an LC matching network
An LC matching network is a tiny circuit of inductors (L) and capacitors (C), often just two parts, that sits between two stages and makes one impedance look like another. Picture a gearbox between a small engine and a big wheel: the gearbox does not add power, it just trades speed for torque so the two sides work together. The LC network does the electrical equivalent, trading voltage for current to transform, say, a 10 ohm transistor input up to a 50 ohm system, at a chosen frequency.
The simplest version is the L-network, one series part and one shunt (parallel) part. The trick is that a reactance in series adds to the impedance while a reactance in parallel does the opposite, and by picking the right two values you can move any resistance to any other resistance at one frequency. A rough worked feel: to match a source resistance Rs up to a load Rp, the network's loaded Q is square root of (Rp/Rs - 1); for 10 ohms to 50 ohms that is square root of (5-1) = 2, which then sets the reactance values you need. Add a third element and you get a pi or T network, which gives you control over bandwidth as well as the match.
LC networks are the everyday workhorse of RF matching because they are cheap, low-loss, and small. They appear at antenna feedpoints, between amplifier stages, and inside almost every transmitter and receiver. Honest caveats: the match is exact only at the design frequency and degrades away from it, so the network's Q trades narrow bandwidth for a sharper match. Real inductors and capacitors at RF have loss and their own parasitics, so the network is never quite ideal, and the higher the transformation ratio, the higher the Q and the touchier the tuning. Layout matters too: at high frequencies the traces between the L and C are themselves part of the network.
To match a 50 ohm source to a 200 ohm load at 100 MHz, an L-network needs Q = square root of (200/50 - 1) = square root of 3 = 1.73. That sets a series inductor and a shunt capacitor whose reactances are about 86 ohms and 115 ohms, values you then realise as a specific nH and pF at 100 MHz.
An L-network's Q follows from the impedance ratio and fixes the L and C values.
A higher transformation ratio forces a higher Q, which means narrower bandwidth and touchier tuning. The exact match holds only at the design frequency, and real-part loss in the L and C makes it imperfect.