Why does capacitive coupling require a base resistor in an emitter follower?

Images from Art of Electronics 1st edition, p. 58 (Horowitz and Hill)

Both of these circuits accept an AC signal that is centered around 0V.

The first (Figure 2.16) is supposedly coming from a previous amplifier stage where the output voltage is conveniently situated between the positive and negative voltage supplies.  The second (Figure 2.17) has been capacitively coupled and so (I presume) it is also sitting around 0V and fluctuating into both positive and negative territory.  The only difference between the two as nearly as my novice eyes can see is that one is coupled and one is not.  Horowitz and Hill state:

Warning: You must always provide a DC path for base bias current, even if it goes only to ground.  In the preceding circuit it is assumed that the signal source has a DC path to ground.  If not (e.g., if the signal is capacitively coupled), you must provide a resistor to ground. (Art of Electronics 1st Ed, p. 58)

I don't understand why both circuits don't need this resistor, and why the capacitor makes it necessary.

Capacitor in series needs a charge path but a discharge path also. The Rb ensures both part of this condition.

Without Rb the capacitor is charged through base only during positive cycle. But nothing allows to discharge the cap back during negative cycle. So the cap becomes fully charged after few periods and no signal passing.

In the first picture (2.16), no source is shown so the DC level is unknown, but as you refer to,

In the preceding circuit it is assumed that the signal source has a DC path to ground.

some kind of DC path is existing.

In figure 2.17, where a capacitor is explicitly shown, some kind of DC bias path is required. It could have been a standard voltage divider between Vcc and Vee with the base connected inbetween.

The capacitor prohibits the flow of DC current so if it is necessary to establish a DC operating point, some kind of DC bias path is required. In this case a single resistor is used. If no base current is flowing no collector current will flow. Base current would only flow in the beginning unttil the capacitor is charged up.

Here are CircuitLab simulations of the two circuits. I will add my explanations later.

Vin = 0 V



simulate this circuit – Schematic created using CircuitLab

Vin = 1 V AC



simulate this circuit



Vin = 0 V



simulate this circuit

Vin = 1 V AC



simulate this circuit