• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 50 subscribers
  • Views 358 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THS4541: Impedance match differential amplier and Balun

Jeiner Alvarado Fonseca
Prodigy 50 points
Part Number: THS4541

I am using the THS4541 after a Balun 1-8 www.minicircuits.com/.../TC8-1 .pdf I was checking Noise analisys table on page 31 of the THS4541 datasheet, and Circuit example on Figure 74, page41.

I need to adjust in the best way possible the impedance between the secondary terminal of the balun and the THS4541, I really need reduce the noise all possible on this circuit. The input signal on the primary terminal of the Balun is around 10Vpp. In this case, the relation between primary and secondary is 8.

The input of the primary is 50ohms, this should be 400ohms on the secondary. So, will I require set a gain of 12 on the THS454 based on the table of page 31 to match impedance?

  • 0
    TI__Mastermind 26675 points

    Hello Jeiner,

       For 1/8 turn ratio, you would need turns ratio squared ((1/8)^2) which will be the impedance ratio across the balun. The balun will take care of the 50 ohm matching at input source, and you would need a differential RG termination of 50/((1/8)^2) which will be 3.2k ohms or RG = 1600 ohms.

       We can look into adjusting the resistor ratio for your desired output range at the amplifier. What is your desired gain settings? 

    Thank you,
    Sima 

  • 0
    Prodigy 50 points

    Hello Sima,

    The gain 2 or 3 time maximum, because I was checking on datasheet and gain noise also increased. So, we need a low gain here

    Thank you for your message,

    Best regards,

    Heiner

  • 0 in reply to Jeiner Alvarado Fonseca
    Guru 48395 points

    Morning all, 

    Well deep in my early article archives (this one is 1996, right after I joined BurrBrown) is one option for single ended input to single ended output very low NF HF amplifiers. Figure 10 we built and got <3dB noise figure. Of course much better devices are available now for this approach, Figure 10 would be a good place to start. Darn, not letting me paste in snips - oh well, Figure 10 shows a 1:4 turns input Balun where the 50ohm match is achieved with feedback with an imbedded inverting CFA stage. Lot's of detail in the article, but just trying Figure 10 with newer parts would be an idea. It is about 26dB gain to the final matched load with >40MHz BW. The missing page here was an ad. Using a resistor for the balun secondary match limits you to a min 6dB noise figure, this one gets down below 3dB using an active input match. 3dB NF maps to an input referred spot noise of 0.45nV/rootHz.

    It is odd that you are trying to use an FDA on the input after the Balun then going single ended with the next stage. This one just stays single ended all the way through. 

    Composite Balun input low NF Microwaves and RF article .pdf

  • 0 in reply to Michael Steffes
    Prodigy 50 points

    Hello Michael,

    Thank you for the information. The articule is very helpful.