Calculating Current Gain of Op Amp for Converting Picoamps to Volts?

[d.sonali20]

current gain of an op amp?

Homework Statement

need to convert current(output of a detector) in the range of pico amps to voltage between 1-5V

Homework Equations

(to connect to an adc)

The Attempt at a Solution

I've connected an op amp(for current amplification) to a resistor(to convert the amplified current into voltage)and grounded it and a friend suggested using TL071 op amp.but i don't know much about op amps.
what i want to know is for a certain input current and for a certain op amp how can i calculate the output current?
only by knowing that i can calculate the value of the resistor to be used
the data sheet gives voltage gain but not current gain.
please help me out

 

[berkeman]

Homework Statement

need to convert current(output of a detector) in the range of pico amps to voltage between 1-5V

Homework Equations

(to connect to an adc)

The Attempt at a Solution

I've connected an op amp(for current amplification) to a resistor(to convert the amplified current into voltage)and grounded it and a friend suggested using TL071 op amp.but i don't know much about op amps.
what i want to know is for a certain input current and for a certain op amp how can i calculate the output current?
only by knowing that i can calculate the value of the resistor to be used
the data sheet gives voltage gain but not current gain.
please help me out

Welcome to the PF.

Amplifying picoAmp signals is very non-trivial, unless you can assume an ideal opamp (with no input bias current and no input offset currents or voltages). If you want to use a real opamp, you will need to find one with very little input bias current. Have a look at the TL071 opamp's spec for input bias current and input offset current and voltage, and see if you can find a better candidate.

Still, even with a very good opamp, it is very hard to amplify currents in the picoamps. Leakage currents alone on the PCB will be on that same order, unless you do some special things to the PCB (both in layout and in coatings). And there is also the issue of bandwidth -- what is the bandwidth of the input current?

Can you say more about the problem statement? Can you assume an ideal opamp? If so, look up "current to voltage converter opamp" with Google Images (leave out the quotes) to see the typical circuit that is used. I'd also recommend using several stages of gain -- it is not practical to go from picoAmps to Volts in a single gain stage. After the first current-to-voltage converter stage, the subsequent gain stages can just be voltage amplifiers.

 

[jegues]

Homework Statement

need to convert current(output of a detector) in the range of pico amps to voltage between 1-5V

Homework Equations

(to connect to an adc)

The Attempt at a Solution

I've connected an op amp(for current amplification) to a resistor(to convert the amplified current into voltage)and grounded it and a friend suggested using TL071 op amp.but i don't know much about op amps.
what i want to know is for a certain input current and for a certain op amp how can i calculate the output current?
only by knowing that i can calculate the value of the resistor to be used
the data sheet gives voltage gain but not current gain.
please help me out

You didn't provide us with a schematic or a sketch so it's not very clear what you're talking about, is this essentially what you have?

If so, it's going to take an impractically large resistance value to convert a mere few pico amps to suitable range of say 1-5V.

 

[d.sonali20]

@jegues
its pretty much the same...but the output is not grounded

 

[d.sonali20]

@berkeman
multiple gain stages is a good idea..
can you suggest a circuit?
and is there no way to find the exact current gain given by an op amp??

 

[berkeman]

@jegues
its pretty much the same...but the output is not grounded

His sketch does not ground the output of the opamp. It represents an opamp output voltage that is referenced to ground (like the load resistance is between the opamp output and ground). That is standard notation.

 

[berkeman]

@berkeman
multiple gain stages is a good idea..
can you suggest a circuit?
and is there no way to find the exact current gain given by an op amp??

jegues showed you the first gain stage already -- that is what you would have seen if you would have used my Google Images suggestion.

And you have not answered any of my questions from my initial reply. Your circuit will never work unless you address my initial questions.

 

[d.sonali20]

SORRY for this late reply...
@berkeman
oops!that was what i wanted to say...what a blunder!

and about your initial questions...the op amp cannot be assumed to be ideal because i have to build this circuit.
and about the bandwidth of input current...
i searched for it but couldn't find it...i didn't know till you told me that current can have a bandwidth.can you please explain how current, which is nothing but a stream of electrons,can have a bandwidth?