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18th November 2019, 04:12 #1
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Determining output impedance in LT SPICE
Hiya everyone,
I am currently using LTSPICE to perform few analog designs, and have a few queries regarding the tool.
* Is there a way to measure the output impedance and gm of a transistor whilst performing DC analysis of a circuit?.
* whilst performing AC analysis, how does one separate the phase and the gain plot?.
* how does one determine the values of transistor parasitics apart from the typically used formula ?.
Many Thanks.
P.S
I have been using cadence, and trying to work with LTSPICE is very finicky.

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18th November 2019, 21:13 #2
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Re: Determining output impedance in LT SPICE
For gm, you can vary Ib and do waveform arithmetic of Ic/Vbe (rightclick on the waveform title).
For output impedance, you can vary the collector voltage and do waveform arithmetic of Vce/Ic for a fixed Ib.
To plot either the phase or the gain, leftclick on either the gain or phase values and select "Don't Plot Gain/Phase" respectively.
What transistor parasitics are you interested in?Zapper
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18th November 2019, 21:50 #3
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Re: Determining output impedance in LT SPICE
Thanks.
I'm sorry, but all of the above mentioned characteristics are for MOSFETs not BJTs.
Gm can be determined by hand calculation, but whilst performing simulations due to the presence of body effect, the overdrive voltage tends to change thereby varying the GM. The above mentioned procedure can be employed for discrete devices and not full fledged circuits, hence is there an alternative to this?.
Same thing with respect to rout, how does one determine early voltage or lambda (as lambda is not present in the model file )in order to determine output impedance.
The parasitics that I would like to determine are CGS,CGD,CDB and CSB. I do not want to use the PDK or model files for this procedure.

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19th November 2019, 07:48 #4
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Re: Determining output impedance in LT SPICE
I'm sorry, but all of the above mentioned characteristics are for MOSFETs not BJTs.
So you'll have to pardon me If I didn't realize you were referring to MOSFETs.Zapper
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19th November 2019, 07:55 #5
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Re: Determining output impedance in LT SPICE
Sorry for the lack of info.

19th November 2019, 09:06 #6
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Re: Determining output impedance in LT SPICE
gm=derivative(Ids/Vgs)(whilst Vds=constant)
Obtain Ids vs Vgs and take the derivative of this curve..

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19th November 2019, 18:51 #7
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Re: Determining output impedance in LT SPICE
LTSpice is freeware, Cadence Spectre is the highend, operating point reading of DC sweep is a luxury comfort I guess. Like AC stability analysis.
But with some struggling you can characterise complex circuits in LTSpice too. Once I saw a matlab script which used LTspice as the core simulator to get swept parameter serults. I can imagine that is the only way to sweep DC operating point analysis in LTSpice, and it is applicable to get some internal smallsignal parameter of transistor, from any kind.
Use a .op spice directive on your schematic to run the analysis then open the View menu/SPICE Error Log to see gm, gmb, Cgs, Cds, gds, and so on. As I know it can show these parameters, not sure, and I don't have the matlab script which can handle multiple runs."Try SCE to AUX." /John Aaron/

19th November 2019, 19:45 #8
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