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Post subject: Cascaded gain-multistages PA
postPosted: Thu Jun 23, 2005 4:49 am
Joined: Tue Mar 15, 2005 11:43 pm
Hi all :
I need help urgently.
3 stages PA for 3.4-3.6GHz. The overall gain is around 24 dB. The first
stage gain is happen to be around 8dB. And I check the second-third
stages (combined) is 16dB. I was pretty confident that it was fine.
But when I looked at it more carefully, I simulated second and third
stage individually and I found with 50 Ohm termination the second stage
gain is only 6 dB while third stage is around 0dB.
I thought it maybe mismatch at the second-third interstage, so I terminated
second stage output and third stage input with certain port impedance
and optimized the value to get the best gain for both stages.
It shows that the best gain for second stage is only 4dB and third
stage is also 4 dB with gamma termination 0.8.
So I lack of 8
dB, can anyone help me what's happening here??
and I also take
a look at the stability of each stage, second and third stage individually
stable, but when I combined second and third stages, it's oscillates.
I suspect something wrong here, though I am not sure where the error
Just wondering whether biasing line might be the reason?
I'll appreciate your advice.
Please enlighten me.
Unread postPosted: Thu Jun 23, 2005 9:41 pm
There could be few causes to your problem:
1) Check that your bias line doesn't load the output matching, do
you use quarter wavelength lines and capacitor to short the RF signal
to GND at the output of each stage?
2) Could be a wrong bias
level for the second and third stage.
3) Have you checked that
the input power to the second stage is not high enough? there could
be a possibility that the second stage and hence the third stage are
driven into compression.
In the simulation you have to do an
intersection not just between stability circles but also between gain
circles and find an area on the unity circle that will allow you to
get the highest possible gain yet with a stable operation.
luck, should you need more help please let me know.