Adjusting Differential Amp's Gain by Using MOSFETs

[cengizsamanci]

Hello

I would like to select gain of differential amp's gain by a controller. I used N channel Mosfets and you can see on attached sch. each of them is working individually. But when I connect them together, the output singal becomes sine wawe. Op amp is TL082. Power supply of opamp is +-18V and I apply -18 and +18V to gates of mosfets.

Could you please tell me if there would be another mosfet connection to select gain?

thank you

 

[FvM]

To analyze circuit stability, you need to know the MOSFET capacitances.

I assume a TL082 in inverting amplifier configuration isn't stable with MOhm range feedback resistors due to the input capcitance generated pole.

 

[cengizsamanci]

but each of feedback circuits works properly when they are not connectoed parallel to eachother by mosfets. But the connection of mosfet's are correct? In addition this sytem will generate negative output as well. So current direction will change. Does Mosfet let current flow in both direction? Maybe this is the problem

 

[FvM]

Does Mosfet let current flow in both direction?

MOSFETs basically do. The schematic suggests that you are using MOSFETs with separate substrate terminal. If they are biased correctly, they can work as bidirectional switch.

I fear the discussion becomes meaningless without detail information about the MOSFETs.

 

[cengizsamanci]

The mosfet which I use is
IRLML2502. So the problem could be capasitance?

 

[Borber]

What about DG1412 Ic instead simple MOSFETs.

 

[cengizsamanci]

That's going to be expensive solution..

 

[crutschow]

If you need a differential amp with adjustable gain then you could use an instrumentation amp which controllable gain such as this or this.

 

[FvM]

IRLML2502 is absolutely unsuitable for the application. It has a huge output capacitance (100 - 200 pF) and a substrate diode shorting reverse voltage above 0.5V.