ac dc circuit (no ac signal) and abnormal amplifier (huge output voltage)

[bluefish]

Hi everyone,

I have built an ac sine waveform to rectangle waveform circuit, which the Multisim file is attached below. AC power supply (120 Vrmp and 60Hz) is connected with this circuit. A rectangle waveform (Vpeak: 12v and -12v; 60Hz) is generated. In practice, I have obtained a rectangle waveform from this circuit.

When I simulate this circuit on Multisim, I meet two problems.

I used Multisim oscilloscope to watch V (1) (oscilloscope terminal: +: V (1), -: V (5)). A sine waveform can be seem at V (1) point.  When the sine signal passed through LM833N amplifier, the result becomes a kilo voltage (oscilloscope terminal: +: V (11), -: V (0); VDC ground).

First question, how can I make this amplifier working normally?

 The DC power supplies of this amplifier are only +12V and -12V. Therefore, the output voltage should be between 12V and -12V.

Second question, do I miss any setting? Why can I not obtain the same rectangle signal from Multisim simulation?

Thank you for your help,

[NatashaB]

Hi Bluefish,

The model you have chosen to use in your design is actually a simplified model that does not model the power supply rails; in some cases, designers like to use these models in complex designs to increase simulation speed.  As a result of this though, you will not be able to use the model as a comparator because the signal will not be clipped by the power supplies and you will achieve very high and unrealistic gains!  There is, however, another LM833 model you can use -- the LM833_2.  The power supplies are modeled in this model, so you will see the clipping at +/- 12 V.

To change the model, right-click on each opamp and select Replace Components.  You will notice that under the LM833N there are several models to choose from.  Choose the LM833_2 model and click OK.  FYI: You can click on View Model to see how many terminals are modeled in the .subckt statement of each model.

That should give you a good starting point, keep us posted on your design!

Message Edited by NatashaB on 03-23-2009 12:55 PM