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offset with a variable gain amplifier

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Hi all,

I want to design a analog variable gain amplifier. So I use a JFET (n channel) used in a variable resistor. (cf SCH.ZIP).

Into the sch.zip, you can find the result from the simulation (wave.jpg). The gate voltage is polarized @4Vdc. The V1 source is a square 100Hz, 100mV PP with no offset (eg +50mV, -50mv).

As you can see, there is an offset on Vout (100Hz, 960mmV PP with 25mV offset (eg +468mV, -493mv).

Does anyone know why there is an offset? I don't understand.

Best regards,
Christophe.[/img]
 

First of all, the circuit is not a varaible gain. Put the fet connected as R6.

Does the input of your OpAmps have an input bias current?
 

Hi Humungus,

On the Picture, the voltage input source is V1, as said above. There is a probe calle V_IN, this is a voltage probe. You can forget it.

A desagree with you, the ciruit as a variable gain wich is a function of V5. The simulation results confirm.

I use a LMC6041 OpAmp fron national. The input bias current is very low (some fA).

I think, I have found why there is an offset. The fet, when used in resistor has not the same properties when it is used is first quadran (U>0 & I>0) and when it is used is third quadran (U<0 & I<0). I have to compensate the non linearities with a feedback resistor.
 

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