Power amplifiers blow-up in a bi-directional system - RF Cafe Forums
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Post subject: Power amplifiers blow-up in a bi-directional system Posted: Thu Dec 08, 2005 7:22
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!
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.
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.
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?
Post subject: Posted: Thu Dec 08, 2005 3:22 pm
Class A. Isolation supposedly at
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?
Post subject: Posted: Sat Dec
10, 2005 7:16 am
Joined: Mon Jun 27, 2005 2:02 pm
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.
Post subject: Posted: Sat Dec 10, 2005 1:31 pm
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.
Post subject: Posted: Sat Dec 10, 2005 3:30 pm
Joined: Mon Jun 27, 2005 2:02 pm
Hi again johnny,
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