# op-amp problem

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#### tarik321

##### New Member
Hi Everyone,
I have an ac current source of the order of 30 nA pp, whenever I use a resistor 500k to convert the current to voltage I get an output signal ( ac voltage ) but when I used a current to voltage converter using an op-amp 741cn and a resistor 500k I don’t get any signal, can u help me figure out this dilemma ?

thanks a lot

Tarik

#### ericgibbs

##### Well-Known Member
Hi Everyone,
I have an ac current source of the order of 30 nA pp, whenever I use a resistor 500k to convert the current to voltage I get an output signal ( ac voltage ) but when I used a current to voltage converter using an op-amp 741cn and a resistor 500k I don’t get any signal, can u help me figure out this dilemma ?

thanks a lot

Tarik

hi,
Whats the frequency of the 30nAppk signal.?

Can you post your 741 circuit drawing.

#### tarik321

##### New Member
the signal frequency is 1kHz

thanks

Tarik

#### ericgibbs

##### Well-Known Member
the signal frequency is 1kHz

thanks

Tarik

hi,

#### Hero999

##### Banned
30nA?

It sounds like you need a FET imput opamp, I doubt the lame old 741 will be good enough.

#### tarik321

##### New Member
Hi,

this is the circuit with 741CN OP-AMP

thanks

Tarik

#### Attachments

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#### BrownOut

##### Banned
What is the signal source? What is the opamp output? Where is your input resistor?

#### audioguru

##### Well-Known Member
The input bias current of a lousy old 741 opamp is typically 80nA but is a max of 500nA at room temperature. Look at its datasheet.

The input bias current of a Fet-input TL071 opamp is typically only 65pA but is a max of 200pA at room temperature, it is low noise and its frequency response is much better so it has more internal gain at any frequency.

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