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h_a 8th December 2008 03:53 PM

Threshold's optobias
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I'm currently trying to wrap my head around Nelson's patent for the optobias (# 4752745) as I like the idea to add a sliding bias output stage to my current project amp.

The concept is simple yet elegant (typical for Nelson) and works about as follows:

R21 provides a measure of the driver bias; its voltage drop is about 1.2V or larger (we're talking GaAs diode); if it's larger, R23 is necessary to reduce current through the diode 23.

The larger the current trough R21, the larger the current in diode D23; this diode drives the base of Q21 with a current transfer ratio of, say 5 (means the current through Q21 is 5 times the diode forward current). So more current through the diode yields more current in Q21.

So far so good; however how does that yield a larger VBE for the drivers?

There's a current source drawn - Q25 -, however that would reduce the collector-emitter voltage across Q21 if the current through Q21 increases - so does exactly the opposite of what one wants.

Unfortunately I couldn't find a single schematic with part values that includes optobias, so I hope somebody could enlighten me!

It's a pity that this patent is not as elaborate as the first smart bias patent (which even included 3 full amp schematics), it even mentions I1 and I2 which are not drawn at all...

Have fun, Hannes

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