Chapter 13 Output Stages and Power Amplifiersocw.snu.ac.kr/sites/default/files/NOTE/ch14_2017.pdf · Power Amplifier Characteristics Experiences low load resistance. Delivers large

Chapter 14 Output Stages and Power Amplifiers

14.1 General Considerations

14.2 Emitter Follower as Power Amplifier

14.3 Push-Pull Stage

14.4 Improved Push-Pull Stage

14.5 Large-Signal Considerations

14.6 Short Circuit Protection

14.7 Heat Dissipation

14.8 Efficiency

14.9 Power Amplifier Classes

1

Why Power Amplifiers?

Drive a load with high power.

Cellular phones need 1W of power at the antenna.

Audio systems deliver tens to hundreds of watts of power.

Ordinary Voltage/Current amplifiers are not suitable for

such applications

2CH 13 Output Stages and Power Amplifiers

Chapter Outline


Power Amplifier Characteristics

Experiences low load resistance.

Delivers large current levels.

Requires large voltage swings.

Draws a large amount of power from supply.

Dissipates a large amount of power, therefore gets “hot”.


Power Amplifier Performance Metrics

Distortion - Linearity

Power Efficiency

Voltage Rating – Transistor Breakdown voltage


Emitter Follower Large-Signal Behavior I

As Vin increases Vout also follows and Q1 provides more current.


1

For 8 ,

1/ 0.8 , thus, 32.5 mA

v L L m

L

m C

A R R g

R

g I

For Vin=4.8V, Vout=4.0V, IL=500mA, IE1=532.5mA

Emitter Follower Large-Signal Behavior II

For Vin=0.8V, Vout=0V, IL=0mA, IE1=32.5mA

For Vin=0.7V, Vout=-0.1V, IL=12.5mA, IE1=20mA

When all current (32.5mA) flows to the load, Vout=-0.26V


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

Example 14.1: Emitter Follower

For 0.5

211 mV by a few iterations

in

out

V V

V


1 1 1

1 1

1

,

ln( / )

1ln

outin BE out C

L

BE T C S

outin T out

L S

VV V V I I

R

V V I I

VV V I V

R I

Vin=0.5V, Vout=?

Example 14.1: Emitter Follower

11 1ln C

in T C LS

IV V I I R

I


For what Vin, Q1 carries only 1% of I1?

1 1For 0.01

390 mV

C

in

I I

V

Linearity of an Emitter Follower

As Vin decreases the output waveform will be clipped,

introducing nonlinearity in I/O characteristics.


Push-Pull Stage

As Vin increases, Q1 is on and pushes current into RL.

As Vin decreases, Q2 is on and pulls current out of RL.


Example 14.2: I/O Characteristics for Large Vin

For positive Vin, Q1 shifts the output down and for negative Vin,

Q2 shifts the output up.

Vout = Vin-VBE1 for large +Vin

Vout = Vin+|VBE2| for large -Vin


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.


Example 14.3: 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.


Example 14.4: 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.”


Improved Push-Pull Stage

With a battery of VB inserted between the bases of Q1 and Q2,

the dead zone is eliminated.

VB = VBE1 + |VBE2|


Implementation of VB

Since VB=VBE1+|VBE2|, a natural choice would be two diodes in

series.

I1 in figure (b) is used to bias the diodes and Q1.17CH 13 Output Stages and Power Amplifiers

Example 14.6: Current Flow I

Iin


1 1 2in B BI I I I

1 2Usually unless 0

flows even when = 0.

B B out

in out

I I V

I V

Example 14.8: Current Flow II


1 20 when 0 if in outI V I I

→1D BEV V

out inV V

Addition of CE Stage

A CE stage (predriver) is added to provide voltage gain from

the input to the bases of Q1 and Q2.20CH 13 Output Stages and Power Amplifiers

Bias Point Analysis

For bias point analysis for Vout=0, the circuit can be simplified

to the one on the right, which resembles a current mirror.

VA=0Vout=0


, 1

1 3

, 1

S Q

C C

S D

II I

I

Small-Signal Analysis

Assuming 2rD is small and (gm1+gm2)RL is much greater than 1,


4 1 2

4 1 2 1 2 1 2

4 1 2 1 2

( 1)

1

v m L

m m m L

m m m L

A g r r R

g r r r r g g R

g r r g g R

Example 14.9: Output Resistance Analysis

3 4

1 2 1 2 1 2

||1

( )( || )

O Oout

m m m m

r rR

g 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.23CH 13 Output Stages and Power Amplifiers

Example14.10: Biasing


1 2

Predriver (CE stage): 5

OutputStage: 0.8 for 8

2 100,

V

V L

npn pnp C C

A

A R

I I

Compute the required bias current.

1 1

1 2 1 2

1 2

1 2

4 1 2 1 2

1

4 4

2 4

Thus, 6.5 mA

|| 400 || 200 133

|| 1 4

133 195 μA

m m m m

C C

m m m L

m C

g g g g

I I

r r

g r r g g R

g I

65uA

135uA

6.5mA

6.5mA

195uA

60uA

125uA

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 (500 mA

for 4 V on 8 ). 25CH 13 Output Stages and Power Amplifiers

Modification of the PNP Emitter Follower

Instead of having a single PNP as the emitter-follower, it is now

combined with an NPN (Q2), providing a lower output

resistance.

2 3

1

1out

m

Rg


Example 14.11: Input Resistance

2 33

1

11

out L

inL

m

v R

vR

gr


2 32 3

3

Comparing with the standard EF,

1 1

1 11

outm

m

rg

gr

Example 14.11: Input Resistance

3

2 3

3 2 3

1

1

1

( 1)

Lin in in

Lm

in L

Ri v v

rR

g

r R r


Additional Bias Current

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. Additional pole at the base of Q2.29CH 13 Output Stages and Power Amplifiers

2

0in

out EB

MinV

V V

Example 14.12: Minimum Vin


2

3 2

in BE

out EB BE

Min V V

V V V

Power Amplifier Classes

Class A: High linearity, low efficiency

Class B: High efficiency, low linearity

Class AB: Compromise between

Class A and B


HiFi Design

As Vout becomes more positive, gm rises and Av comes closer to

unity, resulting in nonlinearity.

Using negative feedback, linearity is improved, providing higher

fidelity.


Short-Circuit Protection

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.33CH 13 Output Stages and Power Amplifiers

Power transistors

Package and heat sink

Small-signal Transistor

Power Transistor in Heat Sink Power Transistor Inside

Emitter Follower Power Rating (Class A)

0

10

2

1 1 1

1

sin1sin

if 2

T

av C CE

TP

CC PL

P P PCC CC

L L

P I V dtT

V tI V V t dt

T R

V V VI V I V I

R R

Maximum power dissipated across Q1 occurs in the absence of

a signal.35CH 13 Output Stages and Power Amplifiers

Pull down is done

by a current source

with huge current!

Example 14.13: Power Dissipation

1 10

1

1sin

T

I p EE

EE

P I V t V dtT

I V


Avg Power Dissipated in the Current Source I1

Push-Pull Stage Power Rating

/2

, 0

/2

0

2

1

sin1sin

4 4

T

av NPN C CE

TP

CC PL

CC P CCP P P

L L L

P I V dtT

V tV V t dt

T R

V V VV V V

R R R

No power for half of the period.



/2

, 0

/2

0

2

1

sin1sin

4 4

T

av NPN C CE

TP

CC PL

CC P CCP P P

L L L

P I V dtT

V tV V t dt

T R

V V VV V V

R R R

No power for half of the period.

Maximum power occurs between Vp=0 and 4Vcc/π.


2

,max 2 when 2CC P CC

av P

L

V V VP V

R


Maximum power occurs between Vp=0 and 4VCC/π.


, , when av PNP av NPN CC EEP P V V

2

,max 2 when 2CC P CC

av P

L

V V VP V

R

, /2

/2

2

1

sin1sin

4 4

T

av PNP C CET

TP

P EETL

EE P P P EE P

L L L

P I V dtT

V tV t V dt

T R

V V V V V V

R R R

Heat Sink

Heat sink, provides large surface area to dissipate heat from

the chip.


Thermal Runaway Mitigation

1 21 2

, 1 , 2

1 2

, 1 , 2

1 21 2

, 1 , 2

1 2

, 1 , 2

1 21 2

, 1 , 2 , 1 , 2

ln ln

ln

ln ln

ln

With the same ,

D DD D T T

S D S D

D DT

S D S D

C CBE BE T T

S Q S Q

C CT

S Q S Q

T

C CD D

S D S D S Q S Q

I IV V V V

I I

I IV

I I

I IV V V V

I I

I IV

I I

V

I II I

I I I I

Using diode biasing prevents thermal runaway since the

currents in Q1 and Q2 will track those of D1 and D2 as long as

their Is’s track with temperature.41CH 13 Output Stages and Power Amplifiers

Efficiency

Power Delivered to Load

Power Drawn From Supply Voltage

out

out ckt

P

P P

Efficiency is defined as the average power delivered to the load divided by the power drawn from the supply

Emitter Follower (Class A)

2

21 1

1

2

2 2

if and4

P LEF

P L CC P EE

P PEE CC

CC L

V R

V R I V V I V

V VI V V

V R


Maximum efficiency for EF is 25%.

Example 14.15: Efficiency of EF


1

2

21 1

2

2

/ 2

With = and

2

2 2

2

2 2

Thus,

16.7%

15P CC

CCPEE CC

L L

P LEF

P L CC P EE

P L

CC CCP L CC P CC

L L

EF V V

VVI V V

R R

V R

V R I V V I V

V R

V VV R V V V

R R

EF designed for full swing operates with half swing.

Efficiency

Push-Pull Stage (Class A or AB)

2

2

2

2

2 4

4

P

LPP

CCP P P

L L

P

CC

V

R

VV V V

R R

V

V


Maximum efficiency for PP is 78.5%.

Example 14.16: Efficiency incl. Predriver


1

2

2

2( )

1

4

1 1

1 14

P L

P

L

P CC PCC EE

L L

P

CC CC

P

CC

I V R

V

R

V V VV V

R R

V

V V

V

V

Documents

Chapter 13 Output Stages and Power Amplifiersocw.snu.ac.kr/sites/default/files/NOTE/ch14_2017.pdf · Power Amplifier Characteristics Experiences low load resistance. Delivers large