# Charge Sensitive and shaping amplifier

## Main Question or Discussion Point

Hi,
we are designing a new radon detector which filled with two parts as CSA&SA(charge sesitive and shaping amplifier) in the total system. I attached a Circuit consisting of those two parts and i need to know the function JFET in CSA. In the given circuit i mentioned one arrow at the end of CSA part and next is SA. the output of CSA is 40mV and input will given as 111mV from function generator. i want to clear my self how the jfet is working over there adn about feed back.

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berkeman
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Hi,
we are designing a new radon detector which filled with two parts as CSA&SA(charge sesitive and shaping amplifier) in the total system. I attached a Circuit consisting of those two parts and i need to know the function JFET in CSA. In the given circuit i mentioned one arrow at the end of CSA part and next is SA. the output of CSA is 40mV and input will given as 111mV from function generator. i want to clear my self how the jfet is working over there adn about feed back.
The attachment didn't seem to go through...?

Now find the attachment of circuit

Last edited:
AD811 is a current feedback amp, you better be careful how you put it as part of the close loop feedback. It is really not a convensional voltage feedback opamp that you can run it in open loop like in the circuit shown. You don't have a feedback resistor to close the loop on the opamp itself.

I don't quite understand the circuit, it almost like sensing the voltage across the R5 and try to feed back to the input. Looks like R3 is supposed to be the trans-impedance resistor so what ever current at the input will be convert into voltage of V=I X R3. Also if it is a closed loop feedback trans-impedance amp with R3 as feedback resistor, C3 will be the killer, this will create a poll and you cannot stablize the amp that easy.

I design a lot of trans-impedance amp in my career, this looks very different to me. Maybe I don't know the input condition. You need to specify what input range to expect and what is you output requirement.

I think the designer put the jFet to minimize the input current. But I can tell you that you can get a jFet input amp and make it simplier. With the C3, don't think the designer is expecting high frequency performance, so it should not be hard to find a JFet input opamp to do the job if that really needed.

Yes, this operation will not conduct at higher frequencies and also the op-amp we are using is ad817.
input parameters by function generator
------------------------------------------
freq-100Hz
amp-111mv
duty cycle-50%
time-5ms

Yes, this operation will not conduct at higher frequencies and also the op-amp we are using is ad817.
input parameters by function generator
------------------------------------------
freq-100Hz
amp-111mv
duty cycle-50%
time-5ms
If you don't need high output current drive requirement, forget the J-FET stuff and just use a simple op-amp circuit. You can do it with one op-amp and two resistors. What is the input current range? I can't find your schematic again!!!! Give me you input and output requirement, I'll come up with something for you. What you given is not enough for design, just enough to tell me this should be a piece of cake!!!