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Suggestions on Amplifier discrepancy needed.

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edf

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Has anyone prototyped Ku Band (~17GHz) amplifiers using package HEMT FET's such as FHC40LG or NE3210S01 and actually realized the gain that the "S parameters" predicted? I've had success at 10GHz, but 2 attempts at 17GHz yielded a peak at ~14.9GHz and only 10dB for 3 stages. I used Rogers RO4003 PCB and Susumu 0.1pF caps (0402 size). I didn't observe any spurious oscillations below 20GHz.
 

Hi,

sorry it is not clear from your post if you had designed 3 stage amplifier or cascaded 3 stages of same amplifier? If the later is the case you should pay attention to matching of your stages becouse cascading will result in sum of gains only if both input and output matchings are excelent (S11 and S22 less than -15dB)

flyhigh
 

Thanks for the reply. I have cascade of 3 identical stages. Persistence paid off. Though I had 1000pF decoupling caps on the drains and no spurious oscillations were visible, after adding a 10uF on drains the gain jump up to 30dB. The peak is still at the lower frequency, but at least I have a fighting chance now!
 

In my opinion, you do not have a prayer of getting a leaded packaged FET to work as an amplifier at Ku band. The source to ground inductance alone will make stability a problem, and pretty much preclude any sort of broadband match.

I have had good luck with internally matched amplifiers, like the Hittite HMC441-LM1.

I suspect if you could find a FET with flip chip ball grid connections, you might have a chance.
 

edf said:
Thanks for the reply. I have cascade of 3 identical stages. Persistence paid off. Though I had 1000pF decoupling caps on the drains and no spurious oscillations were visible, after adding a 10uF on drains the gain jump up to 30dB. The peak is still at the lower frequency, but at least I have a fighting chance now!
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Hi,

now I think that you had a problem with very low frequency of oscillations, maybe around 50-100MHz. It is a common problem with high-freq devices, they tend to have enormous gain of low frequences, and furthermore, I've seen some devices that even have S11 and S22 larger than 1 below 1GHz. If this is the case, you might have to check stability in the simulator using very fine frequency step (5MHz) almost from DC to the end of the S par freq range listed in your data sheet. Unfortunately, it is not enough just to look in the operating range.

flyhigh
 

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