How to design a balun circuit for an RF application?
Input impedance is 50 Ohm Output impedance is 100 Ohm

For an unbalanced 50 Ω to 100 Ω impedance matcher you can use this online calculator: -

Rin is the input impedance and RL is the load impedance.
To make that circuit balanced, you need to split R1 into two resistors of equal value: -