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Wimax power amplifier stability

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lucas_84

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Hi,

I'm looking for some help in designing power amplifier for wimax. Precisely my question is about stability. Because I have to use Nitronex transistor NPT3015 I was utilizing following datasheet **broken link removed** . When I perform small signal stability test (K, B1) I saw that in band 0.1 -5.1 the amplifier is not stable, but I am absolute beginner in designing active devices so maybe I am wrong. My question is,

how to investigate stability in power amplifier (MWO simulator)?

I'll be grateful for any help.

Regards
Lukas
 

You are measuring unconditional stability which means all loads and sources will not produce oscillations. Since you have control on the source and load impedance at all frequencies, you can make them whatever value that is stable.

One trick is to put a few ohm resistor in series with the input. This will lower the gain a little.
 

I have added series and parallel resistor with the input. In effect stability circles in input/output was moved out of smith chart but gain, efficiency degrade. I really don t know what to think about reference datasheet **broken link removed** because it should work.


Regards
Lukas
 

The amplifier is unstable on the evaluation board provided by Nitronex, or on your own board?

Looking to the datasheet of NPT35015 I see that the layout they recommend, there is no ground connection of the bottom source pad to the top layer ground.
For example the layout they recommend for other part NPT25015 looks fine (see the picture attached).

To improve stability, instead using LPF matching networks for both, input and output, an idea would be to use HPF at the input and LPF at the output.
 

I do not have the evaluation board. I have just draw circuit from reference data sheet and start simulating. When I perform small signal stability test (K, B1) I saw that in band 0.1 - 5.1 GHz the amplifier is not stable, in my opinion it should be stable but I am newbie in power amplifiers. So I start thinking about conformity of the reference project.

Precisely my questions are

- is power amplifier need to be unconditionaly stable (in wide frequency band like small signal amplifiers, for example 0.01 - 20 GHz)?
- if above question is true, what is wrong with reference project included in datasheet?

I am extremely grateful for all answers I got so far.


Regards
Lukas
 

If the amplifier is potential unstable, doesn’t mean it will oscillate, but the final circuit must be verified for oscillations in a wide frequency range.

PCB grounding, decoupling, and matching impedances (especially input match) are important getting good stability.

I don’t think is something wrong with the reference project because this is optimized for a relative narrow frequency range.
 

I think a clue is the 0.33 ohm resistor in the drain bias supply, and 10 ohm resistor in the gate bias, to keep them from seeing a mid-frequency short circuit! I have use some of the nirtronex bigger devices, and the demo boards all had that series resistor.
 

Can anyone tell me how to check large signal stability in MWO??

Regards
Lukasz
 

vfone said:
If the amplifier is potential unstable, doesn’t mean it will oscillate, but the final circuit must be verified for oscillations in a wide frequency range.

PCB grounding, decoupling, and matching impedances (especially input match) are important getting good stability.

I don’t think is something wrong with the reference project because this is optimized for a relative narrow frequency range.
Click to expand...

Up this since I have question here.

How do you verify for oscillations? By sweeping a CW or desired signal through low freq close to DC to high end?

Thanks.
 

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