Input impedance refers to the equivalent impedance at the input end of a circuit.
Add a voltage source U to the input, measure the input current I, then the input impedance Rin is U\/I.
You can think of the input as the two ends of a resistor, and the resistance of this resistor is the input impedance.
The input impedance is no different from a normal reactance element, which reflects the size of the current obstruction.
For a voltage driven circuit, the greater the input impedance, the lighter the load on the voltage source, and therefore the easier it is to drive, and will not affect the signal source; For current-driven circuits, the smaller the input impedance, the lighter the load on the current source.
Therefore, we can think of it as follows: if it is driven by a voltage source, the larger the input impedance, the better; If it is driven by a current source, the smaller the impedance, the better (Note: only suitable for low-frequency circuits, in high-frequency circuits, but also consider the impedance matching problem.
In addition, if you want to obtain the maximum output power, you should also consider the impedance matching problem).