Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums Thank you for visiting RF Cafe!

Power Amplifiers Blow-up in a Bi-Directional System - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


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

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Webmaster:
Kirt Blattenberger,
 BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
 AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

height-line