Some fundamental question on ANALOG/RF design.

[al_dddd]

Hey there!
I just have a little fundamental question - why should we work in impedance mismatches in low frequencies (voltage/current gain) and not just trying to match to get the maximum power , like we do in RF? Don't we care about power losses that the reflection causes?
Thanks!

 

[volker_muehlhaus]

Hey there!
I just have a little fundamental question - why should we work in impedance mismatches in low frequencies (voltage/current gain) and not just trying to match to get the maximum power , like we do in RF? Don't we care about power losses that the reflection causes?

It's very easy: the concept of matched transmission lines is relevant where the line is long enough to do some impedance transformation (RF engineer's view) or signal risetime is so fast that reflections matter (SI engineer's view). The signal waveform must be so fast that it has already changed when the reflected signal arrives. Only then, transmission line effects matter.

At low frequency, where the line is much shorter the wavelength and the line delay time is much shorter than the signal risetime, the signal at the input and output of the line are "the same", no matter what line impedance we use.

 

[vfone]

Your question have sense knowing that before any RF development in the early days of electric motors, it was found that to get the most efficient transfer of power from the battery (source) into the motor (load) required that the resistance of the different parts of the circuit be the same, in other words, matched (maximum power transfer theorem).

After many years, impedance matching was used in RF, in principle to match the transmitter output to antenna input.