Opamp strange behaviour?

blitobyte wrote:Hi,
studying opamps I came across an apparently SimulIDE strange behaviour. I was working with an inverter configuration and noticed that if I incremented the feedback resistor (Rf) the gain woulldn't increase as expected (A = - Rf / Ri = -400).

Code:

A =(Aoa*Rf) / ( Rf + Ri – Aoa*Ri )

Code:

A = ( -1000 * 2000000 ) / ( 2000000 + 5000 + 1000 * 5000 ) = -285.51

Defran wrote:Please, In order to staudy the problems, re-edit all your posts attaching your schematics in zip, so We can see the configurations details as well, and so on. Also tell us the version of Simulide and the operative system used. Thanks.

KerimF wrote:

blitobyte wrote:Hi,
studying opamps I came across an apparently SimulIDE strange behaviour. I was working with an inverter configuration and noticed that if I incremented the feedback resistor (Rf) the gain woulldn't increase as expected (A = - Rf / Ri = -400).

As you said, A= minus 400. Try making Vin = -1V. I guess you will get 400V.

On a real circuit, if Vin=1, Vout will be close to zero (close to the opamp ground voltage).
I mean, on your schematic, I am afraid that any negative value at the opamp output is simply a bug to be fixed in SimulIDE.

KerimF wrote:

blitobyte wrote:Hi,
studying opamps I came across an apparently SimulIDE strange behaviour. I was working with an inverter configuration and noticed that if I incremented the feedback resistor (Rf) the gain woulldn't increase as expected (A = - Rf / Ri = -400).

As you said, A= minus 400. Try making Vin = -1V. I guess you will get 400V.

On a real circuit, if Vin=1, Vout will be close to zero (close to the opamp ground voltage).
I mean, on your schematic, I am afraid that any negative value at the opamp output is simply a bug to be fixed in SimulIDE.

vranik wrote:Hi,
the equation A = - Rf / Ri is valid only for amplifiers with infinity gain. If there is finite gain you should use this equation

Code:

A =(Aoa*Rf) / ( Rf + Ri – Aoa*Ri )

where Aoa is gain of operational amplifier - the gain set in properties of operational amplifier. In this case Aoa = -1000.

Code:

A = ( -1000 * 2000000 ) / ( 2000000 + 5000 + 1000 * 5000 ) = -285.51

Therefore there is no mistake in SimulIDE.

Code:

A = - (Aoa*Rf) / ( Rf + Ri – Aoa*Ri )

KerimF wrote:And it seems that the opamp is supplied internally by both a positive and negative voltage having relatively very high values. So, my remark on post #2 is not valid. In this case, the opamp output could be negative and positive as well (I mean, there is no bug in this respect).

I was just confused because in all my designs I try to avoid the necessity to add a negative supply for my opamps. So, I was surprised seeing a negative output which is normal in SimulIDE.

blitobyte wrote:

Defran wrote:Please, In order to staudy the problems, re-edit all your posts attaching your schematics in zip, so We can see the configurations details as well, and so on. Also tell us the version of Simulide and the operative system used. Thanks.

Hi, following your instructions I edited my old post and added a zip, but I cannot see it on the reposted message. Can you see the zip?

KerimF wrote:

blitobyte wrote:

Defran wrote:Please, In order to staudy the problems, re-edit all your posts attaching your schematics in zip, so We can see the configurations details as well, and so on. Also tell us the version of Simulide and the operative system used. Thanks.

Hi, following your instructions I edited my old post and added a zip, but I cannot see it on the reposted message. Can you see the zip?

Perhaps, its size is more than 49 Kbytes.

Alex68 wrote:In all my schemes in the program, op-amps are well simulated in my program.
You just need to prescribe real supply voltages, at least unipolar or bipolar, set the output resistance, gain.
 So everything is fine with the op-amp models!



https://disk.yandex.ru/d/847aUsO0Cvesiw

blitobyte wrote:

Alex68 wrote:In all my schemes in the program, op-amps are well simulated in my program.
You just need to prescribe real supply voltages, at least unipolar or bipolar, set the output resistance, gain.
 So everything is fine with the op-amp models!




On all circuit diagrams with op amps, power sources are always indicated. In order to see them. The program can also be done internally. But better with external pins.
Then there will be fewer questions from developers who do not have this program.


https://disk.yandex.ru/d/847aUsO0Cvesiw

Hi Alex68. A couple of things:

1. Any reason to include the opamp source voltages? The two 12V batteries? For circuit clarity sake? The opamps work without, don't they?
2. What's the triangular 270mV symbol? I caanot find it on my SimulIDE (V1.0.0 R1334)

Alex68 wrote: