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Power amplifiers blow-up in a bi-directional system - RF Cafe Forums

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johnny

Post subject: Power amplifiers blow-up in a bi-directional system Posted: Thu Dec 08, 2005 7:22 am

The transistor of the final stage of the amplifiers in a bi-directional amplifier system often blows-up. The system is the usual bi-directional/repeater system: amplifier in each direction, combining diplexers at each end, middle filters for the isolation in the cross-over band. The amplifiers have high gain – above 90dB. The diplexers and the middle filters supposedly give enough isolation so that there is no closed loop around the amplifier-diplexer loop. The pass band of the diplexers and the other filters is 5MHz and the distance between the 2 bands is also 5MHz. This is anywhere in the 400-500MHz or 800-960MHz. When the power supply is switched on no oscillations occur. One can measure the gain in each direction on a network analyzer. But when you start measuring P1dB or IP3 at some power level one of the final amplifier stages in one or the other direction blows-up. The amplifiers are definitely unconditionally stable. In other systems with the same final stages, but in which the gain is lower and the filters can give much better isolation this never happens. Yes, the isolation in the high gain system is a bit too close to the gain but there are never oscillations. We've come up with the idea that the distortion products cross couple in the opposite bands and somehow an instantaneous oscillation happens through the distortion products and that blows the final stages. It seams that the blow-up happens only when certain level of power is applied at switch-on of the signal generator or when the power is being increased and the generator switches through a internal attenuator power level which is seen on the spectrum analyzer as a broadband pulse.

Comments and advices please!

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Guest

Post subject: Posted: Thu Dec 08, 2005 9:36 am

Have you looked at the current? Approcahing the P1dB point will increase the DC current. Perhaps the part is overheating? Another possibilty is tranisents on the bias line. I would check to see if there are any tranisents.

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johnny

Post subject: Posted: Thu Dec 08, 2005 9:50 am

The amplifiers do not blow up if the loop is open – lets say you disconnect the amplifiers in the other direction. The blowing-up final stages have been tested with input powers higher than the power they deliver in full compression including at frequencies where the filter (diplexer) at the output is rejecting (or fully reflecting back). It's definitely something that goes around the loop of amplifiers-diplexers but we could not model it or fully explain how it happens.

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Peter

Post subject: Posted: Thu Dec 08, 2005 3:16 pm

What is your margin on Isolation? (Both gain summed minus both isolations in one channel).

Are those class A or cass AB amplifiers?

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johnny

Post subject: Posted: Thu Dec 08, 2005 3:22 pm

Class A. Isolation supposedly at least 10dB.

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Joe

Post subject: bidirectional amplifierPosted: Thu Dec 08, 2005 4:10 pm

What is the rejection of the diplexer and middle filter for the other band? Are the amplifiers running saturated? Are the bias lines sufficiently isolated?

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IR

Post subject: Posted: Sat Dec 10, 2005 7:16 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello johnny,

From the description of your problem, it seems that you are right in your idea that the problem can be caused by intermodulation products. If this is the problem, I suggest to add additional filter stage to further supress these products, because from your description it seems that you are working on the edge and the difference between this system to the other systems with lower gain is small. You can also alternatively/in addition put a low value attenuator between the middle stages to the final stage in each direction, that will reduce a bit the gain and provide further isolation.

Please keep us posted if this helps! I will be keen to help you more.

_________________

Best regards,

- IR

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johnny

Post subject: Posted: Sat Dec 10, 2005 1:31 pm

Thanks IR.

My thinking is basically the same and actually your suggestions have been tried already. The attenuator helps but the we can not deliver the promised gain; more filtering helps also but then the system is bulkier and more expensive. But again, that's the only way, so now the right people have to be convinced that that's the only way, which is the most difficult part of the job.

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IR

Post subject: Posted: Sat Dec 10, 2005 3:30 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hi again johnny,

Yes convincing the managers who often aren't engineers is sometimes harder than to solve the problem. I am happy that you found the solution to the problem. I guess that there is no simple solution to such problem but addition of components as attenuators and additional filtering which could somewhat degrade the performance. Good luck in the next steps!

_________________

Best regards,

- IR

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