The gnt library contains the probes that perform test signal injection as well as a large number of examples that illustrate the various gnt analyses and features.

Adding the gnt library to the library list

You can access the library from the following path: <gntpath>/library/OA

Make sure you specify this path in the search path of the Set Library Search Path form or by adding the following line manually to cds.lib (replace with the correct path). Alternatively, you can start Virtuoso and run GNTCreateLibrary() in the CIW.


The probes

The probes perform test signal injection during a gnt analysis. Depending on the desired analysis, insert the correct probe type in the schematic. For NGNT and NEET analysis, insert instances of gntvprobe or gntiprobe, for voltage or current injection, respectively. For 2GFT and 2CT, insert gntprobe, for simultaneous voltage and current injection at the same location. Refer to the following table for probe compatibility:

gntprobe     Y Y
gntvprobe Y Y    
gntiprobe Y Y    

All probes share the following CDF parameter:

  • Direction Hint: shows hints on the symbol to indicate which direction yy-ref and xx-ref need to face for the selected analysis (2GFT, NEET, 2CT).

probe CDF parameter illustration

Tip: hover the mouse pointer over the parameter to show tooltip help.

Symbol: gntprobe


The gntprobe is performs dual voltage and current injection at the same location. It it used for 2GFT and 2CT.

Symbol: gntvprobe


The gntvprobe only performs voltage injection. It can be used in NEET and NGNT.

Symbol: gntiprobe


The gntiprobe only performs current injection. It can be used in NEET and NGNT.


The library contains a number of application examples for each analysis. Where applicable, references to the source and further information is annotated on the schematic.

Extra Element Theorem Most examples are taken from Middlebrook's papers on the 1EET, 2EET and NEET, as well as from Vorperian's book.
General Feedback Theorem Examples from Middlebrook's 2GFT paper, Wiedmann's example from LTSpice, the original GFTManual, audio amplifiers, Vorperian's book, ...
Chain Theorem

Examples from Middlebrook's Structured Analog Design course, ...

Nested analyses and MIMO systems

Multiple-loop feedback circuits: examples from Bode's book, Rob Fox's exampleaudio amplifier from Self, 

The library also contains small test cases to validate various parts of the tool.


Influence on circuit performance

None. Probes are inert during dc and transient analyses and only active when the gnt analysis is performed.

Layout XL compliance

The probes are transparent to the Layout XL connectivity extractor. Terminals xx and yy appear shorted.

Calibre LVS

The CDL netlister netlists the probes as a very small resistor (R) with model SH between nodes xx and yy. Calibre LVS can ignore the probes by adding LVS FILTER R(SH) SHORT SOURCE to the SVRF commands. Other LVS tools have similar rules for filtering out devices.