# Common emitter amp and BJT base-emitter resistance question

Discussion in 'General Electronics Chat' started by linear_shift, Jul 1, 2010.

1. ### linear_shift Thread Starter New Member

Jul 1, 2010
2
0
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.

Thanks,

LS

2. ### bitrex Active Member

Dec 13, 2009
79
4
I don't know if I completely understand your question, but the emitter resistance in the "T" model of the bipolar transistor is defined as $\frac{V_t}{I_e}$ where I_e is the emitter current, not the base current. The resistance defined by the equation you have is the base resistance in the "hybrid pi" transistor model; the voltage drop across this resistance times the small signal transconductance gives the collector current in that model.

3. ### linear_shift Thread Starter New Member

Jul 1, 2010
2
0
bitrex,

Someone on Physics Forums answered my question. I confused the AC parameters in the hybrid-pi model with the DC ones, so there is really no problem.

Thanks,

LS