Matching a differential cascode in 75 ohm to a 50 ohm line

Hi guys,

I have a problem. I need to match a differential cascode amplifier operating in a 75 ohm system to a 50 ohm line at the input. The amplifier has a 75 ohm 2:1 balun at the input, so that it is single ended at the the output of the balun. So it is something like this:

50 ohm line 75 ohm balun
-------------------BALUN-------Differential cascode

single ended differential


I need to match the 50 ohm line to the output of the balun. I cut the trace at the input and put in an SMA connector right before the balun. Now my question is what should I be seeing on the network analyzer if I match it correctly. Also, what would be the best way of doing this? Should I be using LC network, or would it help if I were to use at 50:75 ohm balun? Is there such a thing?

Please help, I would be truly grateful

Thank you.

DK

I'm not sure I fully understand what you have, but a quarter wave of transmission line of length sqrt(50*75) will match 50 to 75 Ohms. Take into account the velocity factor of cable, or effective permittivity of the dielectric, if you use a PCB.

Hi DeboraHarry,

Thanks for replying. What I have is a 50 ohm line going into a transformer that is meant for a 75 ohm system, that then connects to a differential amplifier.

so basically you go into the transformer single ended then after the transformer you have that diff amp, and at the output of the diff amp there is another transformer that converts it back to single ended.

I thought about a quarterwave line, but I dont have board space to work with.

Also, can you tell me, from first principles, what should I be expecting to see on the network analyzer when you have matched a particular port? What would S11 look like when I connect the input to the network analyzer?

thanks,

DK

If it not is a too wide frequency range that need to be matched can it be done with a LC network.
This is how it looks in my VNA when converting a pure 50 Ohm load to 75 Ohm.

Red VSWR is without matching network, yellow is including network.