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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 ?.

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Class a output stage recap l.jpg
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 l.jpg
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.


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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 l.jpg
Class B Current Waveforms

Iout

time

IC1

time

IC2

time


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IC1

A/RL

0

p

2p

Phase, q

Class B Efficiency

Average power drawn from the positive supply:

A sin(q)


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Load power:

Efficiency:

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


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Power Dissipation

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

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


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

1.5

PL

PS

PD

1

Power [W]

0.5

0

0

5

10

15

Peak Output Amplitude, A [V]


Maximum power dissipation l.jpg

maximum when:

Maximum Power Dissipation

PD is a quadratic function of A,


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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.


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Design Example

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

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


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But, with qJC = 1.92 °C/W

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

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.


Slide13 l.jpg

Suggested heatsink

Dimensions, 50mm x 50mm x 9.5mm

Accommodates two devices

Rating 6.5°C/W

Cost 60p inc VAT


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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.


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Actual Input-Output Curve

vout

-VBE

vin

+VBE



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Audio Demo

Undistorted original

Class B amplifier output


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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.


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