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PA performance degradation possibilities - RF Cafe Forums

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balajisu
Post subject: PA performance degradation possibilities Posted: Fri Jul 07, 2006 10:38 am

Captain


Joined: Wed Jul 06, 2005 5:54 pm
Posts: 24
Location: Germantown,MD
I would really appreciate any comments in this regard..

What are the possibile causes for degradation in PA's performance over time?

Thanks
Balaji


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fred47
Post subject: PA performance degradationPosted: Fri Jul 07, 2006 6:10 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!

For a PA's performance to degrade, something has to change.

That something can either be the active device(s), the bias circuitry, or the input/output matching circuitry.

Normally, the bias circuitry doesn't change unless it's underdesigned (for example, running 1/2 Watt dissipation into a 1/4 W resistor).

The tuning of the input or output matching circuitry can vary due to vibration, if adjustable devices are used and vibration is present. This condition is relatively easy to fix.

However, the most likely change is in the active device, as a PA section usually runs hot. Heat is the enemy of semiconductors. Another issue with some active devices is "metal migration" - as current density in the metallization on the active device increases, the time to failure decreases.

Some devices/manufacturers are worse than others as far as degradation over time goes.

Good Luck!
Fred


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IR
Post subject: Posted: Mon Jul 10, 2006 7:49 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

As fred47 suggested, the worst enemy of semiconductors is heat. Heat over time causes internal damage to the transistor's junction and degrades the output power capabilities (P1dB/linearity degradation).

P.S: If you will operate a PA over time you will see that the Ids increases and also the output power, this is a drift of the Ids with temperature.

You will see in many applications that there is a temperature compensation network in the Gate made of thermistor. This forms a negative feedback that reduces Vgs and hence decreasing Ids.

[/quote]

_________________
Best regards,

- IR


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balajisu
Post subject: Posted: Mon Jul 10, 2006 10:26 am

Captain


Joined: Wed Jul 06, 2005 5:54 pm
Posts: 24
Location: Germantown,MD
Thanks Fred and IR.







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