pull-Push The 2

2

• The push-pull output stage has a peak efficiency of 78.5% as VP goes to VCC

• Note, this can’t actually be achieved, as we need some VCE across both Q1 and Q2

• Also, we are excluding the input bias stage

Total Push-Pull Stage Efficiency

52

PCC

PPP

L

PCC

L

CCP

L

P

PP

L

PCC

L

P

P

VVV

RVV

RVV

RV

RVV

RVI

V

41

222

is efficiency stage pull-push total theThus

2ispower stage biasinput The

of swingpeak asupport order toIn

2

1

• Assume that I1=I2 and that they are chosen to allow a peak swing of VP

Class A Power Amplifiers

53

• Each transistor is on for the entire cycle• Emitter follower is a Class A power amplifier

• Display high linearity, but low efficiency (25% maximum)

Class B Power Amplifiers

54

• Each transistor conducts for half the cycle• Simple push-pull stage is a Class B power amp

• Display high efficiency, but low linearity• Bad crossover distortion

Class AB Power Amplifiers

55

• Each transistor conducts for slightly more than a half-cycle

• Improved push-pull stage is a Class AB power amp

• Compromise between Class A and B• Good distortion at slightly degraded efficiency

• Many other PA classes exist (C, D, E, …) and are often studied in grad-level RF circuit design classes

Thanks for the fun semester!!!

56