[SOLVED] PA has low forward power

[Mahruz]

Hi All,
My question is regarding a UHF power amplifier ( https://www.digikey.com.au/en/products/detail/stmicroelectronics/PD85006L-E/2864729 ) output power.

1. I require this component to put out 3 Watts @870MHz at max instead of its rated max; however, it never does. I only get 0.5W.
Below is the schematic of the matching and power supply for the PA(Q2) IC. It is runs on gate which has 5V supply and drain has 13V8. L8 and L4 are shorted with solder by hand (manually).
Is there something wrong in the design/matching?
I know this design works because I have built over 200 of them. Only issue is that out of 200, nearly 4o to 60 fails because of low output power (0.5W).

2. Does anyone have the design files for the above mentioned PA; I check with the vendor but it is so old that they removed it from their system.



Thanks,

 

[BigBoss]

Have you ever changed you ordinary supplier ? So have you used the same supplier or have you intended to buy this part from different supplier-for instance aliexpress-.?
Because some suppliers sell " second hand" so de-soldered components ( especially chinese suppliers ) and some of them absolutely are damaged. Especially MOS like components.
This might be true for passive components and they have counterfeit clones. Of course they are terrible and no specification is satisfied.

 

[Mahruz]

Hi Big Boss,

I got the component from suppliers such as digikey/element 14/mouser long time ago (last buy).
Then I had it in stock, unused and within the correct packing. It was taken out from the packing whenever it was required.

Is it possible, there could be some other hidden issues regarding the IC?

Thanks,

 

[G4BCH]

Compare the input and output match between a good unit and the bad units, it may give some clues.

 

[BigBoss]

Hi Big Boss,

I got the component from suppliers such as digikey/element 14/mouser long time ago (last buy).
Then I had it in stock, unused and within the correct packing. It was taken out from the packing whenever it was required.

Is it possible, there could be some other hidden issues regarding the IC?

Thanks,

It's possible. Because last samples may have defects or damages. But I suspect the passive components as well. Because there are tons of counterfeits in the market and only original manufacturer can distinguish those fake or not.
ST has especially a bad reputation on their quality according to my past experiences.
Is DC Operating Point OK ? All currents and Voltages at Quiescent regime ?

 

[vfone]

Years ago I used as a PA driver a PD85006L (bought from Arrow) and I remember was sensitive to the input and output impedance matching. Otherwise put 5W output power with no problems.
As you posted, if you don't use any input/output matching networks, no wonder you get in this situation.
Set the Idq to 200mA, which means Vgs = 4V.

 

[biff44]

i am wondering if the amplifier design is potentially unstable. Sometimes if a FET bursts into oscillations, it does some really weird bias things and destroys itself.

so the 40 or so amplifiers with "low gain" might have been fine for the first 5 seconds of its life, but then the transistor got damaged, and the gain dropped way down?

I have seen some odd bias stopping measures....like small resistors in series with the bypass capacitors, etc, to keep the thing from oscillating at 500 KHz or so.

 

[Audioguru]

Mosfets have a wide range of Vgs sensitivity to conduct. Some work fine with a couple of volts but others need many volts, even when they all have the same part number.
The designer has threechoices:
1) Test all the Mosfets and pick only the ones that conduct enough. Throw away all the Mosfets that are too sensitive or are not sensitive enough.
2) Add a trimpot that needs to be adjusted to the sensitivity of the Mosfet.
3) Design the circuit with some DC negative feedback (a source to ground resistor parallel with a capacitor).

 

[mtwieg]

Hi Big Boss,

I got the component from suppliers such as digikey/element 14/mouser long time ago (last buy).
Then I had it in stock, unused and within the correct packing. It was taken out from the packing whenever it was required.

Is it possible, there could be some other hidden issues regarding the IC?

Thanks,

Ok those vendors should mean your parts aren't counterfeit. But you mentioned they are somewhat old, so you may be having solderability issues.

Have you checked the DC biasing conditions on the bad units?