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Old 30th September 2003, 08:59 PM   #11
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Shaun, not quite right. The resistor to ground creates a DC path crucial if the input has a DC blocking capacitor. Without this resistor the amp won't work at all.
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Old 1st October 2003, 07:32 AM   #12
Shaun is offline Shaun  South Africa
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Default Let's get on the same page first

I'm probably in error in referring to the wrong resistor as the input resistor.

R1 is the actual input resistor, but I was really referring to R2 (as the discussion had originally been about DC offset). Please see the diagram.

Which resistor has greatest impact on input impedance (my own amp has a 100k at R1), and am I wrong about R2 being the DC sense point for the amp input?



PS- I'm not being difficult here, I just need to know for sure.

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Old 1st October 2003, 08:22 AM   #13
Electrons are yellow and more is better!
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The R1 is optional but I use to have some pull down, 100k - 1 Mohms, just preventing the cap to pick up charges. Both resistors have influence but only R2 has influence on the offset. R1 can be any high value but R2 must be "tweaked" not to disturb to much. The value is dependent of how much bse current you have, how much offset and how low input impedance you can take.

Note that base current through R2 creates a voltage.

If you want to get a picture of the problem, check datasheets and especially the offset trim section for a "bad" opamp like NE5534 which has rather large input bias current and also rather high offsets.

If you have understood the theory behind the problem you can easily use the knowledge at your discrete amp.

Good reading here
http://focus.ti.com/docs/apps/catalo...tName=sboa092a
Can anyone give me a hint where I can find the 300 page document from texas in the subject?
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Old 1st October 2003, 08:30 AM   #14
Shaun is offline Shaun  South Africa
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Default Thanks, I can sleep better now

That is exactly as I understood it. Sorry for the wrong terminology in my previous posts.

Just a question about R1: does this (high) value contribute much to noise?

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Old 1st October 2003, 08:43 AM   #15
Electrons are yellow and more is better!
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Think like this: ALL resistances between the signal source and the amp. If you have both series and parallel resistors you must convert them to a single one. The noise resistance is what the input (the input transistor) "sees". Usually ONLY the signal source resistance and .... the feedback network have influence on the noise (including the noise internally in the amp).

The answer is: NONE to your specific question.
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Old 1st October 2003, 08:46 AM   #16
Shaun is offline Shaun  South Africa
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Default Oops- you've edited your post

Yes, Peranders. I think I understand.

The heavier the base current, the more important it is for the dc impedances (looking out of the base) to be equal, in the case of a differential input amp. Else the resultant imbalance in the input standing currents will cause a dc offset at the output.
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