impedance matching concept is based on that the load impedance should be the conjugate of the impedance of the source that is the general concept for low power excitation (small signal mode) for some cases the load changes depending upon the excitation (large signal mode). This is phenomena appears clearly on devices. For example the model for the GaAs MESFET device elements behaves linearly as the excitation nearly less than unity (1 Volt) as the excitation exceeds this limit  the capacitors of the model behaves nonlinear and the inter-modulation distortion appears.

Adding to what has been said by professor El-Rabaie, every source has an internal or a source impedance ZS .Irrespective of the power of the source, In order to draw the maximum power from this source one has to make ZL= ZS conjugate.  In many cases ZL do not satisfy this condition and one has to add matching circuit between the load and the the source. This matching circuit must be made from nearly lossless circuit elements such as inductors , capacitorors and transformers.So, there is L SECTION , T CIRCUIT , Pi CIRCUIT, and transformer matching,

There are also electronic active matching circuits such as the buffer amplifiers and the emitter and source followers that are normally used for matching from high impedenace source to lower impedance load.

There are also, electronic switching matching circuits used in DC to DC converters to match a varying impedance source such as photvoltaic generators  to fixed loads. The switching circuit acts as a varialble turns ratio transformer. This accomplished by varying the duty ratio of the converter.

Thank you Sir El-Sayed Mahmoud El-Rabaie and Sir Abdelhalim Zekry for your answers!

Regards,

Suhas. D

There is yet another angle to matching at low power: noise matching. The best noise of the most amplifiers is usually not coinciding with the best power transfer.

The classic way is to draw noise figure circles in a Smith chart, and find optimum matching from there.