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.
I've designed 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.
So firstly 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.
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 is.
whether biasing line might be the reason?
Please enlighten me.
Thanks so much.
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.
Good luck, should you need more help please let me know.