Transfer Function Analysis (.tf spice directive)

Hello everyone, I am new to QSPICE and it’s amazing!

I’d like to know if there is a way to get in the output file the transfer function analysis results (those given by the .tf directive on the schematic).

I find it very useful for equivalent resistances seen by capacitors, calculate loop gain, input/output resistance (and how they are modified by feedback) and so on.
It would be nice to have it in QSPICE, but I can get anything in the output.

Thanks in advance for helping me and best regards.

I cannot find .tf directive in Qspice, but possibly you should achieve get what you want with either .op or .ac directive.

Here is a simple circuit as an example. For dc analysis, if you want to get V(out)/V(in) ratio, in theory, C1 becomes open circuit and V(out)/V(in) = 0.5. Therefore, you can run a dc operation point (.op) and setup a .meas to calculate V(out)/V(in). The result is given in Output Window.

For ac analysis, in this example, V1 is set to AC 1, which means V1 is a 1V ac voltage source. .ac directive sweep frequency from 1Hz to 10kHz and plot V(out) (this plot means V(out)/V1)
This allow you to get frequency response plot.

** I use a same schematic file and mask directive to switch between dc (.op) and ac (.ac) analysis in this example.

hope this give you some hints to start with.

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Wow, thank you for the detailed response.

I come from Orcad Pspice, and I’m used to the GUI for measurements. I’m just starting now with Spice directives written on the schematic. I didn’t know about the .meas directive (which, by the way, is well-documented on the web). I started playing with it, and it’s powerful!
It’s nice that QSPICE guides you in its usage as well.

I’ve also tried finding equivalent impedances seen by capacitors using this method, and it works. The .tf directive now seems much less essential to me.

Thank you very much for helping me.

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