New head amp design

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Postby Steve Anderson » Wed Feb 24, 2010 12:49 pm

Virtually all pinning diagrams for discrete transistors are a bottom view, i.e. the leads pointing at you. This is a hangover from the valve/tube days where in the pinning diagram pins were numbered clockwise from some form if index, a keyway or a missing pin.

Chips of the calibre we use here are numbered anti-clockwise looking from the top from some form of index, which amounts to the same thing really.

Of course there are the inevitable exceptions!

If your multi-meter has an hfe function confirm those diagrams actually do apply to the devices in your hand.

40nF for a solar-cell capacitance comes as no surprise and like photo-diodes is roughly proportional to the cells' active area. Hence where they have been used the pre-amp has a rising response to compensate. With the signal already at a very low level this could result in excessive HF noise. But others have used them with success.

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Postby gary » Wed Feb 23, 2011 6:17 pm

Did you get anywhere with this Lawnboy?
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Postby Lawnboy » Sat Feb 26, 2011 9:53 pm

honestly i havent touched it in a long time. has it really been a full year? the rig is still sitting on the shelf along with the unfinished mirror screw and mirror drum. been busy working on a color monitor for Dom's NBSC, but i will post my results when i get to working on it again.
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Re: New head amp design

Postby Lawnboy » Tue Dec 30, 2014 6:49 am

Gentlemen, I believe I have finally located a part number for the club's beloved dome sensor. It appears to be a VTP1188SH. More info here: http://www.excelitas.com/Lists/Photodio ... aspx?ID=46
and here: http://www.newark.com/excelitas-tech/vt ... dp/79K2522

-Byron
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Re: New head amp design

Postby Steve Anderson » Tue Dec 30, 2014 3:59 pm

Well done. If it is the club's 'Dome Sensor' the datasheet (downloadable from Newark) points out its rather large internal capacitance, some 2-300pF compared to around 70pF or so for a BPW34. The capacitance of the BPW34 decreases to around 15pF under a reverse bias of ten volts. This may explain why some have observed a rather lethargic response in the past, but it's usable as many have found out.

It does have the advantage of a larger sensitive area compared to the BPW34 though.

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Re: New head amp design

Postby Panrock » Thu Jan 01, 2015 4:44 am

Steve Anderson wrote:.... points out its rather large internal capacitance, some 2-300pF compared to around 70pF or so for a BPW34. The capacitance of the BPW34 decreases to around 15pF under a reverse bias of ten volts.

I am also having to be capacitance-aware at present! The Hamamatsu S5973 I am using for my "45 MHz subcarrier" testbed (presently under construction) claims a capacitance of under 2pF at the reverse bias I shall be using. Sticking to VHF construction techniques as much as possible, to give it a chance...

Steve O

P.S. Anyone know how fast photomultipliers can potentially be?
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Re: New head amp design

Postby Steve Anderson » Thu Jan 01, 2015 3:14 pm

PMT's can/are very fast. Even the humble 931A can resolve 100's of MHz...given appropriate construction methods. Basically a trans-impedance amplifier which eliminates the effects of its internal capacitance and other strays (wiring, socket etc.). The downside...several hundred volts, aging, and as I found out, noise.

And if being considered for this application their general lack of response to red or IR.

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