Input impedance is defined as the voltage divided by the current, where voltage and current are as shown in Figure 1.

Figure 1

An amplifier with a gain of +2 is connected as shown in Figure 2.

Figure 2

Questions:

1. What is the input impedance, Z?

2. What is the particular property of the small-signal resistance that a switching regulator presents to its power source?

3. What is the particular property of the small-signal resistance that a low I_Q LDO regulator presents to its power source?

— Question 1 Contributed by Charles Ratcliffe, Sancrafter Co.

Answer