Op-amp help (adding two waveforms)

Thread Starter

Clive80

Joined Dec 23, 2011
9
Sorry for the mass posting tonight but I’m stuck on a problem and wondered if anyone on here could help.
I’m trying to add a positive and negative offset to a triangle wave by adding it to a square wave through a 741 op-amp, the problem is the offset is not constant and the triangle waveform slowly drops. See attached jpeg.
Am I doing something wrong of is there a better way of doing this rather than using a summing op-amp?
Also please see my other post in the projects forum regarding control of a H-Bridge to achieve an AC PWM or modified sine wave output.
Thanks
 

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Ron H

Joined Apr 14, 2005
7,063
If you are probing with a 10X probe, it looks like the probe compensation is screwed up.

EDIT: AC coupling could also do it, but I don't think you would see that much droop at 5ms/div.

EDIT: I thought your scope picture was of an actual circuit. Apparently it is a simulation?

EDIT: Your schematic shows that both inputs are coming from the triangle outputs of the generators.
 

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Last edited:

Thread Starter

Clive80

Joined Dec 23, 2011
9
Thanks for your replies.

Ron H

The signal generators are set up correctly, the leads are connected to the positive terminal.
I can measure both waveforms independantly with the scope before the op-amp.

The circuit is a simulation in multisim (pspice).

Any further thoughts world be welcome.

Thanks
 

Ron H

Joined Apr 14, 2005
7,063
Thanks for your replies.

Ron H

The signal generators are set up correctly, the leads are connected to the positive terminal.
I can measure both waveforms independantly with the scope before the op-amp.

The circuit is a simulation in multisim (pspice).

Any further thoughts world be welcome.

Thanks
OK, I was interpreting the function generator icon incorrectly.
Are you overdriving the op amp? Its output is limited to less than ±20V by the power supply rails.
 

Papabravo

Joined Feb 24, 2006
21,160
The frequency response of a 741 is worse than whale turds at low tide. It is pretty much done by the the time you get to 9 kHz. or so.
 
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