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I'm attempting to draw the following two circuits up in NI Multisim 10, to attempt simulation and confirm my calculations.

However, I'm not sure how to represent the Vin and Vout sources. Also, multisim only seems to have amplifiers with power supplies, but all theory I read seems to draw the amplifiers without. Does that matter for a eventual simulation?

Also, ff there's any better software for the purpose than Multisim, then please tell.



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up vote 3 down vote accepted

Multisim does have ideal opamps, they can be found in the master database under Analog>Analog_Virtual_OPAMP_3T_VIRTUAL, but as Kortuk said, you should strongly consider using an opamp that forces you to include a power supply.

If you have a particular opamp that you know you will be using in a lab, you should see if you can find it in the multisim database. Multisim has very good models for how the opamps will actually act even more then just how you are powering them.

As for your question about Vin and Vout, in Multisim you can go to Simulate>Instruments and select what ever instrument you want to place. So you could put a function generator in for Vin and a Oscope for Vout.

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I'm pretty sure it's quite common to omit an op-amps power connections when doing a circuit schematic, it's just for clarity I think - it's presumed you know they're supposed to be there, it keeps diagram complexity down and makes other connections easier to visualise.

I'm not familiar with Multisim, but I'd imagine - If the option to connect the power rails is there, you should probably hook them up, in theory the op-amps will not function correctly if they don't have the power hooked up, but this may not be the case in the sim-world.

You can always test both - if the simulation still works without the power connection - it may be that it assumes the nominal power supply is connected. The connection may be more applicable for testing power supplies outside the nominal range.

Also - The de-facto circuit simulator that most people seem to use is SPICE, as it's quite common, it has a good community base and there's loads of help and tutorials available for it.

Just my thoughts on the matter, some of it may be helpful :)

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TINA-TI is a free SPICE that works pretty well. focus.ti.com/docs/toolsw/folders/print/tina-ti.html – endolith Oct 2 '10 at 15:41

Your issue is which op amp you are using from their library. You are using an ideal op-amp. Almost always students in class will show me a simulation where they use an ideal op amp and it does not work, or it works too well.

Try selecting a different op amp from the master database that has connections for power. I just opened my library and I have many many options. You need to determine which one fits your applications. They will have power connections, non-infinite gain, not be able to completely hit the rail, so forth so on.

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As a side note, multisim is more than able to let you do this kind of simulation. – Kortuk Oct 2 '10 at 17:23

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