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Op amp, I want to use LM358
- Thread starter monika.duccoli
- Start date
I don't understand if it is correct or it's wrong
It's unclear what you are doing there. The outputs seem to go down and stop. That's doesn't make sense.
It would probably be much better if you just tried a different simulator like LT Spice.
I'm using LTspice, but I've found that to be a limitation of the opamp
I have another question, I'm making the circuit but I get the output signal
It looked different than LT Spice that's why I mentioned it, but it looks like you are working on Apple so the appearance will be a little different.
Try going through it stage by stage and see if you can spot which stage is causing the problem. That's a basic troubleshooting technique.
Hello again,
Here is the spice model I tried in the LT Spice simulator. It seems to work ok even with a pulse like you were testing earlier.
If you want to try it you can put the file under the "Lib" sub-directory of the simulator directory.
I named the file simply LM358.mod so you can rename the file after you download it.
Here is the spice model I tried in the LT Spice simulator. It seems to work ok even with a pulse like you were testing earlier.
If you want to try it you can put the file under the "Lib" sub-directory of the simulator directory.
I named the file simply LM358.mod so you can rename the file after you download it.
Hello again,
Going back to your post #55 with the unusual waveform that was not yet explained, it immediately looks like crossover distortion. The wave distortion itself looks like that with the combined fact that it looks like those intermediate 'steps' are roughly 4us long each. That's the time for one of the internal signals to ramp to the right level to switch between upper and lower output transistors. That's where the crossover distortion comes in. This is always a consideration with the LM358 type op amps.
To get rid of crossover distortion, we have to 'bias' the output so that it forces either the upper or the lower transistor to supply the output for the entire time of operation. This can sometimes be accomplished by simply adding a resistor (like 10k) from the output to ground, which keeps the upper transistor in the linear mode all the time and the lower transistor is never used. It may also be possible to bias from the output to +Vcc which would keep the lower transistor in the linear mode all of the time and with the upper transistor never being used.
The key to this type of bias is that the current through the added resistor has to do all the work of the transistor that is not being used, so it has to be the right value.
Going back to your post #55 with the unusual waveform that was not yet explained, it immediately looks like crossover distortion. The wave distortion itself looks like that with the combined fact that it looks like those intermediate 'steps' are roughly 4us long each. That's the time for one of the internal signals to ramp to the right level to switch between upper and lower output transistors. That's where the crossover distortion comes in. This is always a consideration with the LM358 type op amps.
To get rid of crossover distortion, we have to 'bias' the output so that it forces either the upper or the lower transistor to supply the output for the entire time of operation. This can sometimes be accomplished by simply adding a resistor (like 10k) from the output to ground, which keeps the upper transistor in the linear mode all the time and the lower transistor is never used. It may also be possible to bias from the output to +Vcc which would keep the lower transistor in the linear mode all of the time and with the upper transistor never being used.
The key to this type of bias is that the current through the added resistor has to do all the work of the transistor that is not being used, so it has to be the right value.
