Hello,
I am currently getting familiar with Qspice. My first impression is that it is a bit clunky compared to LTSpice. I am attempting to make use of an operational amplifier. During my google search, it was recommend to use the rail to rail op-amp. Well, I did incorporate an RROp-amp but for some reason, it is not working as expected. I have the same circuit that is working fine with LTSpice but not with QSpice.
Note that the RROp-amp model has no pins for either Vcc or gnd. This is a bit odd.
Can someone please advise how to configure it such that it works as expected.
Thank you.
Qspice RRopAmp is a Ã-device, which is build-in behavorial device to describe Rail-to-Rail opamp.
It contains at least 5 pins in Qspice default symbol, including Vdd and Vss pin.
It is unclear what you are currently doing. Are you refer to using opamp in LTspice?
opamp is ideal opamp model with description of AVOL and GBW, it subcircuit looks like this
.subckt opamp 1 2 3
G1 0 3 2 1 {Aol}
R3 3 0 1.
C3 3 0 {Aol/GBW/6.28318530717959}
.ends opamp
If you are looking for ideal opamp model, I have different level of ideal opamp model in my custom library (https://github.com/KSKelvin-Github/Qspice/tree/main/Symbols-KSKelvin/devices/opamps)
Here is a comparison of using RRopAmp, Level 0 and Level 1 ideal model for your reference. This screenshot with input signal at 100kHz.
opamp.qsch (13.5 KB)
You will aware their difference if increasing signal frequency. Here is 2MHz.
As you only provide very general description, if it cannot address your problem, consider to give a simple example of what you are currently working on and with screenshot or circuit upload.
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If you need more detail information of RRopAmp, I have an unofficial guideline in my Github. This is direct download link
Qspice/Guideline/Qspice - Device Reference Guide by KSKelvin.pdf · KSKelvin-Github/Qspice
Discussion of Ã-device is currently from p.287, and RRopAmp from p.306. (it subject to change if document update)
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Hello,
thank you for replying to my post. When I performed a search for an op-amp, this is the one that comes up. Notice that there are no Vcc or Vss pins on either side.
The opamp is the main difference between the circuit that I tested in LTSpice and the one that I am testing using QSpice. In LTSpice I used the UniversalOpamp2 and my simulation works fine there.
From your screenshot, the opamps appear to be from your own library. A novice like me would not have their own personal library of op-amps to choose from. Does Qspice require advanced training to use? Or semi expert level training to first create your personal library prior to using QSpice? I don’t see QSpice having an extended built-in library of op-amps like LTSPice to choose from.
You just pull a RRopAmp from Symbol & IP Browser and the symbol without Vdd and Vss? Right click your symbol > Show Symbol Properties, and check if anything is filled into the net. Qspice will hide the pin if you fill a net name for a pin directly. This is the only reason I know you can mask a pin.
Have to expect all simulation software requires a learning curve.
LTspice belongs to AD and they includes library for AD devices, that why many devices there.
Qspice belongs to Qorvo… I guess after Qorvo merge with Skyworks, you may see more devices there. But SPICE is a general simulation, you can do simulation without MFG device model, or by import MFG model.
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Thank you for providing this reference.
I will review it.
Much appreciated!
From the screenshot, how would one know which one is the “+” and the “-” pins since there is no identifier on the symbol? Why were the symbols vague? When I click on any of the small dots, I don’t see the corresponding number highlighted to cross reference with the pinout.
From your screenshot, do you want to generate a PWM signal by comparing a signal with a sawtooth waveform? In general, you don’t do this comparison with an op-amp, but rather with a comparator.
If your schematic is not confidential, consider uploading it for review. You may learn faster this way. However, uploading files requires a basic user trust level, and here is how you can earn it.
Qspice Forum - New User to Basic User (File Upload) - QSPICE - Qorvo Tech Forum
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Ok, here is a screenshot. I tried uploading the actual QSpice schematic file but I got a message that because I am a new user, I am not allowed to upload a file (apparently they allow you to upload .jpeg/.png files).
Here is how to identify the pin or mask a pin if you force a netname.
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Appears that the creators of this new version did not put much effort into making the user interface more friendly. Maybe their focus was primarily only on creating a faster engine and adding more complex functionality. Sorry, just venting. Was a little bit frustrating.
Well, apparently, the power lines were the missing piece to the puzzle. It appears to now be working.
You have been a lot of help with getting this to work. I greatly appreciate your time.
Thank you.
It seems like people from LTspice are always complaining about Qspice, but I also came from LTspice. Both LTspice and Qspice were created by Mike Engelhardt, but he redid what he didn’t like in the old design. People who are used to LTspice just don’t like any change.
It is interesting that even LTspice only has very basic devices if you don’t count manufacturer models. Building up a custom library is really more of a community effort.
You can check out my custom library, as my focus is to make Qspice easier to use by providing an intuitive library.
Qspice/Symbols-KSKelvin/README.md at main · KSKelvin-Github/Qspice
Well, this is a working version. Regardless of loop stability, replacing the op-amp with a comparator allows the simulation to run now. All devices here are native Qspice components.
It is easier to support if you increase your trust level in this forum and upload schematic, definitely can save a lot of time for the review.
In actual circuit design, you also use a comparator instead of an op-amp in this location.
buck-feedback.qsch (16.3 KB)
The loop is stable LOL. I tested the circuit in both LTSpice and Simplis. No issues observed there.
In the following screenshot, the one on the left is from LTSpice (4.5 A step) and the one on the right is from Simplis (7.3 A step):
Btw, I did a search for current pulse (I want to perform a current step) in the guide that you provided, I only found “pulse” as it relates to a voltage source. I don’t see a current source on the schematic that you provided. How did you perform the load step test?
Fyi, the caps should be 110 uF with 8 mOhm ESR.
Oh, I just added a TTOL to the comparator to reduce the timestep in the simulation at PWM toggling. The timestep was too aggressive in the previous setting.
buck-feedback-TTOL.qsch (16.3 KB)
Ok, I will try the Schmitt Trigger type op-amp. In the meantime, can you please provide details how you performed the load step.
Regarding building custom libraries, I still have a way to go with first familiarizing myself with the basic functionalities of QSpice before creating any that are worthwhile on my own. 
Just a standard current source with pulse can do the job. There is no different of pulse command for voltage or current source, that why I don’t mention it in detail for current source in my guideline.
You can add instance parameter Vsat, this allows current source not to source power. In this schematic, with or without Vsat is not an issue. Just to demonstrate for current source to act as an electronic load, it would be better to add Vsat.
buck-feedback-pulseload.qsch (16.9 KB)
How did you add the “Vsat=0.1”. What exactly does it do? What is it? What is its purpose? For the spice directive beginning with “.model”, if I press CTRL + Enter, I go to the following line. Not so with the line beginning with “pulse”.
I configured the load for a pulse without the Vsat line. The voltage jumps all the way to ~21 V then decays to 1.8 V.
This is strange to say the least.
From your Qspice screenshot, you should know what an instance parameter is. Adding Vsat is done in the exact same way as adding IC. Did you create this schematic yourself?
Vsat is explained in my Device guideline, in Current source section. Currently is p.114.
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Yes, I created the schematic. I have never used the parameter Vsat. Did not have to use it for either of my other two simulations (LTSpice/Simplis), so not sure why there is this requirement in QSpice.
If you have access to the designers of QSpice, please let them know that the orientations currently available are a bit limited. The ones from Simples are ideal. There they have Mirror and Flip. These options appear to work very well. In the case of QSpice, if you use, for example, Rotate -90, you’re barred from reusing it. This makes no sense.
Ctrl-R is rotate, Ctrl-M is mirror.
