+ Post New Thread
Results 1 to 8 of 8

18th November 2019, 04:12 #1
 Join Date
 Apr 2019
 Location
 India
 Posts
 3
 Helped
 0 / 0
 Points
 131
 Level
 1
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.

Advertisement

18th November 2019, 21:13 #2
 Join Date
 Feb 2012
 Location
 L.A. USA Zulu 8
 Posts
 3,691
 Helped
 878 / 878
 Points
 19,190
 Level
 33
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
Curmudgeon Elektroniker
1 members found this post helpful.

18th November 2019, 21:50 #3
 Join Date
 Apr 2019
 Location
 India
 Posts
 3
 Helped
 0 / 0
 Points
 131
 Level
 1
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.

Advertisement

19th November 2019, 07:48 #4
 Join Date
 Feb 2012
 Location
 L.A. USA Zulu 8
 Posts
 3,691
 Helped
 878 / 878
 Points
 19,190
 Level
 33
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
Curmudgeon Elektroniker

19th November 2019, 07:55 #5
 Join Date
 Apr 2019
 Location
 India
 Posts
 3
 Helped
 0 / 0
 Points
 131
 Level
 1
Re: Determining output impedance in LT SPICE
Sorry for the lack of info.

19th November 2019, 09:06 #6
 Join Date
 Nov 2001
 Location
 Turkey
 Posts
 4,514
 Helped
 1359 / 1359
 Points
 30,764
 Level
 42
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..

Advertisement

19th November 2019, 18:51 #7
 Join Date
 Nov 2013
 Posts
 734
 Helped
 199 / 199
 Points
 4,973
 Level
 16
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
 Join Date
 Jan 2008
 Location
 Bochum, Germany
 Posts
 45,804
 Helped
 13925 / 13925
 Points
 262,035
 Level
 100
+ Post New Thread
Please login