Power Amps 6

8/14/2019 Power Amps 6

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Cross-Over DistortionThe non-zero turn-on voltage of a transistorcauses cross-over distortion in a class B

output stage.

Approximatetransistor

response.

vin

vout

VBE0

Ideal

response


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Eliminating Cross-Over Distortion

vin

vout

NPNresponse

NPN responsefor vB=vIN+0.7

PNPresponse

PNPresponse forvB= vIN-0.7


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Class AB Output Stage

Eg. Positive half cycle:

biasinB Vvv

1

BEbiasinBEBout

BEB

VVvVvv

Vv

1

1If

inout

BEbias

vv

VV

If


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Practical Class AB Stages In practice, there isnt an exact turn-on

voltage (VBE).

Vbiasis set slightly high so that there is a non-zero quiescent collector current.

Each transistor will now conduct for slightly

more than 180 - i.e. Class AB operation.


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Class AB Efficiency Slightly more power is dissipated using a

class AB stage compared with a class B due

to the non-zero quiescent collector current. In a well designed circuit, this extra power

should be insignificant so the class B

efficiency calculations are still valid.

I.e. maximum efficiency = 78 %.


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Thermal EffectsThe quiescent collector current depends on VBEand alsoon the junction temperature. So, in designing the biasing

network, thermal effects must be considered.

Net result is that if VBEis fixed, ICrises exponentiallywith temperature.

kTqVII

kT

qVII

GGS

BESC

exp

but

exp V6.0BEV

V2.1GV


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Thermal Effects

20 30 40 50 600

0.4

0.8

1.2

Temperature [C]

C

ollectorCurrent[mA](VBE=0.5

V)


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Thermal RunawayCollector Current Flows, so

power is dissipated

Temperature rises

Collector current rises

Power dissipationincreases


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Suppressing Thermal Runaway Fit a bigger heatsink.

Use series emitter-resistors.

Use a temperature dependent bias voltage.

The latter two are preferred methods. Both

introduce negative feedback.


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Emitter ResistorsREEBBEbias VVVV 22 21

So, if ICrises, VBEfalls andICis reduced.

Note REshould be smallcompared with RLto minimise

power wasted.

By symmetry:

ECbias

REbiasEBBE

RIVVVVV

21


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Bias VoltageThe VBEMultiplier

2

115.0R

RVbias

2

1121

R

RVRIV BERR

2

1

2

121 1

RRVV

RRVVVV BEBE

BERRbias

Base current is negligible, so:

221 R

V

II

BE

RR

VBE


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VBEMultiplierTemperature Effects If junction temperature rises but ICstays the

same, VBEmust fall causing Vbiasto fall also.

Negative thermal feedback achieved if thetransistor is in close contact with the output

devices.

Especially suitable for integrated circuits

where close thermal contact is guaranteed.


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Design Example(i) RELet RL= 16 Wand Amax= 12 V.(Also assume Vout= 0 throughd.c. feedback).

V3.147.015(max)1(max)1 BEBE VVV

WW 2Let3 EE RR

1216

163.14

maxmax1

E

LE

LE

R

ARR

RV


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Design Example(ii) Ibias

mA10LetmA75.3

20016

12

(min)

(max)

(min)

(max)1

(max)1

bias

bias

L

C

Bbias

II

R

AIII

NB. Ibiasis set well aboveminimum to ensure that a

significant current flows throughthe VBEmultiplier.


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Design Example(iii) VbiasPeak output current = 0.75 A,choose quiescent collector currentto be small by comparison, e.g.

V5.1

2025.027.02

22

mA2521

bias

REBEbias

CC

V

VVV

II


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Design Example(iii cont) VbiasFor constant bias voltage,

mA1chooseso 1

13

R

biasRB

I

III

k12

5.115.0

21

2

1

RR

RRVbias

W

500

5.0

2

212

R

RIVV RBER


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Class ABSummary Class AB achieves the efficiency of a class B

output stage but without cross-over

distortion. Biasing arrangements are more complex,

however, as the threat of thermal runaway

must be eliminated.

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Power Amps 6