offset with a variable gain amplifier

[tibs]

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]

 

[Humungus]

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?

 

[tibs]

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.