an electric circuit, whether it's a simple lightbulb or a complex amplifier, has two input terminals that are connected to the two output terminals of the voltage source. the impedance between the two input terminals (often a function of frequency) is the circuit's input impedance. most circuits are designed to have a large input impedance. to see why, suppose you need to amplify the output of a high-pass filter that is constructed with a 1.2 kω resistor and a 18 μf capacitor. the amplifier you've chosen has a purely resistive input impedance.