Power Amplifier vs Voltage Amplifier (impedance matching or impedance bridging?)

[tendo R]

Hi all,
The amplifier input impedance is giving me a headache even I looked for answer from these two posts:
https://www.edaboard.com/threads/29869/
https://www.edaboard.com/threads/230983/.
First of all the situation is for receiver end not transmitter. My question begins with the unfinished discussion in post one. When I want to amplify the received signal form antenna with LNA, why do I need to match it to the antenna but not load it with a infinite impedance so I can have the highest voltage? Since an MOSFET LNA voltage amplifier only concerns input voltage but not input power.
Lower noise does not explain this question because impedance matching is not noise matching.

On the contrary, I find all audio amplifiers and most biosensors use opamp with huge input impedence. Yet they all can be considered as "power" transfer.

So when on earth should I concern maximum power transfer or maximum voltage transfer?

Please, someone help me!

 

[FvM]

Raising the question too general doesn't help to get a reasonable answer.

Impedance matching is mostly applied in RF applications, e.g. > 10 or 100 MHz. Rarely in audio or other low frequency application.

 

[Borber]

If cable is not terminated with it's characteristic impedance (matched loading) then depending on frequency, cable length and loading impedance you will get in one case 0V and in other 2 times generator voltage. To avoid such random situation cable must be terminated with it's characteristic impedance. Audio amplifiers or DC amplifiers have high input impedances because frequencies are too low that kilometers of cable length bring request for matching. It is proportional to wave length of frequencies used. As FvM noted.

 

[Warpspeed]

why do I need to match it to the antenna but not load it with a infinite impedance so I can have the highest voltage?

Which do you think would work better, a jefet gate tied directly to the antenna.

Or the antenna fed into the tapping of a coil, which was part of a resonant tuned circuit that magnified the voltage amplitude many times (noise free gain) before it reaches the input of the first amplifying device ?

If you play around with the LC ratios of the tuned circuit, and the input coupling, you can indeed get maximum power transfer by impedance matching and resonance, and best signal to noise ratio, not to mention some attenuation of troublesome strong out of band signals.
Its all well worth the trouble.

 

[volker@muehlhaus]

On the contrary, I find all audio amplifiers and most biosensors use opamp with huge input impedence.

If you try this with an RF amplifier, and you connect some cable between the antenna and the amplifier, what is the input impedance seen by the antenna?

For the low frequency amplifiers, where the cable is much shorter than the wavelength, the input impedance into the cable is the same as the input impedance into the amp. But for the RF case, where the cable is long compared to the wavelength, the cable will do some impedance transformation. Your high input impedance, seen through a cable with 1/4 wavelength, will transform into a very low impedance (almost short). Now make the cable much longer, and the input impedance will loop around the Smith chart and present all different impedances to the antenna. There is no way to make this high impedance (at the antenna) for all different cable length and over some frequency range. Only matched impedance will maintain a flat response with arbitrary cable length.