Why is there an offset on my oscillator?

[george2525]

Hello,

Im hoping the experienced of you can help here as im still learning

I have made an oscillator that goes from about 0.1 to 23 Hz

it works pretty well but theres an imbalance of voltage swing on the output. like there's a negative DC offset happening but I cant work out why.

Im using TL084's because thats all I had. They are not rail to rail but I assumed they would at least be the same magnitude in either direction?

Anyway heres the circuit and the real output. Any pointers would be great. Thankyou!

 

[Alec_t]

The input signal of a TL084 can't be as close to the negative rail as it can to the positive rail, so there will be asymmetry in the output waveforms if the opamp inputs are driven close to the rails.

 

[george2525]

The input signal of a TL084 can't be as close to the negative rail as it can to the positive rail, so there will be asymmetry in the output waveforms if the opamp inputs are driven close to the rails.

Ok so should I be looking for rail to rail input op amps?

 

[Alec_t]

Try increasing R3, or reducing R1, to reduce the amplitude of the waveforms.

 

[george2525]

Try increasing R3, or reducing R1, to reduce the amplitude of the waveforms.

Thanks but that only affects the triangle and sine. the square will still be the same

 

[Alec_t]

You can offset/reduce the opamp output levels by adding resistors a,b,c like this :-

 

[george2525]

Thankyou

I just tried it and it works well

im not sure what Ra is for though. if I use it the output is good if I take the output from the op-amp (not from the node as you did)

Or if i remove Ra it seems good taking the output from where you said.

what is the purpose of Ra? to reduce magnitude only right? for some reason the offset remains when I use it

Thanks for the help. very useful idea

 

[davenn]

Thankyou

I just tried it and it works well

good

I assume you have made some drawing errors on your diagram around U2A VCC = +V not -V

 

[Alec_t]

Glad it works for you. The ratio of Rb:Rc affects the offset. Ra (in conjunction with the output impedance of the opamp) primarily enables the amplitude to be tweaked.