BFU730F is not as high gain as given in the application note

[pragashsangaran]

We have tried to get 20dB gain from BFU730F but failed. Our Layout is designed to minimize parasitic capacitance and parasitic inductance. However, the peak gain we could get out of BFU730F is only 12dB! There is an 8dB difference! Is BFU730F really can deliver 20dB gain as per "AN11006" attached?

on our layout, we connected BFU730F and multiple 0ohms in series (no matching components). bias are supplied to get Vce = 2V and Ic = 17mA. even without any matching components, BFU730F can only deliver 12dB. We don't believe it's a poor layout because we have designed the layout to minimize the parasitics.

We think it's either the gain shown in AN11006 is inaccurate or BFU730F part to part variation is too much, or BFU730F is faulty. If the parts/BFU730F we got are faulty, it will be great to get the good part sent to us. 

[walton42]

Clearer pictures of schematic:

Left

Right

[walton42]

Here is the corresponding schematic.  The base bias is 860mV and collector is 2.5V.  When measuring with VNA, I used two DC power supplies to apply that voltage.  I also tried varying the base and collector voltage and could not get anything near simulated.

Best,

Alex

[walton42]

Hello,

I am using the same transistor and had the exact same issue.  I attached images of my design, simulations on ADS and measured results with calibrated VNA.  Achieved 28dB gain on ADS and then measured 9dB with   I do not think vias would effect the gain this much.   I'd appreciate support.

 

[pragashsangaran]

I think I resolved the issue but I'm not sure because I will only know after the measurement. for stability, we need to check Mu_source and Mu_load from DC up to 10GHz / 20GHz even when you are operating at a particular frequency. looking at your layout, the parasitic inductance should be minimal. you might not get high gain due to RC feedback or feedback circuit implemented. it will kill the gain for stability.  

[walton42]

Thanks for the reply.  Wouldn't that all be accounted for in the ADS simulation?  The ADS layout simulated is the exact same as the fabricated board.

[pragashsangaran]

as I mentioned, Mu_source and Mu_load need to be simulated wideband for stability. can you correct your stability simulation to see if it's still stable? in my experience, the gain is lower due to parasitic inductance and I also had stability issues. Please check if you have stability issue using spectrum analyzer.

[walton42]

Will do! Did you happen to use the mex or spice schematic symbol? 

[pragashsangaran]

im not really sure but i think it's mex. 

[LPP]

NXP board differs from yours by multiple ground vias along all the RF path. Poor grounding increases parasitic plane inductance. 0.5nH spread inductance at the source will cause 8dB gain drop.

Pavel

[pragashsangaran]

Hi Pavel,

I replied to this commend earlier, and I don't know if you read my answers before. Let me rephrase it again. We have ground vias along the RF path. I have snapped a clear pic using a better camera for your information!

Disclaimer: you might not see all the vias due to the silkscreen (ref des). 

[pragashsangaran]

Hi TIC, thank you so much for replying on the weekend. I was waiting for a reply for a long time—my response to your comments.
1. yes, I did that, and it looks fine.
2. we also have R3 and R4.
3. solder mask preventing you from seeing via stitches along the RF path. We have via stitched along the RF path from RFin to RFout without any gap.
4. we bought from the Digikey. Shall we return to Digikey?

My prime suspect is device soldering temperature. The maximum emitter solder temperature is 90-degree Celsius. At this temperature, our solder/solder paste is not melting. We don't have a heat sink designed on the bottom side of the PCB.
My second suspicion is Digikey ships wrong devices. On top of the BFU730F devices, written "D6t". Could you please clarify if this is the right code for BFU730F?
The reason why I don't think its layout problem is because for BFU790F we can get 12dB stable gain. For BFU790F, this is really good and almost MSG at 2.5GHz. But for BFU730F, we are getting the same gain which doesn't make any sense! I don't think parasitics will be bad for BFU730F and good for BFU790F for the same layout! It's doesnt work that way. As I mentioned earlier, the layout is designed by minimizing parasitics, so BFU730F performance is not acceptable.

[LPP]

1.
NXP provides measured S-parameters. You can use then to evaluate the maximum gain that you can get from BFU730F. MSG is more than 20bB @ 3GHz.

https://www.nxp.com/products/rf/rf-discrete-components-low-power/rf-wideband-transistors/6-18-ghz-wi...

2.
AN11006 provides performance data as measured on real board. You can order this board through our distributors to validate performance vs your design.

https://www.nxp.com/products/rf/rf-amplifiers-low-medium-power/wideband-amplifiers/low-noise-amplifi...

Note, this transistor is conditionally stable and requires stability enhancement. The design in the AN11006 improves stability by means of R3 and R4.

3.
I do not think that device is faulty. Most probably, the lack of gain in your design is caused the layout.

At the first glance the NXP board differs from yours by multiple ground vias along the RF path and regular vias distributed over the board. See AN11006 Figure 6.

Do you use layout with controlled trace impedance?

4.
You can return device for failure analysis. Please contact the distributor who sold you these parts.

Have a great day,
Pavel
TIC