# What is load impedance?

Like resistance for DC, Impedance is for AC. But what is load resistance. How does it used in a circuit.

Had this doubt when reading a line from the book foundations of electronics by D Chattopadhyay

"A voltage amplifier is one that is designed to amplify voltage waveforms in applications where almost no power is drawn by the load impedance"

On AC signals the impedance depends on the resistance, the inductance and the capacitance of the circuit. This also depends on the signals frequency.

The load impedance is the impedance you put on the output of the circuit (in this case the amplifier).

In this case you can see almost no power is drawn this means that the output impedance has to bee very high. This means that you can connect a high input impedance amplifier on the output of the first amplifier. You cannot connect for example a speaker since it's impedance is normally really low (between 4Ω and 16Ω).

There are many applications where the impedance really matters, for example there are some differential signal drivers which need a 120Ω resistor on the differential output and also on the differential input. simulate this circuit – Schematic created using CircuitLab

This is just an op-amp example, the resistors can have any other values depending on the specific case.

• Could u post a circuit diagram along with this expalination Jun 21, 2015 at 13:49
• Ok, I'll try, let me see if I'm able to.
– Falk
Jun 21, 2015 at 13:56