PA and linearity question

[kwanmajun]

Hi,

Here is a question about the PA design and linearity issue.
" how we could input linearity of Class-AB PA? How about using a filter design in the output load, is that possible to using filter to filter out the 3rd harmonics? if yes, then use high pass filter or low pass or bandpass?

Then how about class-E pa, is that filter design at output load help the class-E pa linearity???"

thanks. please answer with details.

 

[ROGER33]

Class A is very linear
Class AB also

But what linearity? from DC to what? 10KHz? 25 Khz?1Mhz?

In general after 20Khz due capacitance in stages, Output level down...

For best Linearity you need pre-emphasis slowly BEFORE amplifier for Best results.

 

[kspalla]

To reject the unwanted signal better use notch filter or band stop filter

 

[WimRFP]

Hello,

As you are mentioning class E also, I suppose you dealing with RF power amplifiers.

Class E amplifiers can only be used for modulation schemes that have constant envelope (FM, constant envelope PSK/FSK, etc).

Filtering out the harmonics is good to remove the harmonics, but does not remove unwanted sidebands due to intermodulation products.

Class E amplifiers can be used for AM (and QAM, OFDM, etc) by modulating the supply voltage. As varying the supply voltage may introduce some phase shift between in- and output also, a kind of pre-distortion of the input signal is used to cancel the distortion introduced by the PA.

Filtering of harmonics in wide band amplifiers can be done by a LPF (switchable in case of very wide BW/fcenter). For single frequency amplifiers PBF are mostly used as this reduces the number of components to get the required suppression of unwanted signals.

In one sentence, adding an LPF or BPF, does not increase the linearity of the RF amplifier (as intermodulation product remain).

 

[kwanmajun]

thanks a lot for all the answers, i think i got it~