OP-Amp circuit problem

[jazzalex]

Hello all,

I have a simple OP-Amp circuit, which is supposed to work as a bandpass amplifier. I designed and simulated it with qucs. The results are attached to this post (circuit and simulation).

For some reason, however, it does not work properly as shown in the third attachment: I used a sine as the input waveform (yellow) and the output (green) shows significant distortions of high frequencies.

Does anyone know how to debug this problem ?

Thanks in advance

Alex

[fcb]

20R as feedback resistor!
50R load for an '071!

Try increasing your resistors by of at least x10 (probably better at x100).

[kg4arn]

fwiw. The design is unstable (oscilatting) due to the large capacitance at the neg op amp input.
Try a different architecture like something from "op amps for everyone" which is free to download.

[dannyf]

Does anyone know how to debug this problem ?

The ac gain increases with frequency and is only modulated by the device's own gpbw.

[jazzalex]

Thanks for your help ! I will try to redesign the circuit and will definitely read through the OP-Amp book you mentioned above !

[jazzalex]

Update: I changed the 20 R resistor to 2000 R and reduced the capacity from 220 nF to 1 nF. This results in a stable circuit with almost the same S-parameter-characteristic.

Thanks again

Alex