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Class A emitter follower power amp

paulmdrdo

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What is the role of Diode and Resistor R here? Thanks.
2020-05-22 17_38_48-(The Oxford Series in Electrical and Computer Engineering) Adel S. Sedra, Ke.png
 

FvM

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Obviously a current mirror biasing the emitter follower.
Missing -VCC label at diode cathode.
 

Akanimo

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Hi,

The diode and resistor R are voltage divider components. The diode serves to turn on transistor Q2 base-emitter junction. R alongside the diode serves to divide -Vcc and to also limit current flowing through the branch.
 

d123

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Hi,

Can I ask: Isn't the diode also for Vbe temperature compensation? I'm sure I've seen AC amplifiers with diode at base like that in tutorials and some bjt app note or other, not just in emitter leg or reverse biased on base to ground.
 

Akanimo

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The diode would drop about the same voltage as the transistor Vbe and so helps in eliminating the crossover distortion that would otherwise be present in the output.
 

Audioguru

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The diode would drop about the same voltage as the transistor Vbe and so helps in eliminating the crossover distortion that would otherwise be present in the output.
This circuit is class-A that conducts a very high current all the time. Only the top transistor is driven with the signal, then it does not produce any crossover distortion. A class-B push-pull circuit produces crossover distortion because both transistors are driven with the signal.
 
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