THS3491: Impedance matching to drive a 50 ohm transmission line

Why do you have that 2nd 50ohm on the output, it would seem to me just get rid of that and use a 12.5ohm source R into the balun and you would get a matched driver without the extra 50ohm. 

here is what I had in mind, the bus thinks it sees a 50ohm termination looking back through the balun. here I used one that seems to have about the right frequency span - you probably need a blocking cap on the input side to keep DC current at 0 - driving right into the Balun will create a lot of DC current due to output offset voltage. I did change the R values around the THS3491 to increase loop gain, you have it real bandlimited with the values you used. Also, on the output side you need a DC current path - for the sim at least. 

Oops, I forgot to change the load on the secondary to 50ohm, here is an 8.8Vpp swing at 10MHz, I also changed this to +/-7.5V supplies, 

And here is the SSBW response with correct load, rolled off about 0.6dB at 20MHz, 

THS3491RGT through balun to 50ohm.TSC

Hello Carl,

I am taking some time to look over your circuit and the writeup; please keep us updated in the thread if you simulate and/or adopt Michael's suggestion.

Best,

Alec

Hey Carl, I got to ruminating on other ways to do this and I remembered this very ubiquitous (but somewhat secret) active impedance differential driver that is used throughout the DSL industry. Here, with cross coupled feedback, we can create more output apparent impedance (I call it synthetic impedance) than the physical buildout Ro. This was done early on in DSL line drivers to get more power to the load and less wasted in the buildout R. Most folks have not seen this, but those of us in DSL lean on it everywhere. The point is with a part like the dual OPA2677, you could easily deliver your desired load power on +/-6V or maybe even lower, I will wait to see where you think I have already gone astray before I try to fire up those design spreadsheets I developed many years ago - I think the last time I looked at them was maybe 2012. Here is the basic topology, and the analysis is very involved, but the results are powerful. I have found those tools, but will not waste any time on them if this is going the wrong direction. The balun would go where Rload is and the required differential input could just be another Balun. This circuit is always driven by a complementary output DAC in the DSL world through a passive filter into the op amp inputs - and the OPA2677 is quite commonly used, not only do you get this nice push/pull doubling of the output voltage swing, but you don't lose 6dB through Ro element anymore. 

Hi Michael,

the audio industry is also using these cross-coupled circuits. The datasheet of DRV134 shows a similar circuit.

These circuits are a bit critical because they either introduce a positive feedback path or a dual feedback topology which only work stably with a limited range of load capacitance. These circuits can beautifully work with a fixed and constant load, but can become instable with varying loads.

When I had to do with audio electronics some decades ago, I decided against cross-coupled differential outputs because of stability issues which I observed when a cross-coupled differential output was loaded "non-differentially" (with one output becoming connected to signal ground). In some difficult applications I even used good old audio transformers.

Kai

There is probably an issue using a VFA topology, the CFA works very well to widely varying loads as the DSL line impedance goes all over the place, 

Hi Michael, 

thank you for the replies, I'm looking at all this during the current morning.

The reason I have a second 50 ohm on the output may be unclear with my provided schematic. The driver is terminated by a 50 ohm because of the requirements when the bus (coax) isn't connected and the second 50 ohm at the end represent the terminator I'm using when I plug the bus (coaxial cable). It makes sense that it causes an impedance mismatch, however I'm unsure about how to match the impedance with a "floating" driver and a "connected" bus with the coax cable. I will look at your provided schematic below, I'm guessing there is a way to "actively" match an impedance depending if the bus is connected or not? 

Regards

sounds good, I will give it a try and come back to you soon

thanks

Hi Carl,

Thank you for keeping us updated.

Best,

Alec