S-Parameters to spice? - Page 1
Forum section: electronics / rf-microwave

author: dmowziz
author_rank: Contributor
timestamp: December 12, 2022, 06:00:09 am
content: Hi...I have the s parameter data for a BFP640Modelled an equivalent input circuitI also have the spice model. I biased it at the same with S-parameter but it is not accurate.Please what can I do to get accuracy? Need to do harmonic balance simulationThanks

author: RoV
author_rank: Regular Contributor
timestamp: December 12, 2022, 04:58:16 pm
content: There can be some differences between the two: the S-parameters are measured on a real device (or perhaps measured on some devices and then averaged in some way, I don't know); the Spice model of RF components normally consists in a model of the intrinsic BJT, surrounded by a circuit that models the effect of bond wires and pins. Also the Spice model is fitted to measurements on real devices, but since it has to fit all possible bias cases and also nonlinear operation, it is normally less accurate than S-parameters taken for a specific bias.But what are you exactly doing? You say "modelled an equivalent input circuit". I hope you are not assuming that the input impedance in CE is S11, because this is incorrect, unless the output of the device is "seeing" an impedance of exactly 50 ohm (assuming this is the reference impedance for S-params, that should be for a small device like the BFP640).

author: dmowziz
author_rank: Contributor
timestamp: December 12, 2022, 06:58:11 pm
content: Quote from: RoV on December 12, 2022, 04:58:16 pmThanks!A hybrid-PI input model (with base inductance)... The input plot of this is very close to the input of the S-parameter for transistor.I also applied a small voltage 10mV to both circuits : accurateWhen I bias a BFP640 at the bias point 2V 12mA (output into 50 ohms), it is totally different.So I want to know how maybe I can change the spice file so it matches up or any way to make it accurate.Yeah the input is not S11.. But please even if the output is seeing 50 ohms, the input impedance is still not S11, no?S11 is 20log(T)... input impedance = 50 * (1+T)/ (1-T)

author: dmowziz
author_rank: Contributor
timestamp: December 12, 2022, 07:32:36 pm
content: I was making a mistake...Spice model agrees with S-parameter .All good now.

author: mawyatt
author_rank: Super Contributor
timestamp: December 12, 2022, 08:13:17 pm
content: As mentioned Spice models can vary from simple to extremely complex, and not all the same. Generally a Spice model is used for DC Bias, Transient Analysis, Temperature Effects and Linear AC Analysis and attempts to achieve a reasonably accurate representation for all these different Analysis and bias conditions including highly non-linear regions.Bipolar device Spice models can be Behavioral models, device Physics based models, Measurement based models, Curve Fit models, and all of these "blended" together. Examples are Ebers-Moll, Gummel-Poon, VBIC, MEXTRAM, HiCuM for bipolar transistors, and MOS models are usually BSIM 3, 4, 5 and 6 with various levels within and PSP based.Spice model development for modern integrated devices is extremely expensive, and semiconductor fabs can spend many man-years developing a single device model. This is necessary because most of the highly advanced chips of today can't be "breadboarded" and the designers must rely solely on their experience, the models, and the simulators to achieve a working chip. Mistakes are extremely expensive and career limiting, so few engineers/scientists choose this as a career path!!Best,

author: RoV
author_rank: Regular Contributor
timestamp: December 12, 2022, 09:36:02 pm
content: Quote from: dmowziz on December 12, 2022, 06:58:11 pmOf course, sorry I was incorrect in terms! When talking about impedance, I was implicitly assuming the bilinear transform from S to ZHappy that you solved.

author: dmowziz
author_rank: Contributor
timestamp: December 12, 2022, 09:50:58 pm
content: Quote from: RoV on December 12, 2022, 09:36:02 pmActually, the book I'm learning from calls S11 the input impedance...When you asked that, for a second, I thought maybe you're "reading" the same book as well