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Cascaded gain-multistages PA - RF Cafe Forums

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Post subject: Cascaded gain-multistages PA
Unread postPosted: Thu Jun 23, 2005 4:49 am
User avatar

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
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.

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 is.

Just wondering whether biasing line might be the reason?

I'll appreciate your advice.
Please enlighten me.
Thanks so much.



Post subject:
Unread postPosted: Thu Jun 23, 2005 9:41 pm

Hi Jean,

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.

Posted  11/12/2012
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