Base to emitter voltage of BJT in spice - Page 1
Forum section: electronics / beginners

author: edba
author_rank: Contributor
timestamp: February 21, 2021, 09:36:11 am
content: I have done a DC operating point simulation of a AC127 germanium transistor with model as follows:*AC127 NPN Germanium Transistor Spice Model*.MODEL AC127_X NPN IS=1.41f ISC=0 ISE=0 IKF=80m IKR=0 ITF=0.4+NC=2 NE=1.5 BF=70 BR=4.977 RB=10 RC=2.5 CJC=9.728p CJE=8.063p+TR=33.42n TF=179.3p FC=0.5 EG=1.11 VJC=0.2 VJE=0.2 VTF=4+MJC=0.5776 MJE=0.3677 XTB=1.5 XTF=6 XTI=3It should have a base to emitter voltage of 0.2V indicated by VJE=0.2. However I have measured this 0.7671V? I know there could be base spreading resistance etc as the gummel and poon model is rather complicated, however even when you set VJE=0.0 it has no affect on the voltage?Can anyone help?Edward

author: mvs
author_rank: Frequent Contributor
timestamp: February 21, 2021, 11:35:47 am
content: LTspice is just using default npn values rather then your model.It is not clean solution, but you can add your model data to standard.bjt file and reload LTspice.Then you will be able to use "Pick New Transistor" button.

author: edba
author_rank: Contributor
timestamp: February 22, 2021, 12:14:20 am
content: HiOk I tried doing saving the models to the standard.bjt file but the transistor still did not show in 'Pick New Transistor' button.I still don't understand why the .include statement would not work?

author: rfclown
author_rank: Frequent Contributor
timestamp: February 22, 2021, 01:25:04 am
content: I don't where you got your model from, but it doesn't look like a germanium transistor.VJE does not set Vbe. VJE/PE sets the curvature of junction capacitance. Vbe is set by IS.IS = Ie*(e^(-q*Vbe/(k*T)))where:k= 1.38e−23 (Boltzmann constant)T=293 (temperature in Kelvin)q=1.60e−19 (electron charge)So for 5mA of emitter current, to get Vbe=0.2v, IS=1.8e-6LTspice results:--- Operating Point ---V(n001): 5 voltageV(p001): 0.0676074 voltageV(vb): 0.208829 voltageIc(Q1): 0.00493239 device_currentIb(Q1): 0.000479117 device_currentIe(Q1): -0.00541151 device_current

author: edba
author_rank: Contributor
timestamp: February 22, 2021, 06:25:12 pm
content: rfclown thankyou.So you have to go back to the shockley equation. Do you know of a good tutorial on the spice gummel and poon model that you use?

author: edba
author_rank: Contributor
timestamp: February 23, 2021, 12:36:54 am
content: I have been developing a behavioural model instead of using the built in gummel and poon model, just as an interest but it is simpler and more practical and you can convert a data sheet parameter straight into this model. It is the standard DC and AC small signal model from most textbooks and university lectures.

author: edba
author_rank: Contributor
timestamp: February 23, 2021, 12:43:26 am
content: Note R1, R2, V1 and V2 are external to the NPN transitor. Vbe is the DC 0.7V base to emotter voltage. V2 is the 25mV of the dynamic resistance re. B1 is Bdc and B2 Bac

author: rfclown
author_rank: Frequent Contributor
timestamp: February 23, 2021, 12:58:15 am
content: Quote from: edba on February 22, 2021, 06:25:12 pmI don't know of a tutorial. I would Google. I only knew the answer to this question off the top of my head because I made a similar mistake many years ago and found that VJE wasn't what I had assumed it was based on its name (Base-emitter built-in potential). At the time, I Googled on the NPN Spice model and found my answer.