Class A Output Stage - Recap

Class A output stage is a simple linear current amplifier.

It is also very inefficient, typical maximum efficiency between 10 and 20 %.

Only suitable for low power applications. High power requires much better

efficiency.

Why is class A so inefficient ?

Single transistor can only conduct in one direction.

D.C. bias current is needed to cope with negative going signals.

75 % (or more) of the supplied power is dissipated by d.c.

Solution : eliminate the bias current.

Class B Output Stage Q1 and Q2 form two unbiased

emitter followers Q1 only conducts when the input

is positive Q2 only conducts when the input

is negative Conduction angle is,

therefore, 180° When the input is zero,

neither conducts i.e. the quiescent power

dissipation is zero

Class B Current Waveforms

Iout

IC1

IC2

time

time

time

Class B EfficiencyAverage power drawn from the positive supply:

1CSve IVP

LL

C RA

RA

II

0

2

0

C11 dsin21

d21

IC1

Phase,

A/RL

0 A sin()

L

Sve R

AVP

By symmetry, power drawn from +ve and –ve supplies will be the same. Total power, therefore:

L

SveveveS R

AVPPPP

2

2

Load power:

LLL

outL R

AR

tAR

vP

2sin 2222

Efficiency:

SS

L

LS

L

VA

AVR

RA

PP

422

2

%5.784/ NB. SVA

Power Dissipation

To select appropriate output transistors, the maximum power dissipation must be calculated.

LL

SLSD R

AR

AVPPP

2

2 2

Just need to find the maximum value of PD to select transistors/heatsinks

0 5 10 150

0.5

1

1.5

Peak Output Amplitude, A [V]

Pow

er [

W]

PL

PS

PD

E.g. VS = 15 V, RL = 100

L

SS R

AVP

2

LL R

AP

2

2

PD is a quadratic function of A,

Maximum Power Dissipation

0dd APDmaximum when:

LL

SD R

AR

AVP

2

2 2

L

S

L

S

L

SD R

V

R

V

R

VP 2

2

2

2

2

2

(max)

224

02

LL

S

RA

RV

SVA

2

Efficiency / Power Dissipation

Peak efficiency of the class B output stage is 78.5 %, much higher than class A.

Unlike class A, power dissipation varies with output amplitude.

Remember, there are two output devices so the power dissipation is shared between them.

Design Example

Design a class B amplifier which will deliver up to 25 W into a 4 load.

V 1.14252

252

ARA

PL

L

Supply voltages must be larger than Amax so choose Vs = 15V.

W7.52 W4.112

2

2

max L

SD R

VP

Each of the two output transistors must be able to safely dissipate up to 5.7 Watts. Using a TIP120 & TIP 125:

ADJAJDJAAJ TPTPTT

But, with JC = 1.92 °C/W

C/W20

92.1

CA

CACAJCJA

i.e. Either two heatsinks rated at less than 20°C/W are required or a single heatsink rated at less than 10°C/W.

C/W22

150257.5

JA

JA

maxmax JADJA TTP

Suggested heatsink

Dimensions, 50mm x 50mm x 9.5mmAccommodates two devicesRating 6.5°C/WCost 60p inc VAT

Cross-Over Distortion A small base-emitter voltage is

needed to turn on a transistor Q1 actually only conducts when

vin > 0.7 V Q2 actually only conducts when

vin < -0.7 V When 0.7 > vin > -0.7, nothing

conducts and the output is zero.

i.e. the input-output relationship is not at all linear.

Actual Input-Output Curve

vin

vout

BEinout Vvv

BEinout Vvv

+VBE

-VBE

Effect of Cross-Over Distortion

Audio Demo

Undistorted original

Class B amplifier output

Class B Summary

A class B output stage can be far more efficient than a class A stage (78.5 % maximum efficiency compared with 25 %).

It also requires twice as many output transistors…

…and it isn’t very linear; cross-over distortion can be significant.