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Old 2nd October 2006, 02:24 PM   #1
mikeks is offline mikeks  United Kingdom
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Default Good stuff...

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Old 2nd October 2006, 04:06 PM   #2
Jan Dupont is offline Jan Dupont  Denmark
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Free Schematic and Service Manual downloads www.audio-circuit.dk, Spare time company (just for fun): www.dupont-audio.com
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Old 2nd October 2006, 04:49 PM   #3
TOINO is offline TOINO  Portugal
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Old 2nd October 2006, 10:04 PM   #4
TomWaits is offline TomWaits  Canada
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Old 2nd October 2006, 10:49 PM   #5
anatech is offline anatech  Canada
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Hey Mike,
Thank you.

Everything is worth a read, especially app. notes.

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Old 2nd October 2006, 10:56 PM   #6
TOINO is offline TOINO  Portugal
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Old 2nd October 2006, 11:03 PM   #7
lpm76 is offline lpm76  Denmark
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Okay that is going to take a while to digest, and even longer to understand :-)
Perception is reality, not truth.
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Old 2nd October 2006, 11:16 PM   #8
lineup is offline lineup  Sweden
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thanks michael
I downloaded, too, like many others
.. a bit slow compared to downloadss from my own Apache http:// / Serv-U ftp:// servers
but okay, I got the PDF

Another common mistake made by many first- time IA users is shown in Figure 16. This circuit
uses two silicon PIN photodiodes to detect a modulated LED light beam. For best speed of
response and linearity, the detectors are reverse biased. An instrumentation amplifier is used to
amplify the difference between the two detectors’ outputs. Capacitors AC couple the signals to
the IA inputs while blocking the detectors’ DC bias voltage.

The circuit is missing one thing-- without a bias current return path, the IA bias current creates
a huge offset and its output is driven to the rail.

Consider Ohm’s Law:
The input offset Vos = Ib x Rin where Ib is the IA input bias current (1nA)
and Rin is the IA input resistance (1010 ohms) in parallel with the 0.47mF capacitor’s insulation
resistance ( about 30,000 megohm · microfarads for a metallized polypropylene film capacitor).

Even if this is not by all considered to be a good thing to do, I take the liberty to quote myself.
Because this was one of the aims for my tutorial on
Active Bias Constant Current Sourcing of Inputs
( does not matter really if is discrete or Op-Amp input transistors.
They are all transistors of Silicon or Metal Oxide ( MOS ) anyway.
I find it a good option to
- be free to use whatver resistors in input, feedback
- keep these resistors almost current-free at idle, no signal input
- avoid DC-servo circuits
- keep correct current balance in input pair, = ideally equal currents

You do not have to use bias current addition to both legs of input differential pair.
Just using for the left, non-inverting is good enough to get zero DC-offset.

When using other methods, you can endup with input pair
does not have equal current and this is not a good thing

Offset Correction using Bias Compensation
An original www.diyAudio.com topic
by member

To correct Ouput DC offset
you can in any amplifier with differential inputs
provide bias current compensation at one or both input transistors.
( I do not mean servo - Only static currents )

This can be done using a constant current source feeding a transistor.
The base current of this transistor provide the bias current to compensate.

>> lineup audio lab <<
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Old 3rd October 2006, 07:50 AM   #9
Nexus is offline Nexus  Netherlands
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does anybody has another link, I can't seem to download from this site.

Thank you in advance:
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