Has the Elektor forum disappeared ?
Well it is not audio related , unless you count the clicker .
Has anyone made this radiation detector with a photo diode BPW 34 ? (Elektor june 2011, and the improved one november 2011).
A while back I bought 2 different BPW34 , Osram and Vishay and the BC849C.
But November 2011 is the improved version with JFET + opamp.
I thought to try the transistor version first , but it has to be shielded and the BPW covered, so why bother , better to go for the better version.
Not sure why Elektor didn't directly go for a JFET opamp or even a CMOS one without the now harder to find JFET BF245 or other.
I know too little of analog circuits to adapt the JFET version to a straight opamp one .
I have 2 good opamps for this: OPA196 in super small Sot23-5 and OPA197 in soic 8.
Don't know how much I can post here , with copy rights and stuff , but it is from 2011 , 12 years ago.
Well it is not audio related , unless you count the clicker .
Has anyone made this radiation detector with a photo diode BPW 34 ? (Elektor june 2011, and the improved one november 2011).
A while back I bought 2 different BPW34 , Osram and Vishay and the BC849C.
But November 2011 is the improved version with JFET + opamp.
I thought to try the transistor version first , but it has to be shielded and the BPW covered, so why bother , better to go for the better version.
Not sure why Elektor didn't directly go for a JFET opamp or even a CMOS one without the now harder to find JFET BF245 or other.
I know too little of analog circuits to adapt the JFET version to a straight opamp one .
I have 2 good opamps for this: OPA196 in super small Sot23-5 and OPA197 in soic 8.
Don't know how much I can post here , with copy rights and stuff , but it is from 2011 , 12 years ago.
Use a LMC-something instead of the 358, and connect directly the photodiode+ 20M resistor (or more) to the + input of the opamp and scrap the FET and its loading resistor.
The foot of the 20M could be returned to a voltage of 1 to 2V instead of GND, to increase the dynamic range
The foot of the 20M could be returned to a voltage of 1 to 2V instead of GND, to increase the dynamic range
Thanks Elvee .
Can it be as simple as this ? Then why did Elektor go to the trouble of adding a JFET.
Not sure what you mean with : " The foot of the 20M could be returned to a voltage of 1 to 2V instead of GND, to increase the dynamic range"
Elektor writes that a higher voltage over BPW34 is better because it reduces the capitance . The text says gives a overall voltage gain of 30.000 , but I only see about 33 x 33 = about 1000 gain. How am I wrong ?
Can it be as simple as this ? Then why did Elektor go to the trouble of adding a JFET.
Not sure what you mean with : " The foot of the 20M could be returned to a voltage of 1 to 2V instead of GND, to increase the dynamic range"
Elektor writes that a higher voltage over BPW34 is better because it reduces the capitance . The text says gives a overall voltage gain of 30.000 , but I only see about 33 x 33 = about 1000 gain. How am I wrong ?
Attachments
It increases the input impedance and lifts the DC voltage by 1 or 2V.
With an opamp like the 358, this is necessary because it requires a bias current of several tens of nA. With a LMCxyz or a TLCzyx, the input current is fA, perfectly OK, but the input will be very close to the GND.
To give it a little more leg room, you can tie the 20M resistor to a divider made of 1K5 and 10K for example.
As it is, the gain of the circuit is 1000 indeed
Still a bit bothered by the 30.000 gain for the JFET version in the Elektor text , now that the tr/tr version is also more than 1000.
Does the JFET step have gain or is it just a 1:1 buffer ? Sorry , I can't tell , I'm not that familiar with JFETs.
Also is R2/C6 necessary ? With the 10k/1,5k , it is a 800 mV drop over R2 . OR replace them with higher value : 100k/15k ?
Since the BPW34 is not AC coupled , can current fluctuations over R11 cause false signals on the output ?
Does the JFET step have gain or is it just a 1:1 buffer ? Sorry , I can't tell , I'm not that familiar with JFETs.
Also is R2/C6 necessary ? With the 10k/1,5k , it is a 800 mV drop over R2 . OR replace them with higher value : 100k/15k ?
Since the BPW34 is not AC coupled , can current fluctuations over R11 cause false signals on the output ?
The jFET has a gain of 0.9, or thereabout.
R2/C6 is certainly necessary with the FET; much less so with the direct connection to the photodiode, but it remains necessary with the foot divider.
100K/15K is certainly OK, going much larger would risk noise collection from both ends of the 20M.
The RC cells with R2 and R11 have a large enough time constant to rule out the possibility of false signals
R2/C6 is certainly necessary with the FET; much less so with the direct connection to the photodiode, but it remains necessary with the foot divider.
100K/15K is certainly OK, going much larger would risk noise collection from both ends of the 20M.
The RC cells with R2 and R11 have a large enough time constant to rule out the possibility of false signals