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Installing and using LTspice IV (now including LTXVII). From beginner to advanced.
Installing and using LTspice IV (now including LTXVII). From beginner to advanced.
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Old 25th July 2021, 05:10 PM   #2971
jan.didden is offline jan.didden  Europe
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There is no 'right' or 'realistic' analysis level in .ac, it's a gain and phase versus frequency, you cannot draw any conclusions from it with regard to large-signal behavior.
You can set the level to 1kV, it won't change the graph one bit.

So 150mV for 35V output is a gain of 233, which very nicely matches the approximately 233V with 1V input in your graph ;-)

Jan
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Old 25th July 2021, 05:17 PM   #2972
madis64 is offline madis64  Estonia
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Thanks for the explanation.

The important thing is that the sim results are either reliable or not. If strange voltages are shown to you and you do not know why it happens then the level of trust may have potential to reduce
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Old 25th July 2021, 05:26 PM   #2973
jan.didden is offline jan.didden  Europe
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Yes, that is crucial. OTOH, you also need to be aware of the limitations. The important thing about a .ac run is that it is strictly small signal, each device linearized around the DC bias point, so it cannot be used to test for any nonlinearity or distortion.

For that you need a .tran which is a large signal simulation using the actual signal levels you specify and exercising all non-linearities of the devices, insofar as they are part of the models.

For instance, it is not uncommon for say an opamp model not to simulate the current drawn from the V+ and V- pins, or not to simulate temperature dependence of the input DC offset. Know your model!

Jan
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Old 25th July 2021, 06:20 PM   #2974
Ray Waters is offline Ray Waters  United States
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@ madis64

It's more common in an AC plot to set the Y-axis to display in decibels instead of volts. Right-click on the vertical axis and this will bring up a dialog box where you can set the Y-axis units. Choose "Decibel" instead of "Linear" or "Logarithmic." The value will still reflect the gain of the stage referenced to whatever AC signal level is applied to the input, but in decibels instead of volts.

Of course, there is no right or wrong way to display the results. LTspice gives you the choice.
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Old 25th July 2021, 06:37 PM   #2975
Mooly is offline Mooly  United Kingdom
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Installing and using LTspice IV (now including LTXVII). From beginner to advanced.
The gain of the A80 (I used to own one) is high using as it does a 110k and 470 ohm the feedback network.

If you set the Analysis sim to show voltage and set the 'Small Signal AC Analysis' amplitude input to 0.017v then you should see 4v just as on the Transient run of the sim. The other setting of 17mv doesn't now apply for the AC analysis run. You can set the input to a million volts and it will still give the correct figure of 4 volts output as it uses the other entry for this function.
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Old 25th July 2021, 06:38 PM   #2976
madis64 is offline madis64  Estonia
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Quote:
Originally Posted by Ray Waters View Post
It's more common in an AC plot to set the Y-axis to display in decibels instead of volts.
I know

The volts just came into the play when we (in one local forum discussion) started to wonder about very high (abnormal) voltages in the simulation results.
And since I have been doing these simulations only for a relatively short time then I just swapped the Y-axis scale to volts to see actually what was presented.

Thanks for the advice anyway.
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Old 25th July 2021, 06:41 PM   #2977
madis64 is offline madis64  Estonia
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Mooly, also thanks.

While we are "at it" - what is the preferred way of getting tansient and ac analysis results simultaneously displaid? Open two copies (separate files) of same model and run one simulation from each of them? Or is there a way to get results from both simulations when using only one opened model?
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Old 25th July 2021, 07:01 PM   #2978
Mooly is offline Mooly  United Kingdom
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Installing and using LTspice IV (now including LTXVII). From beginner to advanced.
I don't think I've ever seen that done tbh. There doesn't seem to be any way of keeping one result open and on screen and then changing the sim editor to do something else and running that.

It would be a new one on me
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Old 25th July 2021, 07:06 PM   #2979
madis64 is offline madis64  Estonia
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There are plot panes in the waveform window available, why not add functionality to display results from different analysis of the same model in different panes?
It would guarantee that - when using all kinds of .step parameters - sims would be run with the same set of values.
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Old 25th July 2021, 07:16 PM   #2980
Mooly is offline Mooly  United Kingdom
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Installing and using LTspice IV (now including LTXVII). From beginner to advanced.
There is a bit about Plot Panes in the help files but it looks like they work with whatever sim function is selected rather than different actual functions.

You'll have to experiment but I've never seen it done.
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