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Oddity in Behavioural Voltage Source

 

Hi all,

I'm toying with an OpAmp design that ultimately shall amplify and clip a sinusoidal voltage (100Vp, 100kHz, 10x Amplification), nothing special except the slew rates are quite high. Now, to verify the functionality that comes _after_ the OpAmp, I've used a behavioural voltage source that I would expect performs exactly the function I want: limit(10 * V(Vin), 0, 10).

However, when looking at the simulation, I see that the behavioural voltage source starts to rise _before_ V(in) crosses zero. I would post an image, but cannot.?

What causes this behaviour? Can it be fixed??
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My Version of LTSpice is 17.1.5 for MacOSX.?
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Thanks a lot for your input and cheers,
Joerg
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Here is the a
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Rather than trying to post an image, upload your .ASC and all other files required to run it. to Files -> Temp and then tell us you did that.

On 2025-05-14 09:57, joerg via groups.io wrote:
Hi all,

I'm toying with an OpAmp design that ultimately shall amplify and clip a sinusoidal voltage (100Vp, 100kHz, 10x Amplification), nothing special except the slew rates are quite high. Now, to verify the functionality that comes _after_ the OpAmp, I've used a behavioural voltage source that I would expect performs exactly the function I want: limit(10 * V(Vin), 0, 10).

However, when looking at the simulation, I see that the behavioural voltage source starts to rise _before_ V(in) crosses zero. I would post an image, but cannot.?

What causes this behaviour? Can it be fixed??
?
My Version of LTSpice is 17.1.5 for MacOSX.?
?
Thanks a lot for your input and cheers,
Joerg
?
Here is the a
?
?
?
?
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

Virus-free.
 

P.S. here are the plots /g/LTspice/album?id=302449
 

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On 14/05/2025 10:57, joerg via groups.io wrote:
I'm toying with an OpAmp design that ultimately shall amplify and clip a sinusoidal voltage (100Vp, 100kHz, 10x Amplification), nothing special except the slew rates are quite high. Now, to verify the functionality that comes _after_ the OpAmp, I've used a behavioural voltage source that I would expect performs exactly the function I want: limit(10 * V(Vin), 0, 10).

However, when looking at the simulation, I see that the behavioural voltage source starts to rise _before_ V(in) crosses zero. I would post an image, but cannot.?

What causes this behaviour? Can it be fixed??
?
My Version of LTSpice is 17.1.5 for MacOSX.?
?
Thanks a lot for your input and cheers,
It looks to me like your issue is a limited number of data points and/or plot compression. You don't seem to have set a maximum time step, so LTspice is guessing when data points should be. Change your .TRAN directive to something like:

.TRAN 0 50u 0 1n

--
Regards,
Tony
 

Thanks John, and sorry for the inconvenience.
Cheers,
Joerg
?
 

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It all seems to work as expected. But you first divide the input signal voltage by 100, then multiply by 21, then multiply by 51, which seems an odd thing to do . The output is a 20 V peak-to-peak square wave.

On 2025-05-14 10:38, joerg via groups.io wrote:
Thanks John, and sorry for the inconvenience.
Cheers,
Joerg
?
--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

Virus-free.
 

Thanks Tony,
that does the trick!

But I'm still confused about the inner workings of LTSpice then: If I define a voltage source to behave like a function of another voltage, shouldn't the timing of the points align?

In other words, if V(D1) is known at time t, why is LTSpice calculating V(G1) = limit(10 V(D1), 0, 10) at some other time? In the plot, one can clearly see V(D1) being negative and V(G2) already being positive, and that should never happen under this function.
(/g/LTspice/photo/302449/3916346?p=Created%2C%2C%2C20%2C2%2C0%2C0)

Thanks for any insights!

Cheers,
Joerg
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