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Tuesday, June 9, 2015

Termination Insensitivity -- Only Three TIAs in the World?

Termination Insensitivity.  Is this like when your girlfriend dumps you and you just don't care? 

No. It's more like this:


Wes Hayward and Bob Kopski developed the Termination Insensitive Amplifier circuit back in 2009.  With a bidirectional rig it is important that the crystal filter face the same impedance in both directions.  Simpler bidirectional amps are impedance "transparent" in that their input impedance depends on what is on the output (and vice versa).  The circuit that Wes and Bob developed solved this problem:  these amps always look like 50 ohms.  That's very useful.  When I was building my earlier BITX rigs, Farhan suggested that I try the termination insensitive amp circuits.  On my latest project I followed his suggestion.  At the same time, Pete was building a new BITX, also using the TIA amps. 

Pete wrote to Wes and Bob and asked if anyone had followed through on their 2009 article by building a rig using this circuit.  Wes said that he was unaware of any TIA rigs.  It turned out that Tom Hall up in New York City had built one (video below). So I guess Pete's rig is TIA 2 and mine is TIA 3!  (But I was thinking, because of Pete's Italian ancestry, shouldn't we call his rig a Zia?)
Please let us know if you know of any other TIAS (or ZIAS!) out there.

Here is Tom Hall's very FB TIA:

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20


  1. Really inspiring work guys! I am working on a manhattan bitx20 with the si5351 but using the standard farhan bilateral amps...

    I almost have a direct conversion receiver working but with an arduino/dds8950 as the vfo... Had a problem that I think can be traced to the output power of the vfo which was not enough p2p to effectively drive my homebrew diode dbm. (thought it may be the transformers that I wound with 30 guage wire wrap wire, so I rebuilt them with magnet wire only to find the same issue... hmm)

    so to fix it i thought I'd put an amp between the vfo and dbm... I chose to build one of the TIA amps, but did not use the "standard" design in wes/bob's paper. instead I used the 2 transistor version labeled as figure 6.... I havent had any luck.. no amplification. I reached point basta and havent made it back to the bench in a few days... likely i'll find some mistake, but it leads me to a question maybe you can help me with:

    Why (in the version labeled fig 6) does the input drive the emitter? this is a different setup than the 'standard' base driven design of farhan/wes and I am trying to wrap my head around it... if you could share any insights on that it would be great!

    Once I get this one going (and get over the first transceiver learning curve) I think I'll use a similar TIA setup that you are using now for v2... thank you all for sharing the progress and keep up the great work with the podcast. you guys have been my virtual homebrew elmers and I really appreciate it! I'll share some of my work with you guys once i get it going. 73

    1. correction -- its the dds9850...

    2. For a "booster amp' used both with the AD9850 and the Si5351 --see http://www.n6qw.com/ and connect to the link on the Lets Build Something. On the detailed LBS build page is the schematic for the booster amp. There is also info on homebrew DBM's. If you check youtube under N6QW there are three videos on building DBM's. BTW my TIA (ZIA) is taking the output direct from the Si5351 without any booster amp -- I believe the use of the SBL-1's versus a homebrew DBM may account for not needing the amp. Although I did find I do not get as much "carrier unbalance" for tune up. But my final approach will use an Arduino generated audio tone at 988 Hz.

      Pete N6QW

    3. Thanks Pete -- I built the booster as suggested over the weekend and it worked great! It looks to have roughly doubled the output of my dds 9850 and now it's driving my homebrew dbm no problem -- I have a receiver running! The audio sounded good even with sub-par conditions on 20 (i could only find stations coming in at 57 on my ft857 to compare to) and a very nearby 50kW am station bleeding in a bit (WNWR).

      I didn't have a 100r pot, so I just used two resistors (56r and 47r) in voltage divider config... Does that pot happen to adjust gain?

      Anyhow thanks again for the advice and cograts on the hall of fame award!

      ...Now onward toward the full transceiver!

  2. I thought farham's minima design used TIAs.
    Nice work regardless guys

  3. http://vk2sja.org/piffle/

    Many have built it, but few have measured parameters other than gain.


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  5. yes, the entire homebrew world revolves around what you are aware of...


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