The basic components of analog circuits are wires, resistors, capacitors, inductors, diodes, and transistors. Passive filters use resistors combined with reactive components such as capacitors and inductors. Resistors, capacitors, inductors, and transformers are all considered passive devices. Analogue circuits can be entirely passive, consisting of resistors, capacitors and inductors. To match electrical impedances, engineers use combinations of transformers, resistors, inductors, capacitors and transmission lines. Only capacitors, inductors, and resistors are considered passive. Capacitors, inductors, and resistors are usually designed to minimise other parameters. LC networks with more inductors and capacitors. In low pass form the topology would consist of series inductors and shunt capacitors. The inductors and capacitors are the reactive elements of the filter. The rectified output is then smoothed by a filter consisting of inductors and capacitors. The design also specifies high quality screened air-gap inductors and film capacitors. This same principle applies to capacitors in series or to inductors in parallel. The mutual inductance, formula 140, is also a measure of the coupling between two inductors. This simplifies the electronics, but the inductors and capacitors tend to be large and heavy.