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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! Top 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.
Top 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.
Top 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?
Top johnny Post subject: Posted: Thu Dec 08, 2005
3:22 pm Class A. Isolation supposedly at least 10dB.
Top 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? Top 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 Top 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.
Top 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 Posted
11/12/2012
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