0
\$\begingroup\$

I have made a common emitter amplifier in PSpice with an NPN transistor. I have set the Beta value to 200 in the model settings, however, as you can see from the picture, the collector current is not 200 times the base current as would be expected.

When I do the DC analysis, I find that the collector and emitter currents are correct, however the base current I am expecting is 3.98uA, not 7.512uA.

common emitter amplifier bias currents

These are the model parameters for the Q2N3904 that I'm using: .model Q2N3904-X1 NPN( Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=200 Ne=1.259 Ise=6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10)

\$\endgroup\$
4
  • 2
    \$\begingroup\$ Beta varies with collector current and Vce and some other things like temperature. How much it varies and how depends on the specifics of your model and its parameters. Just because you set "beta" to something in the model doesn't mean that the relationship will hold for all operating points. \$\endgroup\$
    – John D
    Commented Mar 10, 2022 at 22:15
  • \$\begingroup\$ We'll need to see the complete model specifications you are using. \$\endgroup\$
    – jonk
    Commented Mar 11, 2022 at 2:13
  • \$\begingroup\$ As far as I know the SPICE BJT model does not contain just a simple "beta value". \$\endgroup\$
    – LvW
    Commented Mar 11, 2022 at 8:05
  • \$\begingroup\$ Change Ne=1.259 from to Ne = 1.5. You can also delete Ikf or increase it to for example Ikf = 200m \$\endgroup\$
    – G36
    Commented Mar 11, 2022 at 17:51

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.