questions about RF power amplifiers and non-constant envelope signals

Hi,

I am using a QPSK modulation. Based on the literature I have gone through, it is supposed to be a non-constant envelope due to filtering. So I have to use a linear RFPA to amplifier the signal.

I am curious that since the envelope variance is caused by phase change only, why can't I just put a filter after the PA, so that the envelope can develop there?
By doing this, I only need a switching PA, which is much more power efficient?

All the literature just go ahead to design the PA to be linear. Please enlighten me on this point.

The non-constant envelope is due to the modulation and not due to the filtering.
Polar modulation approach use a phase modulator at the input of the PA, a non-linear PA, and a synchronized AM modulator. In this way a non-constant envelope modulated signal can be amplified by a non-linear PA, with higher efficiency.

Hi,

Could you tell me what type of QPSK modulation is ?
constant-envelope or non?

if it is constant envelope, why can't I use switching PA instead? Because all the literature I have read said otherwise. And I don't understand the real reason behind that.

thanks in advance.


Best Regards,
Whlinfei

hi,
In the case of constant envelope modulation you can use a switching mode power amplifier with high efficiency because the PA is used in the saturation region due to constant envelope.

Show me where you read that a constant envelope signal cannot be amplified by a switching PA.

You CAN have an QPSK driving an amplifier, and then bandpass filter at the output.

In general, you do not want to do that because:

1) you need a narrowband filter to filter out the unwanted intermodulation products, and that means you can no longer tune the transmit frequency (since you would have to retune the bandpass filter center frequency also). One often prefers to bandpass filter at a lower fixed IF frequency, and then upconvert to the final transmit frequency.

2) bandpass filters, in general, do not handle high power well. A bandpass filter at the output of a tranmsitter might be hard to design or very big for the power handling.

3) many systems need very low attenuation at the output of the transmit amplifier. That is because transmiters are expensive and you do not want to overdesign them to handle output loss too. Also, battery life is severely impared if you put loss after your transmit amplifier. Bandpass filters, especially narrowband ones to clean up intermodulation, have higher insertion loss.

Hi,

I understand constant envelope can be amplified using switched mode PA.
But I couldn't find any literature using switching PA to amplifier QPSK signal.

I think biff44's answer is right.

Thank you anyway.

Best Regards,
whlinfei