What is meant by input and output impedance?

Input and output impedance are terms constantly thrown around by hardware engineers but what exactly do they mean?

Impedance is the generalised form of resistance which caters for the complex variety, i.e. composed of capacitive and inductive components as well as resistive.

Input impedance is then the impedance measured across the input terminals of a circuit while output impedance is the impedance measured across the output terminals.  Input impedance refers to the load, the input, attached to the source while output impedance refers to the source, the impedance it has internally.

A simple circuit used as an example could be an NMOS amplifier driving a simple resistive load as shown below:

In this circuit we can determine the output impedance by looking back into the input source circuit, i.e. from the gate of the MOSFET, which would be (R1||R2)||(C1+Rin). For the input impedance we look back into the output terminal and we would see RL||(C2+Rs).

We can use Thevenins Theorem to calculate the input impedance.

We can calculate the output impedance by open circuiting the source circuit and measuring the impedance.

See the Amplifier topics of the Analog Electronics course for further details on the importance of input and output impedance!