the benefits of a high voltage in this photodiode amplifier

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the benefits of a high voltage in this photodiode amplifier

Postby newbie » 12 Sep 2016, 10:06

In an article in LT Journal of Analog Innovation, Linear Technology brags about their high-voltage operational amplifier, the LTC6090, which can operate on ±70 volts. They present an example circuit, a photodiode amplifier:


This is perhaps impressive; I don't know much about photodiode amplifiers.

What strikes me as odd is that, while the feedback loop operates on voltages up to 120 volts, taking advantage of the increased voltage range, they then place a 1:10 voltage divider on the output, seemingly negating the benefits.

Why not replace the 10 MΩ resistor with 1 MΩ and get rid of the output divider? Then you can use a completely average operational amplifier because the output only needs to swing between 0 and 12 volts.

In my simulations, lowering the feedback voltage gave me a number of benefits:

Higher bandwidth (using the same LTC6090)
0.3 pF capacitor can be increased, loosening the layout requirements
Lower output impedance
A larger selection of operational amplifiers are now available
...and so on
I have searched in the article and the datasheet for reasons behind this, but I could not find any. Perhaps it is obvious to people experienced with photodiode amplifiers, but since the "feedback action" makes sure that the photodiode sees the same potential difference, it should not affect its behaviour - there will be 3 volts across it, and the same current will flow.

Why is this circuit improved by a high feedback voltage?
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