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Old 16th November 2006, 01:22 PM   #1
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Default bias capacitor

I am about to replace one, its an electrolytic,

what does it actually do?

its on the output of the bias transistor network.

Its only 1uf, is this sufficient?

thanks
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Old 16th November 2006, 01:30 PM   #2
lineup is offline lineup  Sweden
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Default Re: bias capacitor

Quote:
Originally posted by lt cdr data
I am about to replace one, its an electrolytic,
what does it actually do?
its on the output of the bias transistor network.
Its only 1uf, is this sufficient?
thanks
more details please
What amp?
What bias network?

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Old 16th November 2006, 01:55 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
that capacitor, is it across the bases of the driver transistors? and across the Vbe multiplier?

Usually it is quite low in value, I have seen 47nF upwards.
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Old 16th November 2006, 02:02 PM   #4
Amp_Nut is offline Amp_Nut  India
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Quote:
Originally posted by AndrewT
Hi,
that capacitor, is it across the bases of the driver transistors? and across the Vbe multiplier?

Usually it is quite low in value, I have seen 47nF upwards.
That capacitor is to bypass the Signal accross the transistor ( Vbe multiplier )

The Vbe multiplier is there to create a DC drop that tracks with temperature, but the Vbe multiplier is supposed to be transparent to the signal.

Yes, the capacitor accross the Vbe multiplier can vary from 0 f ( Nothing at all ) to even 100 mfd. It needs to be rated at only a few volts ( the voltage accross the Vbe multiplier which is typically less than 10 Volts DC )
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Old 16th November 2006, 02:04 PM   #5
lineup is offline lineup  Sweden
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Quote:
Originally posted by AndrewT
Hi,
that capacitor, is it across the bases of the driver transistors? and across the Vbe multiplier?

Usually it is quite low in value, I have seen 47nF upwards.
I did some simulation in very a simple push-pull stage.
( Without any VAS, this output stage was a follower buffer )
With and without one 1uF film cap.
Was a bit lower distortion with cap.


It has been discussed several times before in this forum, if such a cap does any difference.

My thinking:
...................................
It depends of the actual circuit, if it contributes anything good, at all.
And as it does not hurt anything,
we may as well put it in across Vbe multiplier in all amplifiers.
At least 1.0 uF.
...................................


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