# Common emitter amp and BJT base-emitter resistance question

Hi,

In a common emitter amplifier circuit, I am considering the base-emitter junction. When you apply a base current, you get the base-emitter resistance, according to thermal_voltage/base_current. But when you get that resistance and place it in it's position in series with the emitter resistor in the amplifier circuit, it changes the base current. What gives? What keeps it at the same value (for given thermal voltage, beta/Hfe, etc)? Certainly I'm missing something here (probably something elemental ;D ). I know base-emitter resistance isn't supposed to matter much in a typical common emitter amp, but this question has been bugging me for some time now. :P

Thanks,

LS

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I think you are refering to small-signal models, the hybrid pi or the T model. Those models relate to the AC behavior of the amplifier. The base-emitter resistance is what a small amplitude AC signal would see.

The base-emitter resistance seen from the base side of the bjt is called "r_pi". Seen from the emitter side, it is called "re".

r_pi = hfe/gm = hfe*Vt/Ic. But we must be careful & remember that "Ic" is the dc or quiescent value of collector current. The small signal, or ac part of the collector current is denoted by "ic". We must not confuse the two.

By definition, r_pi = vbe/ib, being sure to observe the lower case "v" in "vbe", & lower case "i" in "ib". Referring to the above equation, r_pi = hfe*Vt/Ic, we must remember that r_pi is a small signal ac parameter. But, the "Ic" in the equation is the dc bias value. So the dc value of collector current determines the ac small signal resistance value.

The "hfe" value denotes small signal ac conditions. So "Ic/hfe" is not really the base current. Ic = hFE*Ib, since "Ic, Ib, & hFE" are all dc values. Likewise ic = hfe*ib (ac).

The re value is Vt/Ie. Again Ie is dc value, but re is ac. Also, re = vbe/ie = alpha/gm, all ac values. I think that is where the confusion can take place. Have I helped or made matters worse.

Claude

Last edited:
cabraham,

That makes sense now, the two values Ic and ic (or Ib and ib) are separate. Ic (DC) determines the AC resistance, so the AC circuit is different (adding r_pi), than the DC circuit determining r_pi which does not see r_pi (so we don't have the "looping" r_pi calculation). Wikipedia does not explain things well. :P

Thanks for all of your help,

LS