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Mystery scientific preamplifier - Reverse engineering

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David Hess:

--- Quote from: Kleinstein on February 16, 2020, 03:38:39 pm ---
--- Quote from: David Hess on February 16, 2020, 03:24:49 pm ---
The AD744 is an odd choice unless they needed higher slew rate than a bipolar part would provide.  I suspect they picked it because it uses external compensation.  I wonder if they original intended to only use the AD744 and changed it later by adding the discrete JFET input stage.
--- End quote ---

My though is more like starting with an BJT based OP (e.g. NE5534 was popular in combination with external JFETs) and than switched to the AD744 for higher speed / slew rate.
--- End quote ---

The AD744 actually has two advantages over a bipolar part in this circuit.  The JFET input stage inherently roughly triples the slew rate because of lower transconductance, although bipolar operational amplifiers can use transconductance reduction to do the same thing, and the external compensation allows for decompensated operation.

I suspect the AD744 included external compensation for use in high gain microphone amplifiers.  That feature is very rare in any modern part; if anything, they usually support overcompensation.

I am always looking for modern parts which support external compensation.  It is too bad that the AD744 is so expensive.

ChristofferB:
Thanks for all the info!

I tried (reversibly) chopping away at the feedback network, since I really want all my shaping done later in my signal chain.

At low gains (2M feedback resistor shorted or reduced to 50K) it oscillates. at around 250K the gain is significantly lower, to the level where all but the largest signals are within 0-5V range. See attached. In the picture the signal has also been run through my shaping amplifier, but they look similar out of the preamp itself.

I'm unsure where the massive undershoot comes from. It was present before I modified anything too. Also unsure if it will be an issue in a multi-channel analyzer.

ChristofferB:
Since I have three of these boards I'm also considering harvesting one of the nice opamps for a very bare-bones charge sensitive preamplifier, something like this:


but running +-12V as supply. And maybe with a less exotic amp as buffer. Or without buffer entirely.



Kleinstein:
For the amplifier one would not need such a high grade OP. For a first test even TL071 grade should be OK. It would make the pulsed a little wider, but that is about it. OPs with 10 MHz GBW (e.g. OPA172, OPA197) are relatively cheap these days.
A really fast amplifier is mainly needed if one cares about the timing,  e.g. for something like correlation. For the pulse hight the speed is normally set by the multi-channel analyzer, especially the peak shaper part.

It depends on the implementation of the peak hight measurement weather the undershoot really matters, most version should be ok with undershoot.

ChristofferB:
Good to know. I just though the whole point of having stuff like pole zero cancellation in a spectroscopy was to avoid this behavior.

Also, this setup is NOT optimized for impedance. I've just been eyeballing it all the way through, and I think I've read that the PMT should have a T with a termination right at the output. I'll work it out.

thanks for the interest and help!

--Chris

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