# 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!