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LMH6629: Survey of opamps based on a SiGe process for cryogenic applications

Part Number: LMH6629
Other Parts Discussed in Thread: OPA836, , OPA837, THS4551, OPA838, THS4541, OPA855, THS4303, OPA211

I'm investigating the options for a cryogenic (~50K) DC-10MHz LNA design. The source is low impedance (2 Ohm typical at 30K) but does not like bias currents exceeding 5uA.

I know that TI does not specify it's parts at low temperatures, but there are reports about successful use of LMH6629 and OPA836 below 77K in the scientific literature. Both are based on a SiGe process as far as I understand. I do not mind testing a few parts myself, but can not find much information on which other parts are based on a same processes. I would also be interested in FDAs if any are available. Could you please provide some hints?

  • Hello, 

    Not sure about the LMH6629, but the following might be useful for you

    OPA837 (upgraded OPA836)

    THS4551 (FDA)

    OPA838 (decomp like the LMH6629)

    Not really sure that the SiGe thing is what is making this better at lower temps, can you attach your reference of using some of these at 77k?

  • Hello,

    thank you for the list! The 836/837 input voltage noise is quite high, not sure how it will beheave at low temperature... OPA838 could be worth a try... I'm also considering a hybrid with a discrete stage in front

    AFAIK BJT and JFET fail at low temperatures, while SiGe HBT as well as HEMT remain usable (HBT beta in fact rises). For the LMH6629 see the following paper:

  • Thanks, that is a really nice article - I recently surveyed decomp op amps updating my tables with newer parts in this article,

    The OPA838 is SiGe, as is the THS4551 and THS4541, 

    Not sure about the newer bipolar input >4GHz OPA855 type device. 

  • Hello, I started skimming through that article and it seems they might have missed one really important point, 

    Figure 2 shows the amplifier GBP significantly increasing going cold. That means, for transimpedance design, as you generated these figures 10 and the GBP was increasing going cold, the feedback cap needed to be adjusted to avoid response peaking as shown in the noise plot in figure 11 - more room to improve you design methodology it would seem. We tried to simplify this design flow in this presentation. The simple summary is that the ratio of the feedback pole to the Fo described in the ppt is the Q of the closed loop response. Also, I did not see what the Cd was in the article, but adding 10pF on the inverting node is unnecessarily hurting the noise and bandwidth. 

    1856.Transimpedance design flow using high speed op amps.pptx

  • Also, I did get confirmation that the newest OPA855 is also SiGe - just imagine what its GBP will be at 77k!!!

    As those GBP go up, it is almost certain the min stable gain is also going up. 

  • Thank you for the hint - definitely an insightful read!

    I am wondering at what point the GBP would byte you due to the parasitics of the feedback, no matter the gain...

  • Well yes, at higher Rf you eventually get into fempto Farad comp cap solutions, I go to Tee networks at that point, 

  • more specifically, having measured quite a few surface mount resistors I use 0.18pF parasitic as a good guess. If you need lower, at lower values (<20k) Precision Resistive Products makes what we called Red Buf axial resistors that are <.05pF parasitic. They may have surface mount versions now. Those were used in a lot of the early HP network analyzers. Not sure what they use now but would likely be instructive. 

  • Hi,

    I've tested some opamps and discretes at 77 K and below:

    ADA 4897: at 77 K LF noise goes up ~10x

    THS4303: at 77 K LF noise goes up ~2x

    OPA211: noise increases at 77K, does not function at 4K

    For discretes, NXP's SiGe BFU series and the discontinued SiGe NESG3031 work at 10 mK, with increase in beta for large Ic.

    For HEMTs, ATF36136 works at mK temperatures, as do Yong Jin's HEMTs 

    I have a question for Michael Steffes:

    Discrete SiGe transistors work down below 4 K many SiGe opamps I've tested fail. This probably has to due with internal resistors whose values diverge at low temperature. When a VFA is decompensated, you said in your planetanalog article that the degeneration resistor is reduced. This may allow them to work at lower temperatures. Do you know of SiGe op-amps with "minimal" internal resistance?

    Çağlar Girit

  • Well that is going to be the LMH6629 and the OPA855