What Is the Impedance Between the Base and Ground in a Common Emitter Amplifier?

[tomeatworld]

Firstly, sorry if this is in the wrong section. I'll happily move it if it should be somewhere else.

Homework Statement

What is the impedence between the base of the BJT and ground of the common emitter amplifier circuit.

Homework Equations

I_c = I_E

The Attempt at a Solution

So I started by saying that $$\Delta$$V_b = $$\Delta$$I_E R_E and that $$\Delta$$I_E = $$\Delta$$I_B h_fe where h_fe is the gain of the circuit. By then equating the two using $$\Delta$$I_E I find that $$\Delta$$I_B = $$\frac{\Delta V_{B}}{R_{E}h_{fe}}$$ so impedence is R_Eh_fe but this seems very simple considering the circuit. Am I missing anything?

 

[_coyote_]

The impedance between the base and ground of the common emitter amplifier circuit is given by the equivalent resistance from the base to the ground. This is equal to the sum of the resistances of the base and emitter resistors, plus the output resistance of the transistor in its active region, which can be approximated as the Early voltage divided by the current gain of the transistor. Therefore, the total impedance is given by:Z = R_B + R_E + V_A/(h_fe)