Post History
I never would recommend to use the "re model" for a BJT. The reason is as follows: The BJT is a voltage-controlled device (Ic is controlled by Vbe and the base current is an unwanted by-product...
Answer
#2: Post edited
by
LvW
·
2022-06-15T11:15:39Z (almost 2 years ago)
- I never would recommend to use the "re model" for a BJT.
- The reason is as follows:
- * The BJT is a voltage-controlled device (Ic is controlled by Vbe and the base current is an unwanted by-product).
- * Hence, the characteristic transfer parameter is the transconductance gm=d(Ic)/d(Vbe)=Ic/Vt..
- * The obscure quantity re is nothing else than re=1/gm. And it is not correct to say "re is the intrinsic emitter resistance" as we can read in some contributions. This is simply wrong!
- * The quantity re can be used (instead of gm) in an equivalent small-signal diagram - however, this can lead to confusion and misunderstandings because such a resistance in the emitter leg can be mixed with a real ohmic external resistor which provides current-controlled voltage feedback.
* I see absolutely no reason to use such a confusing model which does not reflect the real BJT principle based on the transconduchtance gm-
- I never would recommend to use the "re model" for a BJT.
- The reason is as follows:
- * The BJT is a voltage-controlled device (Ic is controlled by Vbe and the base current is an unwanted by-product).
- * Hence, the characteristic transfer parameter is the transconductance gm=d(Ic)/d(Vbe)=Ic/Vt..
- * The obscure quantity re is nothing else than re=1/gm. And it is not correct to say "re is the intrinsic emitter resistance" as we can read in some contributions. This is simply wrong!
- * The quantity re can be used (instead of gm) in an equivalent small-signal diagram - however, this can lead to confusion and misunderstandings because such a resistance in the emitter leg can be mixed with a real ohmic external resistor which provides current-controlled voltage feedback.
- * I see absolutely no reason to use such a confusing model which does not reflect the real BJT principle based on the transconduchtance gm.
- * As an example for the mentioned confusion look at the 5 small diagrams as given in the first post: We can se "re" and "RE" - both with a symbol for the resistor. Are they identical or not?
#1: Initial revision
by
LvW
·
2022-06-15T07:58:16Z (almost 2 years ago)
I never would recommend to use the "re model" for a BJT. The reason is as follows: * The BJT is a voltage-controlled device (Ic is controlled by Vbe and the base current is an unwanted by-product). * Hence, the characteristic transfer parameter is the transconductance gm=d(Ic)/d(Vbe)=Ic/Vt.. * The obscure quantity re is nothing else than re=1/gm. And it is not correct to say "re is the intrinsic emitter resistance" as we can read in some contributions. This is simply wrong! * The quantity re can be used (instead of gm) in an equivalent small-signal diagram - however, this can lead to confusion and misunderstandings because such a resistance in the emitter leg can be mixed with a real ohmic external resistor which provides current-controlled voltage feedback. * I see absolutely no reason to use such a confusing model which does not reflect the real BJT principle based on the transconduchtance gm-