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Old 19th March 2012, 05:26 AM   #1
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Default Emitter resistor at output stage transistor.

Hi there,
I'm would like to know what the advantage if I replace small resistance(like 0.22ohm) resistors at emitter pin of output stage power transistor.

Do I get amplifier with lower output impedance?
Is this resistor used for prevent power transistor overheat only?
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Old 19th March 2012, 06:03 AM   #2
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Quote:
Originally Posted by smithomo View Post
Hi there,
I'm would like to know what the advantage if I replace small resistance(like 0.22ohm) resistors at emitter pin of output stage power transistor.

Do I get amplifier with lower output impedance?
Is this resistor used for prevent power transistor overheat only?
Hi Smithomo
The emitter resistors are usually placed to reduce Class a/b distortion. See article point 6
Distortion In Power Amplifiers

Cheers / Chris
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Old 19th March 2012, 07:08 AM   #3
65 DegN is offline 65 DegN  United States
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My understanding is that the emitter resistors are used to diminish the importance of perfectly matched output transistor beta. Small variations in beta would cause outputs to have slightly different crossover points.
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Old 19th March 2012, 07:49 AM   #4
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Originally Posted by Chris Daly View Post
Hi Smithomo
The emitter resistors are usually placed to reduce Class a/b distortion. See article point 6
Distortion In Power Amplifiers

Cheers / Chris
Hi Chris Daly,
That a nice article I have to read right away.
So as I ever seen Krell FPB 300/600 class A schematic, it also have the emitter resistors, so can we state that it is mandatory for all transistor amplifier.?
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Old 19th March 2012, 08:40 AM   #5
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Hi Smithomo
The use of a emitter resistor in a circuit has the tendency to stabilize operating point against changes in temperature and β-value. ,as 65 DegN has pointed out.

Its use generally though - which is your question with all amplifiers is not apparent.
However applies to class a/b amplifiers as a good technique to reduce distortion in output stages. Which is to say transistor bias has many forms.

A very early amplifier from 1967 the quad 303 found great merit in what was termed triples, and used emitter feedback from the use of small value resistors similarly placed, here Doug Self is discussing triples it makes great reading.
Audio Power Amplifier Design Handbook - Douglas Self - Google Books





Cheers / Chris

Last edited by Chris Daly; 19th March 2012 at 08:52 AM.
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Old 19th March 2012, 09:26 AM   #6
DF96 is offline DF96  England
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The emitter resistors reduce crossover distortion. To a first approximation, each resistor should be equal to the transconductance of one side of the output stage at quiescent current. Then the zero signal transconductance and full signal transconductance (when one side is cutoff) will be equal. Real life is not quite that simple, but that is roughly how it works.
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Old 19th March 2012, 10:09 AM   #7
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Originally Posted by DF96 View Post
The emitter resistors reduce crossover distortion. To a first approximation, each resistor should be equal to the transconductance of one side of the output stage at quiescent current. Then the zero signal transconductance and full signal transconductance (when one side is cutoff) will be equal. Real life is not quite that simple, but that is roughly how it works.
Thank you so much
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Old 19th March 2012, 10:47 AM   #8
AndrewT is offline AndrewT  Scotland
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I have read more than a few times that the distortion of the output stage reduces as the emitter resistor values reduce.

Taken to the limit (zero resistance for the external emitter resistor) this would indicate that a single pair output stage with just the inherent internal emitter resistance will give less distortion than a stage with added external emitter resistor.

For the minimum crossover distortion the output stage should be set to the optimum ClassAB output bias voltage. This is what Mooly is describing.
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Old 19th March 2012, 10:59 AM   #9
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Using mosfets it also linearise the square law nature of these devices.
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Old 19th March 2012, 11:11 AM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by AndrewT
Taken to the limit (zero resistance for the external emitter resistor) this would indicate that a single pair output stage with just the inherent internal emitter resistance will give less distortion than a stage with added external emitter resistor.
A BJT, unless it has internal ballast resistors, will have an effective emitter resistance which varies with current. There may be some limit caused by device bulk resistance but this is likely to vary between samples and it will be small. To a first approximation you have to provide the same resistance at quiescent current. The lower the emitter resistor, the higher the quiescent current. You could end up in Class A, where an external resistor is really there for overload protection only.
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