It doesn need to at all, it can have any input impedance. Most RF amplifiers have 50 ohm input impedance, pretty low, and the same as the output impedance.
It doesn’t need to at all, it can have any input impedance. Most RF amplifiers have 50 ohm input impedance, pretty low, and the same as the output impedance. Now, an operational amplifier is a different story. If you have an amplifier with feedback like this, you’d like the output to be zero impedance and the input to be very high impedance, so you can divide the output voltage with resistors. If you’re driving a motor or a loudspeaker, you don’t want the load to affect the output or feedback, which zero output impedance let’s you do. If you are using the amplifier to measure something, like a voltage in another circuit, you want it to have the least impact on that circuit, which a very high impedance lets you do. Of course if you are worried about frequency response, you want low output impedance to drive capacitors or inductors, and you want a relatively low input impedance for the same reason. A very high input impedance with even a small capacitance will roll off high frequencies.
