What is the DC behavior of the switched BJT emitter follower configuration
let’s consider linear vs non-linear differences.
We know from datasheet typical curves and min/typ/max specs how hFE varies with Ic, Temperature and Vce.

since Zout is a direct function of hFE, and impedance applied to the base it is important to understand some variables and how impedance control rise fall time into some fixed charge , Q or estimated Ciss which is also Vgs voltage dependent.

When Vce=Vce(sat) at Ic/Ib=10, and hFE typ is 100 we know that hFE has lost 90% of it’s value due to forward conduction of Vcb.  Similar 10% ratio exists for hFE types &gt;500 tested at Ic/Ib=50.  But since hFE is too dynamic, we standardize Vce(sat) at fixed Ic/Ib ratios even though we know Ic is only controlled by Vbe.  But with Re/Rfb ratios known it is easier to compute Vce(sat) from current ratios than otherwise.

the Zout formula is better defined by hFE than gm because gm not given in datasheets.


$$Zo_{emitter}=( Z_{Input}/h_{FE})// Re//Ro. $$

for Q3, Zinput=R6//Rce(Q2), neglecting flyback D1.
We assume R6&gt;&gt;Rce which is the saturation condition =Vce(sat)/Ic,
therefore we know Zo rises when Q2 turns off leaving R6/hFE to define the Zo
When Q2 turns on, it’s Rce drops to 1 Ohm for PN2222A and equivalents. Vb will be &lt;100 mV, then Ve depends solely on the passive pull down load resistance.

This means using a emitter follower will have a faster risetime due to R6/hFE than fall time due to the load resistance unless it is less than R6/hFE.
Since Complementary drivers like this always needs a deadtime to prevent shootthru, the final switch turn off time must be faster than turn on time.  that means emitter followers make poor half bridge drivers so push-pull drivers are used instead with controlled slew impedances.