Capacitance measurement for Spice model parameters - Page 1
Forum section: electronics / projects

author: chilternview
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:23:16 pm
content: I'm in the process of setting up hardware and software for spice model parameter extraction, in particular BJTs but in future JFETs and MOSFETs also.For I/V parameters I've an Agilent 4156C with 41501 for extending the range to 1A/200V, for capacitance measurements either a HP4280A or a E4980A with DC bias option.In order to measure the (small) capacitances though, I'm wondering what sort of test fixture to use. AliExpress sell the blank boxes for standard capacitors, and Fischer make a 4 way TO-5 transistor socket in ptfe which would suit through hole devices. Has anyone else tried measuring transistor C/V parameters? In particular Cbe / Cbc or mos Cgs/Cgd measurements?The more difficult parameters to measure are things like tf / tr (forward and reverse transit times) or noise, which I've not really looked into at all yet.

author: jwet
author_rank: Frequent Contributor
timestamp: October 12, 2025, 04:07:39 pm
content: HP makes really good fixtures for all their analyzers. They are designed to be mode free up to high frequencies as specified and generally use high quality materials.Now, I'm not suggesting you go out to Agilent dealer and shell out $5K for these things new (the prices are shocking). I would look for them used and or copy some of their high points. I've bought some where the plastic disintegrated over time and I 3d printed replacements. 3d printing is good for stuff like this by the way.I use a 16092A Spring Clip Test fixture on top of an impedance test set on my 4195 and 4396 Network/Impedance Analyzers. They're very flexible, one lives on my 4195. These will come up on ebay for reasonable money and I can use them for thru hole or SMT stuff with good repeatability.I got on a kick with S parameters for a while and wanted to characterize transistors. The best solution that I evolved to was to make little PC boards with SMA's and a site for the DUT, I would just solder them down to pads. I got 50 boards made in China for nothing and carved them up, make some calibration/de embedding fixtrures, etc. That I could use beforehand.It all depends on your frequency needs- most of the stuff that I did was around 1 Ghz and below. Its somewhat forgiving but gets less so higher.NB- i went on Ebay for a browse and there are a lot of fixtures for good prices. I just searched for "HP Test Fixtures". By the way, some analyzers do equivalent circut analysis and can figure out a bunch of measurements and fit it to a model. I've used the crystal model on the 4195 before- saves a lot of spreadsheeting.Good topic. If you come up with something clever- share.

author: chilternview
author_rank: Frequent Contributor
timestamp: October 12, 2025, 06:02:30 pm
content: Thanks, I'm mainly interested in low frequency capacitance measurement for the Spice model cje / vje / mje etc which model the b-e and b-c junction capacitances. I do actually have a 4936B and 16092A for impedance measurements but the low freq modelling is likely to be easier too.The main issue I see is the parasitic capacitance of the test fixture. Getreu's book has an iterative way of curve fitting to obtain the model parameters in the presence of parasitics but if they can be avoided by just open/short correction that would be a lot simpler.

author: mawyatt
author_rank: Super Contributor
timestamp: October 12, 2025, 06:44:17 pm
content: These links might be helpful.https://www.eevblog.com/forum/projects/bias-network-for-lcr-meter/https://www.eevblog.com/forum/testgear/cheap-smd-lcr-fixture-the-good-bad-and-ugly!/https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/Best